Application of monofunctional isocyanate in surface blocking modification of PA66 industrial yarn

Abstract

The invention discloses application of monofunctional isocyanate in surface blocking modification of PA66 industrial yarn, and belongs to the technical field of polyamide fibers. Aiming at the problems of high hydrolytic sensitivity and poor long-term stability of PA66 in a humid environment, monofunctional isocyanate can react with an amino-terminated group on the outermost layer of the PA66 fiber to form an alkyl / aryl urea terminated structure, a formed urea bond is more resistant to hydrolysis than an amido bond, and a monofunctional design ensures that a body type cross-linked network is not formed, so that the service life of the PA66 fiber is prolonged, and the service life of the PA66 fiber is prolonged. The modification reaction is limited to occur only on the surface layer, performance loss caused by internal modification can be avoided, the strength retention rate of the obtained surface-terminated modified PA66 industrial yarn in a high-damp-heat environment is high, the industrial application requirement for long-term durability is met, and the application prospect is wide. Therefore, the composite material has a good application prospect in preparation of radial tire cord fabrics, automobile safety belts, industrial conveying belts or high-pressure rubber hose reinforcing layers.

Description

Technical Field

[0001] This invention belongs to the field of polyamide fiber technology, and particularly relates to the application of a monofunctional isocyanate in the surface end-capping modification of PA66 industrial yarn. Background Technology

[0002] Polyhexamethylene adipamide (PA66) industrial yarns, due to their high content of terminal amino groups (typically 40-50 mmol / kg), are highly susceptible to hydrolysis under high temperature and humidity conditions, leading to rapid strength degradation. The hydrolysis mechanism primarily stems from the reversible polycondensation reaction of PA66: the amide bond can reversibly hydrolyze in the presence of moisture into carboxylic acid and amine terminal groups, further resulting in polymer chain breakage, molecular weight reduction, and mechanical property degradation. Specifically, water molecules undergo nucleophilic addition with the carbonyl group in the amide bond, forming a tetrahedral intermediate, which subsequently cleaves to produce new terminal carboxyl and amino groups. This chain-breaking process accelerates moisture diffusion, creating a vicious cycle that causes the fiber's strength retention to drop below 70% after 14 days at 85℃ / 95%RH.

[0003] Existing hydrolysis-resistant methods mainly include melt copolymerization to introduce hydrolysis-resistant monomers (such as aromatic diacids) to reduce end-group density, using difunctional isocyanates (such as hexamethylene diisocyanate) for bulk crosslinking, or applying epoxy resin or silane coupling agents as a post-coating. However, these methods have significant drawbacks: melt copolymerization often increases costs and reduces fiber spinnability; crosslinking of difunctional isocyanates easily leads to fiber embrittlement, decreased elongation, and deterioration of fatigue performance; and post-coatings face problems such as poor adhesion, easy peeling, and poor environmental compatibility. In addition, difunctional isocyanates may form a three-dimensional network, hindering the expulsion of moisture from the fiber and further promoting localized hydrolysis. Summary of the Invention

[0004] In order to overcome at least one of the problems existing in the prior art, one of the objectives of the present invention is to provide an application of monofunctional isocyanate in the surface end-capping modification of PA66 industrial yarn. Surface end-capping modification using monofunctional isocyanate can avoid the negative impact of the three-dimensional cross-linked network, and at the same time, the hydrolytic active sites can be significantly reduced by selectively end-capping terminal amino groups (forming a stable urea structure).

[0005] The second objective of this invention is to provide a surface end-capping modification liquid for PA66 industrial yarn.

[0006] The third objective of this invention is to provide a method for surface end-capping modification of PA66 industrial yarn.

[0007] The fourth objective of this invention is to provide a surface-end-modified PA66 industrial yarn.

[0008] The fifth objective of this invention is to provide an application of the above-mentioned surface-end-modified PA66 industrial yarn.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first aspect of the present invention provides the application of a monofunctional isocyanate in the surface end-capping modification of PA66 industrial yarn.

