A method for testing the content of amino groups at the terminal end of polyamides.

Abstract

This invention discloses a method for testing the terminal amino content of polyamide. The method includes the following steps: (1) preparing a hydrochloric acid-ethanol standard solution; (2) dissolving a polyamide sample in a mixed solvent to obtain a polyamide solution; the mixed solvent includes at least two of phenol, benzyl alcohol, and m-cresol; wherein the phenol accounts for 10% to 80% of the total mass of the mixed solvent; the benzyl alcohol and m-cresol are mixed in any proportion; (3) titrating the polyamide solution using the hydrochloric acid-ethanol standard solution to determine the terminal amino content of the polyamide. This invention, through the synergistic effect of the ternary mixed solvent, significantly shortens the testing time and improves testing efficiency while ensuring the repeatability and accuracy of the test results. Simultaneously, it also reduces testing costs, achieving cost reduction and efficiency improvement in the testing process.

Description

Technical Field

[0001] This invention relates to the field of polyamide property testing technology, and in particular to a method for testing the content of terminal amino groups in polyamides. Background Technology

[0002] Polyamide (PA), also known as nylon, is currently the world's second largest synthetic fiber in terms of production volume, second only to polyester. Its applications have penetrated deep into people's daily lives, including clothing, automobiles, electronics, medical devices, and home appliances. Polyamide materials may undergo oxidative degradation during processing and use, resulting in a decrease in molecular weight and severely affecting the mechanical properties, dyeing properties, and processing performance of the products. The content of terminal amines reflects the molecular weight of polyamide materials and is an important indicator that needs attention in industrial production. Currently, the method for detecting the content of terminal amines generally refers to the national standard GB / T38138-2019, which involves dissolving the polyamide sample in a 2:1 mass ratio m-cresol-isopropanol mixture at 65℃, followed by titration with a hydrochloric acid-ethanol standard solution. Since isopropanol has a boiling point of approximately 82℃, the dissolution temperature for this method cannot be too high. Furthermore, isopropanol has relatively weak polarity. During testing using this method, it was found that the m-cresol-isopropanol mixture has poor solubility for polyamide, and the dissolution process is time-consuming, especially when the sample contains large-sized slices, making dissolution difficult. To meet testing requirements, a large proportion of m-cresol needs to be used in the mixed solvent. m-cresol has strong polarity, but it is expensive, resulting in high testing costs.

[0003] Therefore, there is an urgent need to develop a method that can quickly, accurately, and at low cost test the end-amino content of polyamide materials. Summary of the Invention

[0004] The present invention aims to overcome the above-mentioned problems existing in the prior art and provides a method for testing the content of amino groups at the end of polyamides. This method is low in cost, short in testing time, and produces accurate and repeatable results.

[0005] This invention provides a method for testing the content of terminal amino groups in polyamides. Through the synergistic effect of a ternary mixed solvent, the testing time is significantly shortened and the testing efficiency is improved while ensuring the repeatability and accuracy of the test results. Simultaneously, the testing cost is reduced, achieving cost reduction and efficiency improvement in the testing process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for testing the end-amino content of polyamide, comprising the following steps: (1) preparing a hydrochloric acid-ethanol standard solution; (2) dissolving a polyamide sample in a mixed solvent to obtain a polyamide solution; wherein the mixed solvent includes at least two of phenol, benzyl alcohol, and m-cresol; wherein the phenol accounts for 10% to 80% of the total mass of the mixed solvent; the benzyl alcohol and the m-cresol are mixed in any proportion; (3) using the hydrochloric acid-ethanol standard solution, the polyamide solution is titrated by titration to determine the end-amino content of the polyamide.

[0007] According to one embodiment of the present invention, the concentration of the hydrochloric acid-ethanol standard solution is 0.01 mol / L to 0.05 mol / L.

[0008] According to one embodiment of the present invention, the phenol accounts for 50% to 80% of the total mass of the mixed solvent.

[0009] According to one embodiment of the present invention, the mass ratio of benzyl alcohol to m-cresol is preferably greater than 1:1.

[0010] According to one embodiment of the present invention, the dissolution temperature is 65°C to 220°C; preferably, the dissolution temperature is 120°C to 180°C.

[0011] According to one embodiment of the present invention, the dissolution time is 15 min to 120 min; preferably, the dissolution time is 15 min to 60 min.

