CN118325072A - Preparation method and application of low rheology and low yellowing nylon - Google Patents

Abstract

The present invention relates to a preparation method and application of low rheology and low yellowing nylon in the field of polymer material technology. The preparation method of low rheology and low yellowing nylon may include the following steps: low temperature melt polymerization of nylon salt to obtain oligomers, and then final polymerization of the oligomer melt to obtain the low rheology and low yellowing nylon; wherein the device used for the final polymerization is a self-cleaning reactor. The present invention prepares low rheology and low yellowing nylon by low temperature melt preparation of oligomers and final polymerization in a self-cleaning reactor, which better solves the problems of serious side reactions, high rheological viscosity, and easy yellowing of long carbon chain nylon in the prior art, and achieves better technical effects.

Description

Preparation method and application of low rheology and low yellowing nylon

Technical Field

The invention relates to the technical field of polymer materials, and more particularly to a preparation method and application of low-rheology and low-yellowing nylon.

Background technique

Long carbon chain nylon, such as nylon 1212, nylon 1012, etc., has the characteristics of low density, low water absorption, good dimensional stability, excellent chemical resistance, corrosion resistance, wear resistance, fatigue resistance and good low temperature impact resistance. It is mainly used in the automotive, electrical, machinery and other industries, such as coil skeletons, insulation layers of wires and cables, fuel oil pipelines, hydraulic system pipelines, conduits, etc.

Generally, the preparation of long carbon chain nylon often adopts the method of prepolymerization and solid phase viscosity enhancement, which requires a long reaction time, and has problems such as uneven molecular weight distribution, high rheological viscosity, and easy oxidation causing yellowing. There is also a method of using a twin-screw extruder for viscosity enhancement, but the nylon products prepared by traditional twin-screw extruders have poor shear performance, wide molecular weight distribution, and cross-linking.

Summary of the invention

In order to overcome the problems of serious side reactions, high rheological viscosity, easy yellowing, etc. in the prior art for preparing long carbon chain nylon, the present invention proposes a preparation method and application of low rheological and low yellowing nylon.

One of the purposes of the present invention is to provide a method for preparing low rheology and low yellowing nylon, which may include the following steps:

The nylon salt is melt polymerized at low temperature to obtain an oligomer, and then the oligomer melt is final polymerized to obtain the low rheology and low yellowing nylon; preferably, the device used for the final polymerization is a self-cleaning reactor; more preferably, it is a horizontal self-cleaning reactor.

in,

The device used for the low-temperature melt polymerization is a polycondensation kettle or a polymerization kettle;

Preferably,

The nylon salt is melt-polymerized at low temperature in a polycondensation reactor or a polymerization reactor to obtain an oligomer, and then the oligomer melt is final-polymerized in a horizontal self-cleaning reactor to obtain the low-rheology and low-yellowing nylon.

in,

The nylon salt is prepared from a diamine and a dibasic acid; the nylon salt is preferably a long carbon chain nylon salt;

Wherein, the diamine is selected from C5-C20 diamines, and the dibasic acid is selected from C5-C20 dibasic acids;

The molar ratio of the dibasic acid to the diamine may be 1:(1-1.05), preferably 1:(1-1.01).

specifically,

The nylon salt can be prepared by a method comprising the following steps:

a) mixing the dibasic acid with solvent A, and heating the mixture to obtain solution A;

b) mixing the diamine with solvent B and heating to obtain solution B;

c) adding the solution B into the solution A to react and obtain the nylon salt.

In the step a),

The solvent A may be selected from at least one of methanol, ethanol and propanol;

The weight ratio of the dibasic acid to the solvent A may be 1:(2-10), preferably 1:(3-7);

The heating may be heating to 50-95° C. and maintaining for 10-50 min; preferably, heating to 60-75° C. and maintaining for 20-40 min.

and / or,

In the step b),

The solvent B may be selected from at least one of methanol, ethanol and propanol;

The weight ratio of the diamine to the solvent B may be 1:(1-6), preferably 1:(1-3);

The heating may be heating to 40-90° C. and maintaining for 10-50 min; preferably, heating to 50-70° C. and maintaining for 20-40 min.

and / or,

In the step c),

The reaction conditions may be: reacting at 50-100° C. for 0.2-4 h; preferably, reacting at 70-80° C. for 0.5-2 h.