[0010] PA66, a semi-crystalline thermoplastic polyamide, possesses excellent mechanical strength, heat resistance, and chemical resistance. However, its hydrolytic sensitivity in humid environments limits its long-term stability in high-performance applications. The surface-capping modification solution provided in this invention can specifically address this issue. The reactivity of monofunctional isocyanates (MIs) stems from the electrophilicity of the -NCO group, and their addition rate constant with amine groups can reach 10. 3 ~10 5 L / mol·s (depending on substituents), alicyclic or aryl structures provide steric hindrance to control the reaction depth. This invention uses a monofunctional isocyanate to react with terminal amino groups on the outermost layer of the fiber, forming an alkyl / aryl urea-terminated structure. This reaction is based on the rapid addition of the monofunctional isocyanate to the amino group, forming a urea bond (-NH-CO-NH-). This bond is more resistant to hydrolysis than amide bonds, thus significantly reducing the water absorption rate (by shielding the hydrophilic end groups) and hydrolytic active sites (reducing reversible hydrolysis sites). The modified fiber effectively improves its strength retention under high humidity and heat environments, meeting the requirements for long-term durability in industrial applications.

[0011] In some embodiments of the present invention, the monofunctional isocyanate is used as the only isocyanate component in the surface end-capping modification of PA66 industrial yarn.

[0012] This invention uses monofunctional isocyanate as the sole isocyanate component, which is derived from the nucleophilic addition reaction kinetics of monofunctional isocyanate with polyamide end groups. The substituents of monofunctional isocyanate (such as cyclohexyl, phenyl or alkyl) provide steric hindrance and electronic effects, limiting the reaction depth to the surface layer, thus avoiding performance loss caused by internal modification. The monofunctional design ensures that no three-dimensional cross-linked network is formed, thereby maintaining the flexibility and fatigue resistance of the fiber.

[0013] In some embodiments of the present invention, the monofunctional isocyanate includes at least one selected from cyclohexyl isocyanate (CHI), phenyl isocyanate (PI), n-butyl isocyanate (n-BI), isopropyl isocyanate (IPI), methyl isocyanate (MIC), ethyl isocyanate (EI), or octadecyl isocyanate (OI); in some specific embodiments of the present invention, the monofunctional isocyanate includes at least one selected from cyclohexyl isocyanate, phenyl isocyanate, or n-butyl isocyanate; in some preferred embodiments of the present invention, the monofunctional isocyanate is selected from cyclohexyl isocyanate.

[0014] All of the above-mentioned monofunctional isocyanates can achieve good surface end-capping modification effects in this invention. The substituents in different monofunctional isocyanates (such as cyclohexyl, phenyl or alkyl, etc.) can provide different steric hindrance and electronic effects, thereby adjusting the reaction depth of surface end-capping modification, and thus adjusting the hydrophobicity and stability of the end-capping structure.

[0015] A second aspect of the present invention provides a surface-capping modification liquid for PA66 industrial yarn, the surface-capping modification liquid comprising an isocyanate; the isocyanate being a monofunctional isocyanate.

[0016] This invention uses monofunctional isocyanate as the sole isocyanate component in the surface-capping modification solution. Only monofunctional isocyanate reacts with terminal amino groups on the outermost layer of the fiber to form an alkyl / aryl urea-capped structure. This reaction is based on the rapid addition of monofunctional isocyanate to amino groups, forming urea bonds (-NH-CO-NH-). These bonds are more resistant to hydrolysis than amide bonds, and the monofunctional design ensures that a three-dimensional cross-linked network is not formed, thus maintaining the fiber's flexibility and fatigue resistance. Therefore, the surface-capping modification solution of this invention can significantly reduce the water absorption rate (by shielding hydrophilic end groups) and hydrolytic active sites (reducing reversible hydrolysis sites). The modified fiber's strength retention rate under high humidity and heat environments is effectively improved (e.g., 21-25% higher than unmodified fiber), meeting the industrial application requirements for long-term durability.

