Water-soluble polyamide sizing agent, its preparation method and application

Abstract

The present application relates to aramid fiber surface modification technical field, specifically to a kind of water-soluble polyamide sizing agent and its preparation method and application, the preparation method is: S1, preparation of terminal carboxyl group oligomer: in solvent, under catalyst condition, binary acid, isophthalic acid-5-sodium sulfonate and binary amine are reacted to obtain the system containing terminal carboxyl group oligomer;S2, amphiphilic polyether end-capping: to the system of step S1, add terminal amino polyethylene glycol monomethyl ether, carry out reaction under the condition of ladder temperature and aftertreatment obtain water-soluble polyamide resin;S3, the preparation of sizing agent: the water-soluble polyamide resin is prepared into aqueous solution with water to obtain sizing agent.The sizing agent has good heat resistance and hydrophilic performance, can be uniformly distributed in water without emulsifier and organic solvent, has better film-forming property.

Description

Technical Field

[0001] This invention relates to a water-soluble polyamide sizing agent, its preparation method and application, belonging to the field of aramid fiber surface modification technology. Background Technology

[0002] Aramid fibers (such as poly(p-phenylene terephthalamide), PPTA) are a class of high-performance synthetic fibers, and have become core reinforcing materials in aerospace, bulletproof armor, and high-end sports equipment due to their excellent specific strength, high modulus, and high-temperature resistance. However, the high crystallinity and chemical inertness of aramid fiber surfaces result in weak interfacial bonding with matrix materials such as epoxy resins and unsaturated polyesters, a defect that severely restricts the overall performance of composite materials. Studies have shown that the interlaminar shear strength (ILSS) of untreated aramid fibers with epoxy resins is typically below 30 MPa, and interfacial failure has become the main mode of composite material failure. Therefore, modifying the fiber surface with sizing agents to enhance its chemical bonding and mechanical interlocking with the resin has become a key technical approach to improve the performance of composite materials.

[0003] Traditional sizing techniques primarily rely on solvent-based epoxy resin systems. Aramid sizing agents, with epoxy resin as the main component and acetone as the solvent, improve interfacial adhesion by adding aminosilane coupling agents. While this technology has improved interfacial strength strength (ILSS) to some extent (up to 35-40 MPa), its inherent drawbacks significantly limit its application prospects. First, solvent-based systems release large amounts of volatile organic compounds (VOCs), such as benzene and ketones, during production and use. Second, the hard and brittle coating formed after epoxy resin curing is prone to microcrack propagation under high temperatures (>150℃) or dynamic loads, leading to a sharp decline in interfacial strength. Furthermore, residual solvents in solvent-based sizing agents may penetrate into the fiber interior, causing swelling and reducing the fiber's mechanical properties. These problems have prompted the industry to shift towards the development of water-based, environmentally friendly sizing agents, but the technological challenges remain significant.

[0004] To replace solvent-based systems, hydrophilic nylon (polyamide) sizing agents have gradually gained attention in recent years. For example, patent application CN102875803A proposes a method for preparing water-soluble polyamides using tributyl citrate and diethyl succinate as raw materials. This method is simple and the raw materials are readily available, but hydrolysis under strong alkali is required. Patent application CN103788368A discloses a ternary copolymer waterborne polyamide, which has good film-forming properties, but its heat resistance is insufficient, making it difficult to use at high temperatures. Meanwhile, while the academic community's exploration of bio-based nylon sizing agents (such as gluconic acid derivatives) shows potential in terms of environmental friendliness, their complex synthetic routes and high monomer purity requirements pose serious challenges to industrial scale-up. Existing technologies indicate that single-component or simple copolymer nylon sizing agents struggle to achieve a balance between hydrophilicity, heat resistance, mechanical properties, and process feasibility, while the performance optimization of aramid fiber composites urgently requires an innovative molecular design strategy. Specifically, an ideal environmentally friendly sizing agent should meet the following core requirements: (1) complete water-based, completely eliminating VOC emissions; (2) crystallinity controlled by molecular chain segment design to ensure water molecule penetration and coating uniformity; and (3) introduction of rigid structural units to improve heat resistance and withstand processing temperatures of ≥250℃. However, existing technologies have not yet provided a solution that can simultaneously meet the above requirements, which has become a key bottleneck restricting the development of high-performance aramid composite materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a water-soluble polyamide sizing agent, its preparation method, and its application. The sizing agent has excellent heat resistance and hydrophilic properties, and can achieve uniform distribution in water without the need for emulsifiers and organic solvents, resulting in better film-forming performance.