[0012] According to one embodiment of the present invention, the mass ratio of the polyamide sample to the mixed solvent can be (0.5~2):(45~180).

[0013] According to one embodiment of the present invention, the process of titrating the polyamide solution using the hydrochloric acid-ethanol standard solution by titration includes the following steps: adding an indicator to the polyamide solution and mixing it evenly, and then titrating it using the hydrochloric acid-ethanol standard solution.

[0014] According to one embodiment of the present invention, the indicator comprises dimethyl yellow-methylene blue.

[0015] According to one embodiment of the present invention, the ambient temperature for titration is 24°C to 26°C.

[0016] Therefore, the present invention has the following beneficial effects:

[0017] (1) A mixed solvent of phenol, benzyl alcohol and m-cresol was used to dissolve the polyamide sample. The boiling points of the three solvent components are all above 180℃, which can significantly increase the dissolution temperature of the polyamide sample, shorten the dissolution time, and improve the testing efficiency. At the same time, the stability of the high-boiling-point solvent components avoids the fluctuation of component ratio caused by solvent evaporation, ensuring the accuracy and repeatability of the data;

[0018] (2) Compared with isopropanol and m-cresol, phenol and benzyl alcohol, which are more polar, are selected as the mixed solvent components. This can reduce the amount of m-cresol used, from 2 / 3 of the method in the national standard GB / T38138-2019 to no more than 1 / 2, which greatly reduces the testing cost. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. The specific embodiments listed below are merely descriptions of the principles and features of this invention, and the examples given are only for explaining this invention and are not intended to limit the scope of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0020] The present invention will be further described below with reference to specific embodiments.

[0021] In this invention, unless otherwise specified, all equipment and raw materials are available from the market or commonly used in the industry. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art.

[0022] This invention provides a method for testing the end-amino content of polyamide, comprising the following steps: (1) preparing a hydrochloric acid-ethanol standard solution; (2) dissolving a polyamide sample in a mixed solvent to obtain a polyamide solution; wherein the mixed solvent includes at least two of phenol, benzyl alcohol, and m-cresol; wherein phenol accounts for 10% to 80% of the total mass of the mixed solvent; benzyl alcohol and m-cresol are mixed in any proportion; (3) using the above hydrochloric acid-ethanol standard solution, the polyamide solution is titrated by titration to determine the end-amino content of the polyamide.

[0023] According to the inventors' research, this invention employs a high-boiling-point mixed solvent system composed of at least two of phenol, benzyl alcohol, and m-cresol. Phenol has a boiling point of 182℃, benzyl alcohol 205℃, and m-cresol 202℃, all significantly higher than isopropanol (boiling point 82.4℃) used in the national standard GB / T38138-2019 method. The boiling points of the three solvent components are close and all above 180℃, raising the dissolution temperature to 120℃~180℃, nearly twice the 65℃ dissolution temperature of the national standard method, significantly shortening the dissolution time and greatly improving testing efficiency. Simultaneously, the high-boiling-point solvent components exhibit almost no volatilization at the testing temperature, completely avoiding fluctuations in component ratios and concentrations caused by solvent evaporation, fundamentally ensuring the accuracy of test data and batch-to-batch repeatability. Furthermore, by selecting phenol and benzyl alcohol, which are more polar, as the core dissolving components, the amount of high-cost m-cresol can be significantly reduced. The proportion of m-cresol in the national standard GB / T38138-2019 method is reduced from 2 / 3 to no more than 1 / 2 in this invention, significantly lowering the cost of testing reagents. When the mixed solvent contains phenol, its proportion is controlled at 50% or higher. Utilizing phenol's strongest hydrogen bond-breaking ability, it efficiently disrupts the amide bond hydrogen bond network between polyamide molecules, further improving dissolution efficiency. Simultaneously, since the market price of phenol is much lower than that of benzyl alcohol and m-cresol, increasing the phenol proportion can further reduce the overall raw material cost of the mixed solvent. The mixed solvent system of this invention does not contain toxic or harmful solvents such as toluene, avoiding the health hazards and environmental risks posed by toluene to operators. It also avoids interference from undesirable solvents in the titration reaction, further improving the accuracy of the test results.