After step c), the mixture is cooled to below 40° C., preferably below 30° C., and then filtered and dried to obtain nylon salt.

The nylon salt prepared by the preparation method has the characteristics of uniform particles and equal molar ratio of acid and amine.

In the specific implementation,

The low temperature melt polymerization oligomer is obtained by subjecting components including nylon salt, water, a catalyst, a molecular weight regulator, and an optional antioxidant to a low temperature melt polymerization reaction;

Preferably, the conditions for the low temperature melt polymerization reaction may be:

The reaction temperature may be 150 to 220°C; for example, it may be in the temperature range of 180 to 220°C;

The reaction pressure can be 0.8～2.0MPa;

The reaction time may be 0.2 to 8.0 h; for example, the reaction time may be in the range of 0.5 to 8 h, 1.0 to 8.0 h, etc.

The molecular weight regulator may be at least one of monocarboxylic acid, dicarboxylic acid, monoamine and diamine; preferably, the molecular weight regulator may be used in an amount of 0.05 to 1.0 weight % of the weight of the nylon salt.

The amount of water used can be 20 to 100% by weight of the nylon salt.

The catalyst may be selected from at least one of phosphoric acid, phosphorous acid, hypophosphorous acid, sodium phosphite, and sodium hypophosphite; preferably, the catalyst may be used in an amount of 0.05 to 0.5 weight % of the weight of the nylon salt.

The antioxidant may be selected from at least one of bis(2,4-dicumylphenyl)pentaerythritol diphosphite, antioxidant 1790, antioxidant 3114, antioxidant 1010, and antioxidant 1098; preferably, the antioxidant may be used in an amount of 0.05 to 0.5 weight % of the weight of the nylon salt.

In the specific implementation,

The process of finally polymerizing the oligomer melt to obtain the low rheology and low yellowing nylon may include the following steps:

The oligomer melt is discharged into a horizontal self-cleaning reactor, and the low rheology and low yellowing nylon is obtained through final polymerization.

The final polycondensation reaction specifically includes discharging the oligomer melt reduced to normal pressure into a horizontal self-cleaning reactor, removing moisture at normal pressure at a certain temperature and obtaining a long carbon chain nylon with uniform molecular weight distribution, low rheology and less yellowing through final polycondensation; preferably, the reaction temperature of the final polycondensation reaction is 180 to 280° C., for example, specifically, it can be in the temperature range of 200 to 240° C., and the reaction time is 0.2 to 2.0 h, for example, specifically, it can be in the time range of 1 to 2 h.

The horizontal self-cleaning reactor used in the present invention is a commercially available horizontal self-cleaning reactor, specifically, the SCP-120 horizontal reactor produced by Nanjing Chengmeng Company.

The second object of the present invention is to provide low rheology, low yellowing nylon prepared by the preparation method of low rheology, low yellowing nylon described in one of the objects of the present invention; preferably, under the test conditions of 240°C and 11/s, the rheological viscosity of the low rheology, low yellowing nylon is lower than 1500Pa*s; the molecular weight distribution index of the low rheology, low yellowing nylon is less than 2.0.

The third object of the present invention is to provide the application of the low rheology and low yellowing nylon prepared by the preparation method described in the first object of the present invention or the low rheology and low yellowing nylon described in the second object of the present invention, preferably in automotive oil pipes, hydraulic pipelines, and electrical industries.