[0017] In some embodiments of the present invention, the content of monofunctional isocyanate in the surface-capping modified liquid is 0.3~4wt%; for example, it can be any value or a range between any two of 0.3wt%, 0.5wt%, 0.8wt%, 1wt%, 1.5wt%, 1.8wt%, 2wt%, 2.2wt%, 2.5wt%, 3wt%, 3.5wt%, or 4wt%, such as 1~3wt%, 1.5~2.5wt%, etc.

[0018] In some embodiments of the present invention, the monofunctional isocyanate includes at least one selected from cyclohexyl isocyanate (CHI), phenyl isocyanate (PI), n-butyl isocyanate (n-BI), isopropyl isocyanate (IPI), methyl isocyanate (MIC), ethyl isocyanate (EI), or octadecyl isocyanate (OI); in some specific embodiments of the present invention, the monofunctional isocyanate includes at least one selected from cyclohexyl isocyanate, phenyl isocyanate, or n-butyl isocyanate; in some preferred embodiments of the present invention, the monofunctional isocyanate is selected from cyclohexyl isocyanate.

[0019] The surface-capping modification solution of the present invention can achieve good surface-capping modification effect using the above-mentioned monofunctional isocyanates. The substituents in different monofunctional isocyanates (such as cyclohexyl, phenyl or alkyl, etc.) can provide different steric hindrance and electronic effects, thereby adjusting the reaction depth of surface-capping modification, and thus adjusting the hydrophobicity and stability of the capping structure.

[0020] In some embodiments of the present invention, the surface end-capping modification liquid further includes a solvent.

[0021] In some embodiments of the present invention, the boiling point of the solvent at normal pressure is 120~230℃; for example, it can be any value or a range between any two of 120℃, 126℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 225℃ or 230℃, such as 126~225℃, 150~225℃, 160~170℃, etc.

[0022] The solvent selected in this invention has a moderate boiling point at normal pressure, which facilitates its volatilization and recovery during the heat setting stage.

[0023] In some embodiments of the present invention, the saturated vapor pressure of the solvent at 145°C is ≤10 kPa; more specifically, it is 0.1~10 kPa; for example, it can be any value or a range between any two of 0.1 kPa, 0.5 kPa, 1 kPa, 3 kPa, 5 kPa, 7 kPa, 9 kPa or 10 kPa, such as 0.1~2 kPa, 0.1~0.9 kPa, etc.

[0024] The solvent used in this invention has a low saturated vapor pressure at 145°C, which can reduce the problem of uneven concentration caused by premature evaporation during the impregnation process.

[0025] In some embodiments of the present invention, the retention rate of the -NCO groups after the solvent and the monofunctional isocyanate are mixed at 140°C under sealed conditions for 48 hours is ≥98.5%; more specifically, it is 98.5~100%; for example, it can be any value or a range between any two of 98.5%, 98.8%, 99%, 99.2%, 99.5%, 99.7%, 99.9% or 100%, such as 98.5~99.9%, 99.7~99.9%, etc.

[0026] The solvent selected in this invention exhibits a high retention rate of -NCO groups after mixing with the monofunctional isocyanate at 140°C under sealed conditions for 48 hours, ensuring the stability of the isocyanate. For example, alicyclic or aromatic solvents can effectively inhibit -NCO self-polymerization. This -NCO group retention rate can be verified based on molecular simulations and experiments.

[0027] In some embodiments of the present invention, the solvent has a solubility of ≥80 g / 100 g for the monofunctional isocyanate at 25°C; more specifically, (80~150) g / 100 g; for example, it can be any value or a range between any two of 80 g / 100 g, 90 g / 100 g, 100 g / 100 g, 110 g / 100 g, 120 g / 100 g, 130 g / 100 g, 140 g / 100 g, or 150 g / 100 g, such as (100~150) g / 100 g, (110~140) g / 100 g, etc. Solubility at 25°C refers to the mass of the monofunctional isocyanate that dissolves in 100 g of solvent at 25°C to achieve saturation.

[0028] The solvent used in this invention has high solubility for monofunctional isocyanates at 25°C, which can ensure uniform dissolution.