[0024] For example, the mass ratio of phenol to the total mass of the mixed solvent can be 10% to 80%, such as 10%, 20%, 40%, 60%, 80%, or any combination thereof. A phenol content of not less than 10% provides sufficient hydrogen bonding sites to fully break the hydrogen bonds between the polyamide molecular chains, ensuring complete dissolution of the polyamide sample and full release of free terminal amino groups. This avoids the defects of undissolved solids encapsulating amino groups, leading to lower titration results and larger parallel errors. Controlling the phenol content to an upper limit of 80% avoids the problems of high freezing point (approximately 40°C) and easy crystallization at room temperature in pure phenol systems, which can cause operational difficulties. It also reduces the skin and respiratory irritation of high-concentration phenol to operators, improving the safety and convenience of experimental operations.

[0025] Specifically, benzyl alcohol and m-cresol are mixed in any proportion. This gives the solvent system excellent versatility and formulation flexibility, allowing adjustments based on the differences in molecular structure and crystallinity of different types of polyamides. The boiling points, polarities, and solubility of benzyl alcohol and m-cresol are highly matched, and mixing in any proportion can maintain the homogeneity and stability of the solvent system, without problems such as stratification or uneven volatilization.

[0026] As one specific implementation, the preferred mass ratio of phenol to the total mass of the mixed solvent is 50-80%, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any combination thereof. Further limiting the mass ratio of phenol to 50%-80%, with a ratio not less than 50%, helps maintain a low cost basis for the solvent system. A ratio not higher than 80% helps ensure a sufficient proportion of benzyl alcohol and m-cresol in the mixed solvent, guaranteeing the ease of operation and uniform mixing of the solvent at room temperature, so as to further improve the dissolution efficiency of polyamide.

[0027] A hydrochloric acid-ethanol standard solution was prepared according to the preparation method in national standard GB / T 601-2016. The specific method is as follows: Hydrochloric acid solvent was mixed with a 95% (w / w) ethanol solution to obtain the hydrochloric acid-ethanol standard solution; anhydrous sodium carbonate was dissolved in deionized water to obtain a sodium carbonate solution; bromocresol green-methyl red indicator was added dropwise, and the solution was titrated with the above hydrochloric acid-ethanol standard solution until the sodium carbonate solution changed from green to dark red, then boiled; after boiling, it was cooled to room temperature, and titration continued until the sodium carbonate solution turned dark red again. The concentration of the hydrochloric acid-ethanol standard solution was measured.

[0028] In one specific embodiment, the concentration of the above-mentioned hydrochloric acid-ethanol standard solution is 0.01 mol / L to 0.05 mol / L, for example, 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L, or any combination thereof. The concentration of the above-mentioned hydrochloric acid-ethanol standard solution refers to the molar concentration of hydrochloric acid in the hydrochloric acid-ethanol solution. Controlling the concentration of the hydrochloric acid-ethanol standard solution within the above range helps to further ensure the accuracy of endpoint determination, reduce volume reading errors, and simultaneously consider compatibility with polyamide mixed solvent systems, which is beneficial for the efficient and accurate detection of terminal amino group content.

[0029] In one specific implementation, the mass ratio of benzyl alcohol to m-cresol is >1:1. Benzyl alcohol, as an alcohol solvent, may differ from m-cresol in polarity, viscosity, and interaction with polyamides. A mass ratio >1:1 for benzyl alcohol and m-cresol helps to partially replace the more expensive m-cresol with the relatively lower-cost benzyl alcohol, further reducing the cost of the tested solvent while maintaining or optimizing solubility. Simultaneously, this ratio may also affect the overall polarity and boiling range of the mixed solvent, which is beneficial for maintaining the relative stability of the solvent components and consistent solubility at high temperatures.

[0030] As one specific implementation, the dissolution temperature can be between 65℃ and 220℃, for example, a range of 65℃, 100℃, 150℃, 200℃, 220℃, or any combination thereof. A temperature not lower than 65℃ helps to further improve the dissolution rate of the polyamide sample and shorten the overall testing time; a temperature not higher than 220℃ can save energy while maintaining a high dissolution efficiency of the polyamide sample, and helps to avoid problems such as the potential volatilization, decomposition, or oxidation of solvents (especially phenol, benzyl alcohol, and m-cresol) due to excessively high temperatures, as well as additional thermal degradation of the polyamide sample under prolonged high temperatures, thereby further improving the accuracy of the polyamide terminal amino content test results.