The invention prepares low-rheology and low-yellowing nylon by low-temperature melting and final polymerization in a horizontal self-cleaning reactor, which effectively solves the problems of serious side reactions, high rheological viscosity, and easy yellowing of long-carbon-chain nylon in the prior art, and achieves better technical effects.

The invention provides a low rheology and low yellowing nylon and a preparation method thereof. The low rheology and low yellowing nylon with narrow molecular weight distribution, high strength and good toughness is prepared by low-temperature melting to prepare nylon oligomers and final polymerization of the oligomers in a self-cleaning reactor.

The present invention will be further described below by way of examples.

Detailed ways

The present invention is described in detail below in conjunction with specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the contents of the present invention still fall within the scope of protection of the present invention.

The endpoints and any values of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, the endpoint values of each range and the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this article.

Source of raw materials

The raw materials used in the examples and comparative examples, unless otherwise specified, are all disclosed in the prior art, for example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[Example 1]

1. Nylon salt formation: (1) Add 9000.5g of dodecanedioic acid and 45000g of ethanol to a neutralization kettle, heat to 75°C to dissolve, and keep for 30 minutes; (2) Add 7835g of dodecanediamine and 10000g of ethanol to a dissolution kettle, heat to 60°C to dissolve, and keep for 30 minutes; (3) Pump all the dodecanediamine ethanol solution into the neutralization kettle with a pump, react while stirring, and control the reaction temperature at 78°C. After adding the materials, continue to react for 1.0 hour, cool the neutralization kettle to reduce the temperature to below 30°C, filter the neutralized solution through a filter, and dry it in a drying oven to obtain nylon 1212 salt;

2. Low-temperature melt prepolymerization of nylon 1212: (1) 12000 g of nylon 1212 salt was mixed with 6000 g of water, 12 g of sodium hypophosphite as a catalyst, 12 g of dodecanedioic acid as a molecular weight regulator, and 12 g of antioxidant 1010, and then added into a 30 L high-temperature and high-pressure polymerization kettle; (2) the temperature was raised to 200° C. for 1.0 hour, and the pressure in the kettle was raised to 1.5 MPa, and the temperature and pressure were maintained for 1.0 hour; (3) the reaction temperature and pressure were maintained for 1.0 hour while steam was continuously released to maintain the reaction temperature and pressure; (4) the reaction temperature was kept unchanged and the pressure was reduced to normal pressure for 2.0 hours to obtain nylon 1212 oligomer;

3. Final polymerization of nylon 1212 in a self-cleaning reactor: (1) Nylon 1212 oligomers are continuously discharged into a 12L self-cleaning reactor (SCP-120 horizontal reactor, produced by Nanjing Chengmeng Company) through a melt pump at a feed rate of 6L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 240°C, the residence time of the oligomer melt in the reactor is 1.0 hour, and moisture is removed under normal pressure for final polymerization. After extrusion by a discharge screw, the mixture is cooled and pelletized to obtain a nylon 1212 product.

[Example 2]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 self-cleaning reactor final polymerization step is the same as in Example 1, and the nylon 1212 low-temperature melt prepolymerization step comprises:

(1) 12000 g of nylon 1212 salt was mixed with 6000 g of water, 12 g of sodium hypophosphite as a catalyst, 12 g of dodecanedioic acid as a molecular weight regulator, and 12 g of antioxidant 1010, and then added into a 30 L high temperature and high pressure polymerization kettle; (2) the temperature was raised to 180° C. for 1.0 hour, and the pressure in the kettle was raised to 1.0 MPa, and the temperature and pressure were maintained for 1.0 hour; (3) the steam was continuously released to maintain the reaction temperature and pressure for 1.0 hour; (4) the reaction temperature was kept unchanged and the pressure was reduced to normal pressure for 2.0 hours to obtain nylon 1212 oligomer.