[0006] The technical solution of this invention to solve the above-mentioned technical problems is as follows: a method for preparing a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: In a solvent and under catalytic conditions, a dicarboxylic acid, sodium isophthalic acid-5-sulfonate, and a diamine react to obtain a system containing terminal carboxyl oligomers. S2, Amphiphilic polyether end-capped: Add amino-terminated polyethylene glycol monomethyl ether to the system in step S1, react under stepped heating conditions and then post-treat to obtain water-soluble polyamide resin. S3. Preparation of sizing agent: The water-soluble polyamide resin is mixed with water to prepare an aqueous solution to obtain a sizing agent.

[0007] Furthermore, the dicarboxylic acid is at least one of adipic acid, terephthalic acid, glutaric acid, and succinic acid; The diamine is at least one of ethylenediamine, propylenediamine, butyldiamine, decanediamine, hexamethylenediamine, heptamethamine, octanediamine, nonanediamine, p-phenylenediamine, and o-phenylenediamine.

[0008] Further, the solvent is at least one of N,N-dimethylformamide and N-methylpyrrolidone; The catalyst is at least one of triphenyl phosphite and pyridine.

[0009] Further, in step S1, the molar ratio of the dicarboxylic acid and sodium isophthalic acid-5-sulfonate is (1-3):4; the molar ratio of the total molar amount of the dicarboxylic acid and sodium isophthalic acid-5-sulfonate to the molar amount of the diamine is (1.8-2.2):1.

[0010] Further, in step S1, after adding solvent, catalyst, dicarboxylic acid, and sodium isophthalic acid-5-sulfonate to the reactor and heating to dissolve, a mixture of diamine and catalyst is slowly added, and the temperature in the system is controlled at 85-95℃. After the addition is completed, the reaction is carried out for 1.5-2.0h to obtain a system containing terminal carboxyl oligomers.

[0011] Furthermore, the molecular weight Mn of the terminal amino-terminated polyethylene glycol monomethyl ether is 400~1000.

[0012] Further, in step S2, the system temperature of step S1 is first lowered to 70~75℃, amino-terminated polyethylene glycol monomethyl ether is added, and after being mixed evenly, the temperature is increased stepwise at a rate of 3-5℃ / h. The stepped heating conditions are as follows: heat to 90-95℃, hold for 1-1.5h, continue heating to 105-110℃, and hold for 2-2.5h to obtain water-soluble polyamide resin.

[0013] Further, in step S3, the water-soluble polyamide resin is prepared into an aqueous solution with a mass concentration of 0.5wt%~2.0wt% in water, which is the sizing agent.

[0014] The present invention also discloses a water-soluble polyamide sizing agent, which is prepared by the preparation method described in the present invention.

[0015] The present invention also discloses the application of a water-soluble polyamide sizing agent in the surface treatment of aramid fibers.

[0016] The beneficial effects of this invention are: This invention, through the synergistic design of molecular structure and synthesis process, enables various technical features to work together to solve the technical bottleneck of existing aramid fiber sizing agents that struggle to simultaneously achieve hydrophilicity, heat resistance, and processability. Specifically, this invention simultaneously introduces a rigid aromatic ring (derived from sodium isophthalic acid-5-sulfonate and terephthalic acid, etc.) and an amphiphilic polyether chain (terminated amino-terminated polyethylene glycol monomethyl ether) into the main chain. The rigid aromatic ring imparts high thermal stability to the polyamide main chain (thermal decomposition temperature ≥350℃), while the sulfonic acid groups and polyether segments together provide hydrophilic sites. The two work together at the molecular level: the rigid aromatic ring prevents the polyether segments from excessive thermal degradation at high temperatures, while the polyether segments compensate for the hydrophobicity brought by the rigid aromatic ring. Thus, excellent water solubility is achieved without sacrificing heat resistance, overcoming the technical problem of traditional nylon sizing agents being "heat-resistant but not hydrophilic, or hydrophilic but not heat-resistant."