[0029] In some embodiments of the present invention, the solvent is an inert solvent; in some specific embodiments of the present invention, the inert solvent is an inert solvent that does not contain active hydrogen; specifically, the active hydrogen includes hydrogen from -OH and -NH2. In the present invention, the inertness of the solvent stems from its absence of active hydrogen (such as -OH and -NH2), which prevents pre-reaction with monofunctional isocyanates. Although some polar solvents such as DMF may react slightly at high temperatures, high retention rates can be maintained through optimized conditions.

[0030] In some embodiments of the present invention, the solvent includes at least one selected from butyl acetate, amyl acetate, cyclohexanone, mesitylene, aromatic solvent oil S-150, a diester mixture (DBE), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), or dimethylacetamide (DMAc); in some specific embodiments of the present invention, the solvent includes at least one selected from methyl acetate, cyclohexanone, mesitylene, aromatic solvent oil S-150, a diester mixture (DBE), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), or dimethylacetamide (DMAc); in some more specific embodiments of the present invention, the solvent includes at least one selected from methyl acetate, cyclohexanone, mesitylene, a diester mixture (DBE), or dimethylformamide (DMF). In some preferred embodiments of the present invention, the solvent is selected from mesitylene.

[0031] In some embodiments of the present invention, the solvent content in the surface-capping modification liquid is 96~99.7wt%; for example, it can be any value or a range between any two of 96wt%, 96.5wt%, 97wt%, 97.5wt%, 98wt%, 98.5wt%, 99wt%, 99.5wt%, or 99.7wt%, such as 97~99wt%, 97.5~98.5wt%, etc.

[0032] In some embodiments of the present invention, the surface-capping modified liquid further includes a catalyst.

[0033] In some embodiments of the present invention, the catalyst comprises an organotin catalyst, an amine catalyst, or a combination thereof; specifically, the organotin catalyst comprises at least one of dibutyltin dilaurate, stannous octoate, or dibutyltin diacetate; the amine catalyst comprises triethylenediamine, triethylamine, or a combination thereof; in some specific embodiments of the present invention, the catalyst comprises dibutyltin dilaurate, triethylenediamine, or a combination thereof; in some more specific embodiments of the present invention, the catalyst is selected from dibutyltin dilaurate.

[0034] In this invention, the catalyst is selected based on its promoting effect on the isocyanate-amine reaction. Both organotin catalysts and amine catalysts can achieve good promoting effects. Among them, organotin catalysts (such as dibutyltin dilaurate) can accelerate the formation of urea bonds without initiating side reactions, and have a better overall effect.

[0035] In some embodiments of the present invention, the catalyst content in the surface-capping modified liquid is 0.005~0.08wt%; for example, it can be any value or a range between any two of 0.005wt%, 0.01wt%, 0.03wt%, 0.05wt%, 0.07wt% or 0.08wt%, such as 0.01~0.05wt%.

[0036] In some embodiments of the present invention, the linear density of the PA66 industrial yarn is 800~3000 dtex; for example, it can be any value of 800 dtex, 1000 dtex, 1500 dtex, 2000 dtex, 2500 dtex or 3000 dtex or a range between any two, such as 2000~2500 dtex.

[0037] In some embodiments of the present invention, the terminal amino content of the PA66 industrial filament is 40~50 mmol / kg; for example, it can be any value or a range between any two of 40 mmol / kg, 41 mmol / kg, 43 mmol / kg, 45 mmol / kg, 47 mmol / kg, 49 mmol / kg or 50 mmol / kg, such as 41~49 mmol / kg, 43~47 mmol / kg, etc.

[0038] A third aspect of the present invention provides a method for surface end-capping modification of PA66 industrial yarn, wherein the surface end-capping modification of PA66 industrial yarn is performed using a surface end-capping modification liquid as described in the second aspect of the present invention.

[0039] In some embodiments of the present invention, the surface end-capping modification method includes the following steps: immersing the PA66 industrial yarn in the modification solution to obtain an impregnated yarn; subjecting the impregnated yarn to pre-reaction and heat setting in sequence to obtain a surface-end-capped modified PA66 industrial yarn.