[0031] Preferably, the melting temperature can be 120°C to 180°C, for example, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C or any combination thereof. A temperature not lower than 120℃ can further enhance the swelling and dissolving capacity of the mixed solvent for polyamide, significantly accelerating the dissolution rate of poorly soluble polyamide samples. Simultaneously, it can reduce the viscosity of the mixed solvent system, promoting the full extension of the polyamide molecular chains and ensuring that the terminal amino active groups are fully exposed in the solvent system. This guarantees the complete neutralization reaction between the hydrochloric acid-ethanol standard solution and the terminal amino groups during subsequent titration, further improving the accuracy of the test results. A temperature not higher than 180℃, which is lower than the boiling point (182℃) of phenol, the core component of the mixed solvent, avoids the problem of phenol boiling and large-scale volatilization. This ensures the stability of the component ratio and polarity of the mixed solvent throughout the dissolution process, preventing solvent volatilization from affecting the dissolution effect. It also significantly reduces the environmental risks and reagent losses caused by phenolic solvent volatilization and further avoids side reactions such as solvent oxidation and decomposition, and thermal degradation and yellowing of polyamide samples caused by high temperatures, ensuring the accuracy of the test results.

[0032] As one specific implementation, the dissolution time can be from 15 min to 120 min, for example, a range of 15 min, 30 min, 60 min, 90 min, 120 min, or any combination thereof. By using the above-mentioned mixed solvent, the polyamide sample can be dissolved within a time of 15 min to 120 min, resulting in high dissolution efficiency and contributing to improved testing efficiency throughout the entire testing process.

[0033] Preferably, the dissolution time can be 15 min to 60 min, for example, a range of 15 min, 20 min, 30 min, 40 min, 50 min, 60 min, or any combination thereof. A time of not less than 15 min ensures complete dissolution of the sample; a time of not more than 60 min significantly shortens the testing cycle for a single batch of samples, improves the detection efficiency for multiple batches of samples, and better meets the timeliness requirements of rapid incoming material testing and online quality control in industrial production. Simultaneously, it minimizes the residence time of polyamide samples in high-temperature environments, further reducing the risk of thermal oxidative degradation and molecular chain breakage of polyamide, avoiding falsely high deviations in terminal amino content due to sample degradation, and ensuring a high degree of consistency between the test results and the true terminal amino content of the sample.

[0034] This application allows for the dissolution of polyamide samples using conventional heating devices, and the temperature and time of dissolution can be controlled using conventional methods. For example, the heating device may include a polymer sample dissolving apparatus for holding and dissolving the polyamide sample, an oil bath for heating the sample in the sample dissolving apparatus, and an electric heating plate for heating the oil bath to detect the content of terminal amino groups in the polyamide.

[0035] Furthermore, when heating and dissolving polyamide samples using a mixed solvent, the solvent can be kept in a reflux state. That is, the heating device can also be equipped with a reflux condenser connected to the sample dissolving vessel, allowing the solvent to reflux and condense during the heating and dissolving process of the polyamide sample. This application can use a conventional reflux condenser, as long as it enables the solvent in the sample dissolving vessel to reflux and the distilled solvent to be condensed and recovered; there are no particular limitations.

[0036] In this application, a polyamide sample is heated to dissolve it. After the polyamide sample has dissolved, heating is stopped to obtain a polyamide solution. The polyamide solution is then cooled. The cooling time of the polyamide solution can be from 5 min to 20 min, for example, 5 min, 8 min, 12 min, 16 min, 20 min, or any combination thereof.

[0037] Specifically, the polyamide sample mentioned above may include a polyamide chip sample.

[0038] In one specific embodiment, the mass ratio of the polyamide sample to the mixed solvent can be (0.5~2):(45~180), for example, 0.5:45, 0.5:180, 2:180, 2:45, 1:100, or any combination thereof. Controlling the mass ratio of the polyamide sample to the mixed solvent within the above range helps to provide sufficient mixed solvent to fully wet the polyamide sample, avoiding slow or incomplete dissolution due to insufficient solvent.

[0039] In this application, the standard solution used in the above titration method is a hydrochloric acid-ethanol standard solution. Hydrochloric acid provides the hydrogen ions (H+) required for titration. + The ions react with the amino group (-NH2) at the end of the polyamide chain to neutralize it. Ethanol is chosen as the solvent because it has good compatibility with the titration reaction system and its moderate polarity helps the titration reaction proceed smoothly and the color reaction of the endpoint indicator is achieved.