[0017] The preparation method of the sizing agent described in this invention employs a two-step polymerization process: first synthesizing end-carboxyl oligomers, and then end-capping with hydrophilic end-amino polyethylene glycol monomethyl ether. This results in the concentrated distribution of hydrophilic segments on the main molecular chain and at both ends of the molecular chain, forming a hydrophilic block structure. This retains the mechanical strength and heat resistance of the polyamide main chain while significantly improving water solubility and wetting ability on the surface of aramid fibers through the high density of ether bonds and sulfonic acid groups at the end groups. Compared with single copolymerization or random grafting, this structural design allows the sizing agent to dissolve quickly in water and form a uniform film on the fiber surface. These two aspects promote each other and significantly improve the sizing processability.

[0018] The gradient heating process used in the end-capping reaction of this invention is well matched with the molecular weight selection (Mn=400~1000) of the terminal amino polyethylene glycol monomethyl ether: the appropriate molecular weight of the polyether chain can avoid the risk of thermal degradation caused by excessive chain length, while ensuring sufficient water solubility; the step-by-step heating effectively controls the reaction rate, preventing incomplete end-capping or excessively wide molecular weight distribution caused by local violent reactions. The uniform molecular structure further ensures that the sizing agent can quickly dissolve in water to form a transparent solution and form a uniform film on the surface of aramid fibers, thereby providing a stable and repeatable coating quality for subsequent interface reinforcement.

[0019] The water-soluble polyamide resin prepared by this invention contains an aromatic ring amide structure similar to that of aramid fiber (PPTA). The similar structure enhances the intermolecular interaction between the sizing agent and the fiber surface. Its excellent film-forming properties facilitate the formation of a continuous and complete coating, providing an effective stress transfer medium between the fiber and the resin matrix. It significantly improves the interfacial bonding ability between aramid fiber and matrix such as epoxy resin, and further enhances the interlaminar shear strength compared with traditional solvent-based sizing agents. At the same time, it avoids the swelling damage to the fiber body caused by residual solvent in the solvent-based system and reduces volatile organic compound (VOC) pollution.

[0020] In summary, this invention achieves significant improvements in hydrophilicity, heat resistance, film-forming properties, and interfacial bonding performance within the same material system through the synergistic molecular structure of "rigid aromatic ring-sulfonic acid group-polyether chain", the synergistic two-step polymerization of "terminated carboxyl oligomer-terminated amino polyether", and the synergistic process of "step-heating process-appropriate molecular weight". Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the sizing agent applied to the surface of aramid fiber fabric in Example 1. Figure 2 This is a scanning electron microscope image of the sizing agent applied to the surface of aramid fiber fabric in Example 2. Figure 3 This is a scanning electron microscope image of the sizing agent applied to the surface of aramid fiber fabric in Example 3. Figure 4 This is a scanning electron microscope image of the sizing agent applied to the surface of aramid fiber fabric in Example 4. Figure 5 This is a scanning electron microscope image of the sizing agent applied to the surface of aramid fiber fabric in Example 5. Figure 6 This is a scanning electron microscope image showing the sizing effect of the sizing agent in Comparative Example 1 on the surface of aramid fiber fabric. Figure 7 This is a scanning electron microscope image showing the sizing effect of the sizing agent on the surface of aramid fiber fabric in Comparative Example 2. Figure 8 This is a scanning electron microscope image showing the sizing effect of the sizing agent in Comparative Example 3 on the surface of aramid fiber fabric. Figure 9 Scanning electron microscope image of the sizing agent on the surface of aramid fiber fabric in Comparative Example 4. Figure 10 This is a scanning electron microscope image showing the sizing effect of the sizing agent in Comparative Example 5 on the surface of aramid fiber fabric. Figure 11 This is a scanning electron microscope image showing the sizing effect of the sizing agent in Comparative Example 6 on the surface of aramid fiber fabric. Detailed Implementation

[0022] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0024] A method for preparing a water-soluble polyamide sizing agent, wherein the preparation method comprises: S1. Preparation of carboxyl-terminated oligomers: In a solvent and under catalytic conditions, a dicarboxylic acid, sodium isophthalic acid-5-sulfonate, and a diamine react to obtain a system containing terminal carboxyl oligomers. S2, Amphiphilic polyether end-capped: Add amino-terminated polyethylene glycol monomethyl ether to the system in step S1, react under stepped heating conditions and then post-treat to obtain water-soluble polyamide resin. S3. Preparation of sizing agent: The water-soluble polyamide resin is mixed with water to prepare an aqueous solution to obtain a sizing agent.