[0040] In some embodiments of the present invention, the PA66 industrial filament is dried before impregnation; specifically, the drying temperature can be 90~110°C; the drying time can be 2~4 hours. Drying removes surface moisture from the PA66 industrial filament and activates the end groups, thereby improving the efficiency of subsequent reactions.

[0041] In some embodiments of the present invention, the immersion temperature is 100~135°C; for example, it can be any value or a range between 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C or 135°C, such as 125~155°C, 145~155°C, etc.

[0042] The impregnation temperature is adjusted based on the activation energy of the isocyanate reaction (approximately 50~70 kJ / mol) to ensure surface wetting without excessive evaporation.

[0043] In some embodiments of the present invention, the immersion time is 8 to 50 seconds; for example, it can be any value of 8 seconds, 10 seconds, 20 seconds, 30 seconds, 40 seconds, or 50 seconds, or a range between any two.

[0044] In some embodiments of the present invention, the liquid carry-over rate of the impregnated yarn is 8-25 wt%; for example, it can be any value or a range between 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, or 25 wt%, such as 10-20 wt%, 15-18 wt%, etc. The liquid carry-over rate of the impregnated yarn refers to the percentage of the mass of the modified liquid adhering to the PA66 industrial yarn (unimpregnated yarn) after the PA66 industrial yarn is impregnated with the modified liquid.

[0045] In some embodiments of the present invention, the temperature of the pre-reaction is 120~160℃; for example, it can be any value or a range between any two of 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃ or 160℃, such as 140~160℃, 145~155℃, etc.

[0046] In some embodiments of the present invention, the pre-reaction time is 15 to 90 seconds; for example, it can be any value or a range between 15 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds, 70 seconds, 80 seconds, or 90 seconds, such as 30 to 70 seconds, 40 to 60 seconds, etc.

[0047] In some embodiments of the present invention, the heat setting temperature is 160~200℃; for example, it can be any value or a range between any two of 160℃, 165℃, 170℃, 175℃, 180℃, 185℃, 190℃, 195℃ or 200℃, such as 180~200℃, 185~195℃, etc.

[0048] In some embodiments of the present invention, the heat setting time is 30~120s; for example, it can be any value or a range between any two of 30s, 40s, 50s, 60s, 70s, 80s, 90s, 100s, 110s or 120s, such as 50~90s, 60~80s, etc.

[0049] In some embodiments of the present invention, the total heat treatment time of the surface end-capping modification method is ≤5 min; more specifically, it is 1~5 min; for example, it can be any value or a range between 1 min, 2 min, 3 min, 4 min, or 5 min, such as 1~5 min, 2~3 min, etc.; in some specific embodiments of the present invention, the total heat treatment time of the surface end-capping modification method is ≤3 min; more specifically, it is 1~3 min. The heat treatment includes impregnation, pre-reaction, and heat setting.

[0050] A fourth aspect of the present invention provides a surface-end-modified PA66 industrial yarn, the surface-end-modified PA66 industrial yarn comprising a PA66 industrial yarn and a surface-end-modification layer; the surface-end-modification layer covering at least a portion of the surface of the PA66 industrial yarn; the surface-end-modification layer being formed from a surface-end-modification liquid including the one described in the second aspect of the present invention, or being prepared by a surface-end-modification method including the one described in the third aspect of the present invention.

[0051] In some embodiments of the present invention, the thickness of the surface end-capping modification layer is ≤5μm; more specifically, it is 0.1~5μm; for example, it can be any value or a range between 0.1μm, 0.5μm, 1μm, 2μm, 3μm, 4μm or 5μm.

[0052] In some embodiments of the present invention, the terminal amino content of the surface-capped modified layer is 55% or less of the terminal amino content of the PA66 industrial yarn; for example, it can be any value or a range between 30%, 35%, 40%, 45%, 50%, or 55%. In the surface-capped modified PA66 industrial yarn of the present invention, the terminal amino content of the surface-capped modified layer is significantly reduced, and a three-dimensional cross-linked network is not formed.