[0040] In one specific implementation, the indicator used in the above titration method is dimethyl yellow-methylene blue. Dimethyl yellow-methylene blue exhibits a significant color change within a specific pH range (typically 3-4). During the titration of the terminal amino group of polyamide with hydrochloric acid, the pH of the solution changes as the titration proceeds. Choosing dimethyl yellow-methylene blue as the indicator is beneficial because its color change point is close to the stoichiometric point (equivalence point) of the titration reaction. This facilitates accurate determination of the titration endpoint by observing the abrupt change in solution color, thereby allowing for the calculation of the volume of standard solution consumed.

[0041] As one specific implementation method, the ambient temperature during the titration can be 24℃~26℃, for example, 24℃, 24.5℃, 25℃, 25.5℃, 26℃ or any combination thereof, which helps to improve titration efficiency and has advantages such as mild environmental conditions and ease of operation. In specific implementation, titration can be carried out at an ambient temperature of 25℃±1℃.

[0042] In specific implementation, as one particular method, the titration process of the above titration method may include: using a pipette to transfer an appropriate amount of hydrochloric acid-ethanol standard solution of known concentration, diluting it with ethanol to the required concentration, and storing it in a volumetric flask; standardizing its actual concentration with sodium carbonate solution; and weighing two portions of the same mass (m... 样 The polyamide sample was placed in a stoppered conical flask, and a mixed solvent was added. The flask was heated to dissolve the polyamide sample. A reflux condenser was installed above the flask. After the polyamide sample was completely dissolved, the flask was removed from the hot plate and allowed to cool. After cooling for a certain period of time, dimethyl yellow-methylene blue indicator was added to the flask, and the mixture was shaken well. The concentration used was C. HCl-乙醇 Titrate with a hydrochloric acid-ethanol standard solution until the sample solution changes from green to light purple, which is considered the titration endpoint. Record the volume of hydrochloric acid-ethanol standard solution used, denoted as V1. Simultaneously, add an equal volume of mixed solvent and dimethyl yellow-methylene blue indicator to an Erlenmeyer flask as a blank group, and titrate with the hydrochloric acid-ethanol standard solution. Record the volume of hydrochloric acid-ethanol standard solution used in the blank group, denoted as V0. Calculate the terminal amino content of the polyamide using the following formula:

[0043]

[0044] Among them, X -NH2 V0 represents the content of terminal amino groups in the polyamide; V1 represents the volume of hydrochloric acid-ethanol standard solution consumed in the titration of the blank solution, in mL; V2 represents the volume of hydrochloric acid-ethanol standard solution consumed in the titration of the polyamide sample, in mL; C HCl-乙醇 The concentration of the hydrochloric acid-ethanol standard solution is expressed in mol / L; m 样 The mass of the polyamide sample is expressed in kg.

[0045] In practice, the measured content of terminal amino groups in polyamide can be expressed in mmol / kg.

[0046] The present invention will be further described below through specific embodiments.

[0047] The raw materials used in the following examples and comparative examples are from the following sources: polyamide 6 chips (fiber-grade semi-dull PA6 chips from Hangzhou Yichen Chemical Fiber Co., Ltd.).

[0048] Example 1:

[0049] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0050] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 1:3:6 was added. The conical flask was placed on a hot plate, the temperature was set to 220℃, and the mixture was heated under reflux for 15 min until the polyamide sample was completely dissolved.

[0051] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0052] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (mass ratio 1:3:6) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula.-NH2 The results are shown in Table 1:

[0053]

[0054] Example 2:

[0055] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0056] Weigh out 0.5g of polyamide 6 slices (denoted as m). 样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 3:2:1 was added. The conical flask was placed on a hot plate, the temperature was set to 180℃, and the mixture was heated under reflux for 15 min until the polyamide was completely dissolved.

[0057] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0058] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (mass ratio 3:2:1) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0059]

[0060] Example 3

[0061] Use a pipette to transfer 10 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.01 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.01020 mol / L.

[0062] Weigh out 0.5g of polyamide 6 slices m样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 4:2:1 was added. The conical flask was placed on a hot plate, the temperature was set to 150℃, and the mixture was heated under reflux for 30 min until the polyamide sample was completely dissolved.

[0063] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.01 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0064] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (mass ratio 4:2:1) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0065]

[0066] Example 4

[0067] Use a pipette to transfer 50 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.05 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.04989 mol / L.