[0025] Specifically, the dicarboxylic acid is at least one of adipic acid, terephthalic acid, glutaric acid, and succinic acid; The diamine is at least one of ethylenediamine, propylenediamine, butyldiamine, decanediamine, hexamethylenediamine, heptamethamine, octanediamine, nonanediamine, p-phenylenediamine, and o-phenylenediamine.

[0026] Preferably, the dicarboxylic acid is terephthalic acid.

[0027] Preferably, the diamine is at least one of hexamethylenediamine, butanediamine, and pentanediamine.

[0028] Specifically, the solvent is at least one of N,N-dimethylformamide and N-methylpyrrolidone; The catalyst is at least one of triphenyl phosphite and pyridine.

[0029] Specifically, in step S1, the molar ratio of diamine to (diacid + sodium isophthalic acid-5-sulfonate) is 1 / (1.8-2.2), and the molar ratio of diamine to sodium isophthalic acid-5-sulfonate is (1-3) / 4.

[0030] More specifically, the molar amount of amino-terminated polyethylene glycol monomethyl ether added is at least twice the molar amount of the carboxyl-terminated oligomer.

[0031] Specifically, in step S1, solvent, catalyst, dicarboxylic acid, and sodium isophthalic acid-5-sulfonate are added to the reactor and heated to dissolve. A mixture of diamine and catalyst is then slowly added, and the temperature of the system is controlled at 85-95℃. After the addition is complete, the reaction is carried out for 1.5-2.0 hours to obtain a system containing terminal carboxyl oligomers.

[0032] Specifically, the molecular weight Mn of the terminal amino-terminated polyethylene glycol monomethyl ether is 400~1000.

[0033] Preferably, the molecular weight Mn of the amino-terminated polyethylene glycol monomethyl ether is 500.

[0034] Amino-terminated polyethylene glycol monomethyl ether contains a large number of ether bonds, which makes it prone to thermal decomposition and reduces the overall heat resistance of water-soluble polyamides. Therefore, amino-terminated polyethylene glycol monomethyl ether with a large molecular weight should not be selected. In addition, it should not be selected with too small a molecular weight, as this will affect the water solubility of the entire polyamide.

[0035] Specifically, in step S2, the system temperature of step S1 is first lowered to 70~75℃, amino-terminated polyethylene glycol monomethyl ether is added, and after being mixed evenly, the temperature is gradually increased at a rate of 3-5℃ / h. The stepped heating conditions are as follows: heat to 90-95℃, hold for 1-1.5h, continue heating to 105-110℃, and hold for 2-2.5h to obtain water-soluble polyamide resin.

[0036] Stepwise heating can control the reaction rate between amino-terminated polyethylene glycol monomethyl ether and carboxyl-terminated polyamide oligomers, avoiding problems such as violent local reactions, incomplete end-capping, and uneven molecular weight distribution of the product caused by excessively rapid heating. Two-stage temperature control (90~95℃ and 105~110℃) ensures the end-capping reaction proceeds fully, improving the uniformity of the product structure, while simultaneously preventing thermal degradation of the polyether segments caused by direct high-temperature reaction and protecting the hydrophilic groups.

[0037] Specifically, in step S2, the post-processing method is as follows: after the reaction is completed, precipitant butanone is added to the system and filtered, and the system is washed several times with methanol to obtain water-soluble polyamide resin.

[0038] Specifically, in step S3, the water-soluble polyamide resin is prepared into an aqueous solution with a mass concentration of 0.5wt% to 2.0wt% in water, which is the sizing agent.