[0053] In some embodiments of the present invention, the terminal amine content of the surface-capped modified PA66 industrial filament is ≤23 mmol / kg; more specifically, it is 10~23 mmol / kg; for example, it can be any value or a range between any two of 10 mmol / kg, 12 mmol / kg, 14 mmol / kg, 16 mmol / kg, 18 mmol / kg, 20 mmol / kg, or 23 mmol / kg, such as 16~23 mmol / kg, 18~20 mmol / kg, etc. Specifically, the terminal amine content is determined by potentiometric titration.

[0054] In some embodiments of the present invention, the surface-end-capped modified PA66 industrial yarn exhibits a 14-day strength retention rate of ≥91% under 85°C / 95%RH conditions; more specifically, 91~99%; for example, it can be any value or a range between 91%, 91%, 93%, 94%, 95%, 97%, or 99%, such as 91~95%, 94~95%, etc. Specifically, the strength retention rate is tested according to the GB / T14344-2022 standard.

[0055] In some embodiments of the present invention, the H-pull-out force of the surface-end-modified PA66 industrial filament impregnated with RFL is ≥175 N / cm; more specifically, it is 175~200 N / cm; for example, it can be any value among 175 N / cm, 180 N / cm, 185 N / cm, 190 N / cm, 195 N / cm, or 200 N / cm, or a range between any two, such as 180~190 N / cm, etc. Specifically, the H-pull-out force is tested according to the ASTM D4776-2025 standard.

[0056] In some embodiments of the present invention, the elongation at break of the surface-end-modified PA66 industrial yarn is ≥18%; more specifically, it is 18~23%; for example, it can be any value of 18%, 19%, 20%, 21%, 22% or 23% or a range between any two, such as 19~22%, 20~21%, etc.

[0057] In some embodiments of the present invention, the surface-end-modified PA66 industrial yarn has a disc fatigue strength retention rate of ≥85% after 10,000 cycles; more specifically, 85~95%; for example, it can be any value of 85, 88, 90, 92, 93 or 95% or a range between any two, such as 88~93%, 90~92%, etc.

[0058] The fifth aspect of the invention provides the application of surface-end-capped modified PA66 industrial yarn as described in the fourth aspect of the invention in the preparation of reinforcing layers for radial tire cord fabrics, automotive seat belts, industrial conveyor belts, or high-pressure rubber hoses.

[0059] The surface-end-capped modified PA66 industrial yarn of the present invention has good resistance to damp heat. Products such as radial tire cord fabric, automotive seat belts, industrial conveyor belts or high-pressure rubber hose reinforcement layers have high requirements for high damp heat performance. The surface-end-capped modified PA66 industrial yarn of the present invention can be used to prepare the above-mentioned products with good resistance to damp heat.

[0060] The beneficial effects of this invention are as follows: Addressing the issues of high hydrolysis sensitivity and poor long-term stability of PA66 in humid environments, this invention utilizes a monofunctional isocyanate that reacts with the terminal amino groups of the outermost layer of PA66 fibers to form an alkyl / aryl urea-terminated structure. The resulting urea bonds are more resistant to hydrolysis than amide bonds, and the monofunctional design ensures that a three-dimensional cross-linked network is not formed, limiting the modification reaction to the surface layer and avoiding performance loss caused by internal modification. The surface-terminated modified PA66 industrial yarn obtained by this invention exhibits high strength retention under high humidity and heat environments, meeting the requirements for long-term durability in industrial applications. Therefore, it has promising applications in the preparation of radial tire cord fabric, automotive seat belts, industrial conveyor belts, or high-pressure rubber hose reinforcement layers. Detailed Implementation

[0061] The following specific embodiments further illustrate the content of the present invention in detail. It should also be understood that the following embodiments are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made by those skilled in the art based on the principles described herein are all within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make selections within a suitable range based on the description herein, and are not intended to be limited to the specific data in the examples below. Unless otherwise specified, the raw materials, reagents, or apparatus used in the following embodiments and comparative examples can be obtained from conventional commercial sources or by existing known methods.