[Example 3]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 self-cleaning reactor final polymerization step is the same as in Example 1, and the nylon 1212 low-temperature melt prepolymerization step comprises:

(1) 12000 g of nylon 1212 salt was mixed with 6000 g of water, 12 g of sodium hypophosphite as a catalyst, 12 g of dodecanedioic acid as a molecular weight regulator, and 12 g of antioxidant 1010, and then added into a 30 L high temperature and high pressure polymerization kettle; (2) the temperature was raised to 220° C. for 1.0 hour, and the pressure in the kettle was raised to 2.0 MPa, and the temperature and pressure were maintained for 1.0 hour; (3) the steam was continuously released to maintain the reaction temperature and pressure for 1.0 hour; (4) the reaction temperature was maintained unchanged and the pressure was reduced to normal pressure for 2.0 hours to obtain nylon 1212 oligomer.

[Example 4]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 self-cleaning reactor final polymerization step is the same as in Example 1, and the nylon 1212 low-temperature melt prepolymerization step comprises:

(1) 12000 g of nylon 1212 salt was mixed with 6000 g of water, 12 g of sodium hypophosphite as a catalyst, 12 g of dodecanedioic acid as a molecular weight regulator, and 12 g of antioxidant 1010, and then added into a 30 L high temperature and high pressure polymerization kettle; (2) the temperature was raised to 200° C. for 1.0 hour, and the pressure in the kettle was raised to 1.5 MPa, and the temperature and pressure were maintained for 0.25 hour; (3) the steam was continuously released to maintain the reaction temperature and pressure for 0.25 hour; (4) the reaction temperature was kept unchanged and the pressure was reduced to normal pressure for 0.5 hour to obtain nylon 1212 oligomer.

[Example 5]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 self-cleaning reactor final polymerization step is the same as in Example 1, and the nylon 1212 low-temperature melt prepolymerization step comprises:

(1) 12000g of nylon 1212 salt was mixed with 6000g of water, 12g of catalyst sodium hypophosphite, 12g of molecular weight regulator dodecane dicarboxylic acid, and 12g of antioxidant 1010, and then added into a 30L high temperature and high pressure polymerization kettle; (2) the temperature was raised to 200°C for 1.0 hour, the pressure in the kettle was raised to 1.5MPa, and the temperature and pressure were maintained for 2.0 hours; (3) the steam was continuously released to maintain the reaction temperature and pressure for 2.0 hours; (4) the reaction temperature was kept unchanged and the pressure was reduced to normal pressure for 4.0 hours to obtain nylon 1212 oligomer.

[Example 6]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 low-temperature melt prepolymerization step is the same as in Example 1, and the nylon 1212 self-cleaning reactor final polymerization step comprises:

(1) The nylon 1212 oligomer is continuously discharged into a 12L self-cleaning reactor through a melt pump at a feed rate of 6L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 200°C, the residence time of the oligomer melt in the reactor is 1.0 hour, water is removed under normal pressure and final polymerization is carried out, and the discharge screw is extruded and cooled and pelletized to obtain a nylon 1212 product.

[Example 7]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 low-temperature melt prepolymerization step is the same as in Example 1, and the nylon 1212 self-cleaning reactor final polymerization step comprises:

(1) The nylon 1212 oligomer is continuously discharged into a 12L self-cleaning reactor through a melt pump at a feed rate of 6L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 280°C, the residence time of the oligomer melt in the reactor is 1.0 hour, water is removed under normal pressure, and the final polymerization is carried out. After extrusion by a discharge screw, the product is cooled and pelletized to obtain a nylon 1212 product.

[Example 8]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 low-temperature melt prepolymerization step is the same as in Example 1, and the nylon 1212 self-cleaning reactor final polymerization step comprises:

(1) Nylon 1212 oligomer is continuously discharged into a 12L self-cleaning reactor through a melt pump at a feed rate of 30L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 240°C, the residence time of the oligomer melt in the reactor is 0.2 hours, water is removed under normal pressure, and the final polymerization is carried out. After extrusion by a discharge screw, the product is cooled and pelletized to obtain a nylon 1212 product.