[0039] A water-soluble polyamide sizing agent, wherein the sizing agent is prepared by the preparation method described in this invention.

[0040] Application of a water-soluble polyamide sizing agent, said sizing agent being used in the surface treatment of aramid fibers.

[0041] More specifically, water-soluble polyamide is dissolved in hot water at 50°C to prepare an aqueous solution with a weight fraction of 0.5~2wt%, which is the sizing agent. Then, the aramid fiber fabric is immersed in the sizing agent for 3~5 minutes and repeatedly shaken to ensure uniform immersion. The immersed aramid fiber fabric is then dried in a forced-air dryer at 80~100°C to obtain the aramid fiber fabric sizing with water-based polyamide.

[0042] Example 1: Preparation of a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 12 g of terephthalic acid and 36 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0043] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500) (manufacturer: Shanghai Aladdin Biochemical Technology Co., Ltd.), stir evenly, and then raise the temperature to 95℃ at a rate of 5℃ / h, keep it at that temperature for 1.5h, continue to raise the temperature to 110℃, keep it at that temperature for 2.5h, add the precipitant butanone and filter. After washing several times, water-soluble polyamide resin is obtained.

[0044] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, forming a transparent solution to obtain a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0045] Example 2: Preparation of a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 6.9 g of terephthalic acid and 44.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 2 hours to obtain a system containing terminal carboxyl oligomers.

[0046] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 95℃ at a rate of 5℃ / h. Hold for 1.5h, continue to raise the temperature to 110℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0047] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, forming a transparent solution to obtain a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1.5 wt%.

[0048] Example 3: Preparation of a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 14.7 g of terephthalic acid and 31.7 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0049] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=1000), stir evenly, and then raise the temperature to 95℃ at a rate of 5℃ / h. Hold for 1.5h, continue to raise the temperature to 110℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0050] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, forming a transparent solution to obtain a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 2.0 wt%.

[0051] Example 4: Preparation of a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 10.1 g of adipic acid and 37 g of sodium isophthalic acid-5-sulfonate were added to the reactor. Then, 300 mL of N,N-dimethylformamide was added as a solvent, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the temperature was raised to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 85 °C. After the slow addition was complete, the reaction continued for 2.0 h to obtain a system containing terminal carboxyl oligomers.

[0052] S2, Amphiphilic polyether end-capped: When the system temperature drops to 75℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 90℃ at a rate of 3℃ / h. Hold for 1.5h, continue to raise the temperature to 105℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0053] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, forming a transparent solution to obtain a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 0.5 wt%.

[0054] Example 5: Preparation of a water-soluble polyamide sizing agent, wherein the preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 12 g of glutaric acid and 48.8 g of sodium isophthalic acid-5-sulfonate were added to the reactor. Then, 300 mL of N-methylpyrrolidone was added as a solvent, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of butanediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 90 °C. After the addition was complete, the reaction was continued for 2.0 h to obtain a system containing terminal carboxyl oligomers.

[0055] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 95℃ at a rate of 4℃ / h. Hold for 1.5h, continue to raise the temperature to 105℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0056] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, forming a transparent solution to obtain a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0057] Comparative Example 1: The sizing agent was prepared using the same method as in Example 1, except that the molar ratio of terephthalic acid and sodium isophthalate-5-sulfonate was changed. In Comparative Example 1, the molar ratio of terephthalic acid to sodium isophthalate-5-sulfonate was 2:1. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 23 g of terephthalic acid and 18.5 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0058] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 95℃ at a rate of 5℃ / h. Hold for 1.5h, continue to raise the temperature to 110℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0059] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0060] Comparative Example 2: The sizing agent was prepared using the same method as in Example 1, except that the molar ratio of terephthalic acid to sodium isophthalic acid-5-sulfonate was changed. In Comparative Example 2, the molar ratio of terephthalic acid to sodium isophthalic acid-5-sulfonate was 1:10. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 3.1 g of terephthalic acid and 50.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0061] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 95℃ at a rate of 5℃ / h. Hold for 1.5h, continue to raise the temperature to 110℃, hold for 2.5h, add the precipitant butanone, filter, and wash several times to obtain water-soluble polyamide resin.