[0068] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 8:1:1 was added. The conical flask was placed on a hot plate, the temperature was set to 120℃, and the mixture was heated under reflux for 60 min until the polyamide sample was completely dissolved.

[0069] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.05 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0070] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (8:1:1 by mass) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of the added hydrochloric acid-ethanol standard solution. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0071]

[0072] Example 5

[0073] Use a pipette to transfer 50 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.05 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.05016 mol / L.

[0074] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 3:3:2 was added. The conical flask was placed on a hot plate, the temperature was set to 65℃, and the mixture was heated under reflux for 120 min until the polyamide sample was completely dissolved.

[0075] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.05 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0076] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (mass ratio 3:3:2) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0077]

[0078] Example 6

[0079] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0080] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, and 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent with a mass ratio of 2:1:2 was added. The conical flask was placed on a hot plate, the temperature was set to 80℃, and the mixture was heated under reflux for 80 min until the polyamide sample was completely dissolved.

[0081] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0082] Add 45 mL of a phenol-benzyl alcohol-m-cresol mixed solvent (mass ratio 2:1:2) and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0083]

[0084] Comparative Example 1:

[0085] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0086] Weigh 0.5g of polyamide 6 slice m and place it in a stoppered conical flask as a polyamide sample. Add 45mL of a 2:1 m-cresol-isopropanol mixed solution. Place the conical flask on a hot plate, set the temperature to 65℃, and heat under reflux for 150min until the polyamide sample is completely dissolved.

[0087] Remove the conical flask from the hot plate and allow it to cool to room temperature. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and mix well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0088] Add 45 mL of a 2:1 (w / w) mixture of m-cresol and isopropanol and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank. Shake well and titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0089]

[0090] Comparative Example 2

[0091] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0092] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, 45 mL of phenol solvent was added, the conical flask was placed on a hot plate, the temperature was set to 120℃, and the sample was heated under reflux for 120 min until the polyamide sample was completely dissolved.

[0093] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0094] Add 45 mL of phenol and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0095]

[0096] Comparative Example 3

[0097] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0098] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, 45 mL of benzyl alcohol was added, the flask was placed on a hot plate, the temperature was set to 180 °C, and the mixture was heated under reflux for 30 min until the polyamide sample was completely dissolved.

[0099] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0100] Add 45 mL of benzyl alcohol and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0101]

[0102] Comparative Example 4

[0103] Use a pipette to transfer 20 mL of a 0.5 mol / L hydrochloric acid-ethanol standard titration solution, and dilute it to 500 mL with 95% ethanol to obtain a 0.02 mol / L hydrochloric acid-ethanol standard solution, denoted as C. HCl-乙醇 The solution was standardized with standard sodium carbonate solution, and the concentration of the hydrochloric acid-ethanol standard solution was determined to be 0.02048 mol / L.

[0104] Weigh out 0.5g of polyamide 6 slices m 样 The polyamide sample was placed in a stoppered conical flask, 45 mL of m-cresol was added, the flask was placed on a hot plate, the temperature was set to 180℃, and the mixture was heated under reflux for 30 min until the polyamide sample was completely dissolved.

[0105] Remove the conical flask from the hot plate and cool for 5 minutes. Add 3 drops of dimethyl yellow-methylene blue indicator to the flask and shake well. Titrate the sample solution with 0.02 mol / L hydrochloric acid-ethanol standard solution until the polyamide sample solution changes from green to light purple. Record the volume V1 of hydrochloric acid-ethanol standard solution added.

[0106] Add 45 mL of m-cresol and 3 drops of dimethyl yellow-methylene blue indicator to an empty conical flask as a blank control. After mixing, titrate with hydrochloric acid-ethanol standard solution until the blank sample solution changes from green to pale purple. Record the volume V0 of hydrochloric acid-ethanol standard solution added. Calculate the terminal amino content X of the polyamide sample using the following formula. -NH2 The results are shown in Table 1:

[0107]

[0108] Table 1: Test results of terminal amino content in polyamide samples

[0109]