[Example 9]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 low-temperature melt prepolymerization step is the same as in Example 1, and the nylon 1212 self-cleaning reactor final polymerization step comprises:

(1) Nylon 1212 oligomer is continuously discharged into a 12L self-cleaning reactor through a melt pump at a feed rate of 3L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 240°C, the residence time of the oligomer melt in the reactor is 2 hours, water is removed under normal pressure and final polymerization is carried out, and the discharge screw is extruded and cooled and pelletized to obtain a nylon 1212 product.

[Example 10]

1. Nylon salt formation: (1) Add 9000.5g of dodecane dicarboxylic acid and 45000g of ethanol to a neutralization kettle, heat to 75°C to dissolve, and keep for 30 minutes; (2) Add 6733.8g of decanediamine and 10000g of ethanol to a dissolution kettle, heat to 60°C to dissolve, and keep for 30 minutes; (3) Pump all the dodecane diamine ethanol solution into the neutralization kettle with a pump, react while stirring, and control the reaction temperature at 78°C. After adding the materials, continue to react for 1.0 hour, cool the neutralization kettle to reduce the temperature to below 30°C, filter the neutralized solution through a filter, and dry it in a drying oven to obtain nylon 1012 salt;

2. Low-temperature melt prepolymerization of nylon 1012: (1) 12000 g of nylon 1012 salt was mixed with 6000 g of water, 12 g of sodium hypophosphite as a catalyst, 12 g of dodecane dicarboxylic acid as a molecular weight regulator, and 12 g of antioxidant 1010, and then added into a 30 L high-temperature and high-pressure polymerization kettle; (2) the temperature was raised to 200° C. for 1.0 hour, and the pressure in the kettle was raised to 1.5 MPa, and the temperature and pressure were maintained for 1.0 hour; (3) the reaction temperature and pressure were maintained for 1.0 hour while steam was continuously released to maintain the reaction temperature and pressure; (4) the reaction temperature was kept unchanged and the pressure was reduced to normal pressure for 2.0 hours to obtain nylon 1012 oligomer;

3. Final polymerization of nylon 1012 in a self-cleaning reactor: (1) Nylon 1012 oligomers are continuously discharged into a 12L self-cleaning reactor through a melt pump at a feed rate of 6L/h, and the liquid level is maintained at 50%; (2) The reaction temperature is 240°C, the residence time of the oligomer melt in the reactor is 1.0 hour, and water is removed under normal pressure for final polymerization. After extrusion by a discharge screw, the product is cooled and pelletized to obtain a nylon 1012 product.

[Comparative Example 1]

The nylon salt-forming step is the same as in Example 1, without the final polymerization step. The nylon 1212 low-temperature melt prepolymerization step comprises:

(1) 12000g of nylon 1212 salt is mixed with 6000g of water, 12g of catalyst sodium hypophosphite, 12g of molecular weight regulator dodecane dicarboxylic acid, and 12g of antioxidant 1010, and then added into a 30L high temperature and high pressure polymerization kettle; (2) the temperature is raised to 200°C for 1.0 hour, the pressure in the kettle is raised to 1.5MPa, and the temperature and pressure are maintained for 1.0 hour; (3) the steam is continuously released to maintain the reaction temperature and pressure for 1.0 hour; (4) the reaction temperature is maintained unchanged and reduced to normal pressure for 2.0 hours to obtain nylon 1212 oligomer; (5) nitrogen is added for extrusion, cooling, and slicing to obtain nylon 1212 product.