[0062] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0063] Comparative Example 3: The sizing agent was prepared using the same method as in Example 1, except that the heating rate was increased in step S2 of Comparative Example 3. The heating rate in Comparative Example 3 was 10℃ / h. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 6.9 g of terephthalic acid and 44.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0064] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 95℃ at a rate of 10℃ / h, keep it at that temperature for 1.5h, continue to raise the temperature to 110℃, keep it at that temperature for 2.5h, then add the precipitant butanone and filter. After washing several times, water-soluble polyamide resin is obtained.

[0065] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0066] Comparative Example 4: The sizing agent was prepared using the same method as in Example 1, except that step S2 of Comparative Example 4 did not involve a gradient temperature increase. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 6.9 g of terephthalic acid and 44.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0067] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then raise the temperature to 110℃ at a rate of 5℃ / h. After holding at this temperature for 4h, add the precipitant butanone and filter. After washing several times, water-soluble polyamide resin is obtained.

[0068] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0069] Comparative Example 5: The sizing agent was prepared using the same method as in Example 1, except that step S2 of Comparative Example 5 did not involve a gradient temperature increase. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 6.9 g of terephthalic acid and 44.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0070] S2, Amphiphilic polyether end-capped: When the system temperature drops to 70℃, add 100g of amino-terminated polyethylene glycol monomethyl ether (Mn=500), stir evenly, and then heat to 95℃ at a rate of 5℃ / h. After holding at this temperature for 4h, add the precipitant butanone and filter. After washing several times, water-soluble polyamide resin is obtained.

[0071] S3. Preparation of sizing agent: A water-soluble polyamide sample was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of water-soluble polyamide resin in the sizing agent was 1 wt%.

[0072] Comparative Example 6: The sizing agent was prepared using the same method as in Example 1, except that: in Comparative Example 6, terminal amino-terminated polyethylene glycol monomethyl ether was not added for reaction; instead, terminal carboxyl-terminated oligomers were directly used to formulate the sizing agent. The specific preparation process is as follows: S1. Preparation of carboxyl-terminated oligomers: Under nitrogen protection and stirring, 6.9 g of terephthalic acid and 44.4 g of sodium isophthalate-5-sulfonate were added to the reactor. Then, 200 mL of N,N-dimethylformamide and 100 mL of N-methylpyrrolidone were added as solvents, and 12 mL of triphenyl phosphite was added as a catalyst. After stirring until homogeneous, the mixture was heated to 80 °C until completely dissolved. 12 g of hexamethylenediamine and 30 mL of pyridine were slowly added to a four-necked flask, maintaining the temperature at 95 °C. After the slow addition was complete, the reaction was continued for 1.5 h to obtain a system containing terminal carboxyl oligomers.

[0073] Add methyl ethyl ketone (MEK) as a precipitant and filter. After washing several times, oligomers containing terminal carboxyl groups are obtained.

[0074] S3. Preparation of sizing agent: The sample containing terminal carboxyl oligomers was added to deionized water and stirred to dissolve, thus obtaining a sizing agent. The weight fraction of terminal carboxyl oligomers in the sizing agent was 1 wt%.

[0075] The water-soluble polyamides and sizing agents prepared in the above examples and comparative examples were subjected to performance tests, and the test methods involved are as follows: (1) Solubility test: Place 2g of the prepared water-soluble polyamide sample (comparative example 6 uses an oligomer containing terminal carboxyl groups) in a flask, pour in 20g of deionized water, and heat the solution to 50℃. While stirring, visually observe the dissolution of the water-soluble polyamide. If the polyamide dissolves rapidly within ≤2 min and forms a clear solution, it is considered to have "very good" solubility; if it dissolves relatively quickly (stirring time >2 min and ≤5 min) and forms a clear solution, it is considered to have "good" solubility; if it dissolves slowly (stirring time >5 min and ≤10 min) and the solution is slightly turbid, it is considered to have "good" solubility; if it dissolves slowly (stirring time >10 min and ≤20 min) and is significantly turbid, it is considered to have "poor" solubility; if it dissolves poorly (stirring time >20 min and ≤30 min) and contains a large amount of insoluble matter, it is considered to have "very poor" solubility.