[0110] As shown in Table 1, Comparative Example 1 uses a mixture of m-cresol and isopropanol as a solvent according to the existing national standard method. The dissolution temperature is relatively low, requiring 150 minutes at 65℃ to completely dissolve the polyamide sample. Comparative Example 2 uses only pure phenol as a single solvent, lacking benzyl alcohol and m-cresol to adjust viscosity; it takes 120 minutes to dissolve at 120℃. Comparative Example 3 uses only pure benzyl alcohol as a single solvent, lacking phenol to dissociate hydrogen bonds between polyamide molecules; it takes 30 minutes to dissolve at 180℃. The amino content in the two parallel tests was 45.039 mmol / kg and 45.032 mmol / kg, respectively, with a difference of 0.007 mmol / kg between the parallel data. Comparative Example 4 uses only pure m-cresol as a single solvent, without phenol to provide hydrogen bonding sites; it takes 30 minutes to dissolve at 180℃. The amino content in the two parallel tests was 45.048 mmol / kg and 45.040 mmol / kg, respectively, with a difference of 0.008 mmol / kg between the parallel data. The comparison shows that mixed solvents without phenol, solvents with only a single phenol, and solvents with only a single alcohol all have the drawback of large parallel deviations in detection. The experiment shows that the requirement that phenol accounts for 10% to 80% of the total mass of the mixed solvent and that the mixed solvent has at least two components plays a key role in achieving the optimal dissolution rate. This reveals the synergistic effect between solvent components, rather than a simple substitution, while minimizing costs.

[0111] In all embodiments of this invention, a compound solvent system was used, with phenol accounting for 10%–80% of the total mass of the mixed solvent. Hydrochloric acid-ethanol titrant with concentrations ranging from 0.01 mol / L to 0.05 mol / L, dissolution temperatures from 65°C to 220°C, and dissolution times from 15 min to 120 min were used for testing. The dissolution time was significantly shorter than that of the national standard method. All samples could be completely dissolved, and the overall parallel test error was much smaller than that of the comparative example. Samples with dissolution temperatures of 120°C–180°C, dissolution times of 15 min–60 min, phenol content of 50%–80%, and a benzyl alcohol to m-cresol mass ratio greater than 1:1 showed the best repeatability. The entire set of conditions has a wide applicable window, and the synergistic effect of various parameters allows for accurate determination of the amino group content of polyamides. Optimized parameters can further shorten the detection time and reduce test deviation.

[0112] 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 testing the content of amino groups at the terminal end of polyamides, characterized in that, Includes the following steps: (1) Prepare a hydrochloric acid-ethanol standard solution; (2) Dissolve the polyamide sample in a mixed solvent to obtain a polyamide solution; the mixed solvent includes at least two of phenol, benzyl alcohol, and m-cresol; wherein, the phenol accounts for 10% to 80% of the total mass of the mixed solvent; the benzyl alcohol and the m-cresol are mixed in any proportion; (3) The polyamide solution was titrated by titration using the hydrochloric acid-ethanol standard solution to determine the content of the terminal amino groups of the polyamide.

2. The method for testing the content of polyamide terminal amino groups according to claim 1, characterized in that, The concentration of the hydrochloric acid-ethanol standard solution is 0.01 mol / L to 0.05 mol / L.

3. The method for testing the content of polyamide terminal amino groups according to claim 1, characterized in that, The phenol accounts for 50% to 80% of the total mass of the mixed solvent.

4. The method for testing the content of polyamide terminal amino groups according to claim 1, characterized in that, The preferred mass ratio of benzyl alcohol to m-cresol is greater than 1:

1.

5. The method for testing the content of amino groups at the terminal amines of polyamide according to claim 1, Its features are, The dissolution temperature is 65℃~220℃; Preferably, the melting temperature is 120℃~180℃.

6. The method for testing the content of polyamide terminal amino groups according to claim 1 or 5, characterized in that, The dissolution time is 15 min to 120 min; Preferably, the dissolution time is 15 min to 60 min.

7. The method for testing the content of polyamide terminal amino groups according to claim 1, characterized in that, The mass ratio of the polyamide sample to the mixed solvent is (0.5~2):(45~180).

8. The method for testing the content of polyamide terminal amino groups according to claim 1, characterized in that, The process of titrating the polyamide solution using the hydrochloric acid-ethanol standard solution includes the following steps: adding an indicator to the polyamide solution, mixing thoroughly, and then titrating with the hydrochloric acid-ethanol standard solution.

9. The method for testing the content of terminal amino groups in polyamide according to claim 8, characterized in that, The indicator includes dimethyl yellow-methylene blue.

10. The method for testing the content of polyamide terminal amino groups according to claim 1 or 8, characterized in that, The ambient temperature for titration is 24℃~26℃.

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