[Comparative Example 2]

The nylon salt-forming step is the same as in Example 1, and the nylon 1212 low-temperature melt prepolymerization step and final polymerization step include:

(1) 12000g of nylon 1212 salt is mixed with 6000g of water, 12g of catalyst sodium hypophosphite, 12g of molecular weight regulator dodecane dicarboxylic acid, and 12g of antioxidant 1010, and then added into a 30L high temperature and high pressure polymerization kettle; (2) the temperature is raised to 200°C for 1.0 hour, the pressure in the kettle is raised to 1.5MPa, and the temperature and pressure are maintained for 1.0 hour; (3) the reaction temperature and pressure are maintained for 1.0 hour while steam is continuously released; (4) the reaction temperature is maintained unchanged and reduced to normal pressure for 2.0 hours to obtain nylon 1212 oligomer; (5) the normal pressure is maintained, the temperature is raised to 240°C, and the final polymerization reaction is continued for 1.0 hour, and nitrogen is added for extrusion, cooling, and slicing to obtain nylon 1212 product.

[Comparative Example 3]

The nylon salt-forming step is the same as in Example 1, the nylon 1212 low-temperature melt prepolymerization step is the same as in Example 1, and the final polymerization step comprises:

Nylon 1212 twin-screw extruder final polymerization: (1) Continuously discharge nylon 1212 oligomer into a twin-screw extruder through a melt pump; (2) The reaction temperature is 240°C, the residence time of the oligomer melt in the reactor is 1.0 hour, and water is removed under normal pressure for final polymerization. After discharge, the material is cooled and pelletized to obtain a nylon 1212 product.

The nylon products obtained in the examples and comparative examples were subjected to molecular weight and distribution tests, yellowness tests, and rheological tests. The results are shown in Table 1:

Table 1

It can be seen from the embodiments of the present invention that the embodiment products of the present invention have the characteristics of narrow molecular weight distribution, low yellowness and low rheology.

It can be seen from Examples 1, 2, and 3 that the higher the prepolymerization temperature, the larger the molecular weight, the wider the distribution, and the greater the viscosity and yellowness;

It can be seen from Examples 1, 4, and 5 that if the prepolymerization time is too long, the larger the molecular weight, the wider the distribution, and the greater the viscosity and yellowness; but if the prepolymerization time is too short, the prepolymerization is insufficient and the molecular weight distribution is also wider;

It can be seen from Examples 1, 6, and 7 that the higher the final polymerization temperature, the larger the molecular weight, the wider the distribution, and the greater the viscosity and yellowness;

It can be seen from Examples 1, 8, and 9 that the longer the final polymerization time, the larger the molecular weight, the wider the distribution, and the greater the viscosity and yellowness;

It can be seen from Example 1 and Comparative Examples 1, 2, and 3 that the molecular weight of the prepolymer is very low, and the nylon polymerized directly through a polycondensation reactor and a twin-screw extruder has a wide molecular weight distribution, a large rheological viscosity, and a large yellowness index due to crosslinking and other reasons.

Claims (14)