[0076] (2) Thermal performance test: Take 8 mg of water-soluble polyamide sample (comparative example 6 uses terminal carboxyl oligomer) and place it in a crucible. Use a thermogravimetric analyzer to test the heat resistance of the sample. In a nitrogen atmosphere, heat the sample from 30 °C to 800 °C at a heating rate of 15 °C / min and analyze to obtain the thermal decomposition temperature of the sample.

[0077] (3) Fiber sizing effect test: Aramid fiber fabric (TH51081) was immersed in the sizing agent for 5 minutes, and repeatedly shaken to ensure uniform immersion. The immersed aramid fiber fabric was then dried in a forced-air dryer at 100°C to obtain an aramid fiber fabric sized with water-based polyamide. Scanning electron microscopy was used to observe the fiber sizing, revealing that the sizing agent formed a uniform film on the surface of the aramid fiber fabric, indicating that the water-soluble polyamide prepared in this invention has good sizing properties.

[0078] (4) Interfacial bonding properties of aramid fibers after impregnation with sizing agent: The interfacial bonding properties of aramid fibers, i.e., interfacial shear strength, were determined using a microdroplet debonding method. Single aramid fibers (extracted from aramid fabric of brand TH51081) were glued to both ends of a C-shaped metal frame using epoxy adhesive (Guangxuan 315K). The frames were then preheated at 80℃ for 10 minutes. Epoxy resin (Guangxuan 315K) was gently applied to the fibers using a No. 1 fine-tip pen. After the fibers shrank into small spheres due to surface tension, they were quickly transferred to a controlled-flow oven for curing (160℃ / 15 minutes). Once the samples were prepared, resin microspheres with a wrapping length between 80 and 110 μm were clamped in a fixture for pull-out testing. The samples were moved at a test rate of 0.12 mm / min to remove the resin microspheres from the single fibers, and the interfacial shear strength was measured.

[0079] The specific test results are shown in Table 1 below.

[0080] Table 1 Performance Test Results

[0081] From the performance data and scanning electron microscope images in the table above ( Figures 1-5 As can be seen, the sizing agents prepared by the preparation method of the present invention in Examples 1-5 have good water solubility, film-forming properties, high temperature resistance and mechanical properties. After sizing aramid fibers, the interfacial bonding ability between aramid fibers and resin is enhanced.

[0082] A comparison of the experimental results from Comparative Example 1 and Example 1 shows that if the proportion of terephthalic acid in terephthalic acid and sodium isophthalic acid-5-sulfonate is too high, it will improve the overall rigidity and heat resistance of the chain segments. However, due to the reduced content of sodium isophthalic acid-5-sulfonate, the water solubility of the polyamide decreases, resulting in a poorer sizing effect (e.g., Figure 6 As shown, the film-forming effect is poor, which reduces the interfacial shear strength.

[0083] A comparison of the experimental results of Comparative Example 2 and Example 1 shows that if the proportion of sodium isophthalic acid-5-sulfonate in terephthalic acid and sodium isophthalic acid-5-sulfonate is too high, the rigid aromatic ring structure is insufficient, the heat resistance of the polyamide decreases, the thermal decomposition temperature decreases, and the excessive hydrophilicity leads to insufficient coating strength (e.g. Figure 7 As shown, the film-forming effect is poor, and the interfacial bonding performance deteriorates.

[0084] A comparison of the experimental results of Comparative Example 3 and Example 1 shows that: if the heating rate is increased in step S2, the end-capping rate becomes too fast, the reaction becomes uneven, the end-amino polyether is not completely end-capped, the water solubility of the product decreases, the solution becomes turbid, the molecular weight distribution becomes wider, and both the heat resistance and interfacial shear strength decrease. Figure 8 As shown, the film-forming effect of the sizing agent is poor.

[0085] A comparison of the experimental results of Comparative Example 4 and Example 1 shows that: if a gradient heating is not performed in step S2, and the reaction is directly carried out at 110°C, the temperature in the early stage of the reaction is too high, the end-capping reaction rate is too fast, and the local viscosity of the system increases sharply, resulting in uneven molecular chain growth and insufficient end-capping reaction; at the same time, molecular chain segments become entangled, the water solubility and heat resistance of the final product decrease, and the film-forming effect of the sizing agent is poor (e.g., Figure 9 As shown in the figure, the interfacial shear strength decreases.

[0086] A comparison of the experimental results of Comparative Example 5 and Example 1 shows that if a gradient heating is not performed in step S2, and the reaction is only carried out at 95°C, the end-capping reaction is incomplete, resulting in poor product solubility and film-forming properties, and an inability to form a continuous coating on the fiber surface (e.g., ...). Figure 10 As shown in the figure, the interfacial shear strength decreased significantly.

[0087] A comparison of the experimental results of Comparative Example 6 and Example 1 shows that: without the addition of terminal amino-terminated polyethylene glycol monomethyl ether, the polyamide molecule lacks amphiphilic polyether segments, completely loses its water solubility, and cannot dissolve in water. Simultaneously, it lacks the wetting effect of flexible hydrophilic segments on the fiber surface, resulting in extremely poor interfacial adhesion and interfacial shear strength far lower than in Example 1. Figure 11 It is also clear that the sizing agent cannot form a uniform film on the surface of aramid fiber fabric.

[0088] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0089] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a water-soluble polyamide sizing agent, characterized in that, The preparation method is as follows: S1. Preparation of carboxyl-terminated oligomers: In a solvent and under catalytic conditions, a dicarboxylic acid, sodium isophthalic acid-5-sulfonate, and a diamine react to obtain a system containing terminal carboxyl oligomers. S2, Amphiphilic polyether end-capped: Add amino-terminated polyethylene glycol monomethyl ether to the system in step S1, react under stepped heating conditions and then post-treat to obtain water-soluble polyamide resin. S3. Preparation of sizing agent: The water-soluble polyamide resin is mixed with water to prepare an aqueous solution to obtain a sizing agent.

2. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, The dicarboxylic acid is at least one of adipic acid, terephthalic acid, glutaric acid, and succinic acid; The diamine is at least one of ethylenediamine, propylenediamine, butyldiamine, decanediamine, hexamethylenediamine, heptamethamine, octanediamine, nonadiamine, p-phenylenediamine, and o-phenylenediamine.

3. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, The solvent is at least one of N,N-dimethylformamide and N-methylpyrrolidone; The catalyst is at least one of triphenyl phosphite and pyridine.

4. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, In step S1, the molar ratio of the dicarboxylic acid and sodium isophthalic acid-5-sulfonate is (1-3):4; the molar ratio of the total molar amount of the dicarboxylic acid and sodium isophthalic acid-5-sulfonate to the molar amount of the diamine is (1.8-2.2):

1.

5. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, In step S1, solvent, catalyst, dicarboxylic acid, and sodium isophthalic acid-5-sulfonate are added to the reactor and heated to dissolve. A mixture of diamine and catalyst is then slowly added, and the temperature of the system is controlled at 85-95℃. After the addition is complete, the reaction is carried out for 1.5-2.0 hours to obtain a system containing terminal carboxyl oligomers.

6. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, The molecular weight (Mn) of the amino-terminated polyethylene glycol monomethyl ether is 400-1000.

7. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, In step S2, the system temperature of step S1 is first lowered to 70~75℃, amino-terminated polyethylene glycol monomethyl ether is added, and after being mixed evenly, the temperature is increased stepwise at a rate of 3-5℃ / h. The stepped heating conditions are as follows: heat to 90-95℃, hold for 1-1.5h, continue heating to 105-110℃, and hold for 2-2.5h to obtain water-soluble polyamide resin.

8. The method for preparing a water-soluble polyamide sizing agent according to claim 1, characterized in that, In step S3, the water-soluble polyamide resin is prepared into an aqueous solution with a mass concentration of 0.5wt% to 2.0wt% in water, which is the sizing agent.

9. A water-soluble polyamide sizing agent, characterized in that, The sizing agent is prepared by any one of the preparation methods described in claims 1-8.

10. An application of the water-soluble polyamide sizing agent according to claim 9, characterized in that, The sizing agent is used in the surface treatment of aramid fibers.

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