1. A method for preparing low rheology and low yellowing nylon, characterized by comprising the following steps: The nylon salt is melt polymerized at low temperature to obtain an oligomer, and then the oligomer melt is final polymerized to obtain the low rheology and low yellowing nylon; preferably, the device used for the final polymerization is a self-cleaning reactor; more preferably, it is a horizontal self-cleaning reactor. 2. The method for preparing low rheology and low yellowing nylon according to claim 1, characterized in that it comprises the following steps: The device used for the low-temperature melt polymerization is a polycondensation kettle or a polymerization kettle; Preferably, The nylon salt is melt-polymerized at low temperature in a polycondensation reactor or a polymerization reactor to obtain an oligomer, and then the oligomer melt is final-polymerized in a horizontal self-cleaning reactor to obtain the low-rheology and low-yellowing nylon. 3. The method for preparing low rheology and low yellowing nylon according to claim 1, characterized in that: The nylon salt is prepared from a diamine and a dibasic acid; the nylon salt is preferably a long carbon chain nylon salt; Wherein, the diamine is selected from C5-C20 diamines, and the dibasic acid is selected from C5-C20 dibasic acids. 4. The method for preparing low rheology and low yellowing nylon according to claim 3, characterized in that: The molar ratio of the dibasic acid to the diamine is 1:(1-1.05), preferably 1:(1-1.01). 5. The method for preparing low rheology and low yellowing nylon according to claim 3, characterized in that: The nylon salt is prepared by a method comprising the following steps: a) mixing the dibasic acid with solvent A, and heating the mixture to obtain solution A; b) mixing the diamine with solvent B and heating to obtain solution B; c) adding the solution B into the solution A to react and obtain the nylon salt. 6. The method for preparing low rheology and low yellowing nylon according to claim 5, characterized in that: In the step a), The solvent A is selected from at least one of methanol, ethanol and propanol; The weight ratio of the dibasic acid to the solvent A is 1:(2-10); The heating is heating to 50-95° C. and maintaining for 10-50 minutes; and / or, In the step b), The solvent B is selected from at least one of methanol, ethanol and propanol; The weight ratio of the diamine to the solvent B is 1:(1-6); The heating is heating to 40-90° C. and maintaining for 10-50 minutes; and / or, In the step c), The reaction conditions are: reacting at 50-100° C. for 0.2-4 h. 7. The method for preparing low rheology and low yellowing nylon according to claim 1, characterized in that: The process of obtaining the oligomer by low temperature melt polymerization comprises: The components including nylon salt, water, catalyst, molecular weight regulator and optional antioxidant are mixed and subjected to low temperature melt polymerization reaction to obtain the product; Preferably, the conditions for the low temperature melt polymerization reaction are: The reaction temperature is 150-220°C; preferably 180-220°C; The reaction pressure is 0.8～2.0MPa; The reaction time is 0.2 to 8.0 hours, preferably 0.5 to 8 hours. 8. The method for preparing low rheology and low yellowing nylon according to claim 7, characterized in that: The molecular weight regulator is at least one of monocarboxylic acid, dicarboxylic acid, monoamine and diamine; Preferably, The molecular weight regulator is used in an amount of 0.05 to 1.0 weight % of the weight of the nylon salt. 9. The method for preparing low rheology and low yellowing nylon according to claim 7, characterized in that: The amount of water used is 20 to 100 weight % of the weight of the nylon salt. 10. The method for preparing low rheology and low yellowing nylon according to claim 7, characterized in that: The catalyst is selected from at least one of phosphoric acid, phosphorous acid, hypophosphorous acid, sodium phosphite, and sodium hypophosphite; Preferably, The amount of the catalyst used is 0.05 to 0.5 weight % of the weight of the nylon salt. 11. The method for preparing low rheology and low yellowing nylon according to claim 7, characterized in that: The antioxidant is selected from at least one of bis(2,4-dicumylphenyl)pentaerythritol diphosphite, antioxidant 1790, antioxidant 3114, antioxidant 1010, and antioxidant 1098; Preferably, The amount of the antioxidant is 0.05-0.5 weight % of the weight of the nylon salt. 12. The method for preparing low rheology and low yellowing nylon according to claim 1, characterized in that: The process of finally polymerizing the oligomer melt to obtain the low rheology and low yellowing nylon comprises the following steps: The oligomer melt is discharged into a horizontal self-cleaning reactor, and the low rheology and low yellowing nylon is obtained through a final polymerization reaction; preferably, the reaction temperature of the final polymerization reaction is 180-280° C., and the reaction time is 0.2-2.0 h. 13. The low rheology, low yellowing nylon prepared by the preparation method of low rheology, low yellowing nylon according to any one of claims 1 to 12, preferably, under the test conditions of 240°C and 11/s, the rheological viscosity of the low rheology, low yellowing nylon is lower than 1500 Pa*s; the molecular weight distribution index of the low rheology, low yellowing nylon is less than 2.0. 14. The application of the low rheology and low yellowing nylon prepared by the preparation method according to any one of claims 1 to 12 or the low rheology and low yellowing nylon according to claim 13 is preferably used in automobile oil pipes, hydraulic pipes, and electrical industries.