[0062] In some embodiments of the present invention, the solvents involved and their parameters are shown in Table 1 below. The solvents in Table 1 are inert solvents, as they do not contain active hydrogen (such as -OH, -NH2), thus avoiding pre-reaction with monofunctional isocyanates (MI)I. Although some polar solvents, such as DMF, may react slightly at high temperatures, high retention rates can be maintained through optimized conditions. Among these, solvents such as mesitylene, aromatic solvent oil S-150, DBE, DMF, DMSO, NMP, or DMAc exhibit good stability at high boiling points and low vapor pressures, making them suitable for continuous production.

[0063] Table 1. Solvents and their parameters involved in some embodiments of the present invention.

[0064] The performance tests involved in the embodiments and comparative examples of this invention are described below: 1) The content of terminal amino groups is determined using potentiometric titration. The key principle is that the terminal amino group (-NH2), acting as a weak base, reacts with a strong acid (such as perchloric acid) in a non-aqueous solvent to form a titratable complex. The pH change is monitored using a potentiometer to determine the titration endpoint (equivalence point), and the concentration of the terminal amino group is calculated. Unit: mmol / kg (millimoles per kilogram of sample).

[0065] The required materials and instruments are as follows: Solvent: Benzyl alcohol or other non-aqueous solvents (such as phenol / chloroform mixtures) are used to dissolve PA66 samples. Benzyl alcohol is often chosen because it improves the resolution of weak acid / base titrations and can dissolve polyamides at 140-150°C without causing side reactions.

[0066] Titrant: 0.1 N (equivalent concentration) perchloric acid (HClO4) solution, usually soluble in acetic acid or dioxane to ensure stability in non-aqueous media.

[0067] Instrument: Potentiometer (pH electrode combined with reference electrode), using a three-electrode system (working electrode, reference electrode, counter electrode) to improve accuracy for low conductivity solutions. Brand: Mettler Toledo.

[0068] Sample preparation: Approximately 0.5~1g of PA66 industrial yarn sample.

[0069] Detailed testing steps: 1. Sample Dissolution: Place the PA66 sample (accurately weighed) in benzyl alcohol, heat to 140-160°C and stir to dissolve (approximately 30-60 minutes), ensuring complete dissolution without degradation. Cool to room temperature.

[0070] 2. Titration preparation: Add a known volume of titrant as a blank control.

[0071] 3. Titration process: Slowly add perchloric acid solution, while recording the relationship between potential (mV) and titration volume. Plot the titration curve, identify the inflection point (endpoint), and the corresponding equivalence point of the terminal amino group.

[0072] 4. The calculation formula is: Terminal amino group content (AEG, mmol / kg) = [(V sample -V blank )×N titrant [×1000] / m sample ; Among them, V sample : Sample titration volume (mL); V blank Blank titration volume (mL); N titrantTitrant concentration (mol / L); m sample Sample mass (g).

[0073] Verification: Repeat the test 3 times and take the average value.

[0074] 2) The tests were conducted according to ISO 1419-2019 (hydrolytic stability test, minimum test length of 5 weeks) and BS EN 12447-2021 (hydrolysis resistance index test) standard methods, including wet heat aging (85℃ / 95%RH, 14 days) and dry heat aging (180℃×3 h); the water absorption rate was tested by immersion method (23℃ / 65%RH, 24 h); the strength retention rate was calculated by tensile test (Instron instrument, GB / T 14344-2022 standard); the H pull-out force was tested according to ASTM D4776-2025, using RFL impregnation solution.

[0075] Example 1 A surface-sealing modification solution for PA66 industrial filaments has the following composition: 2.0 wt% cyclohexyl isocyanate (CHI), 0.03 wt% dibutyltin dilaurate, and the balance being mesitylene.

[0076] The above-mentioned surface end-capping modification liquid was used for surface end-capping modification of PA66 industrial yarn. The specific steps are as follows: (1) Preparation of modified solution: Dissolve monofunctional isocyanate MI (cyclohexyl isocyanate) in a special inert solvent (trimethylbenzene), add catalyst (dibutyltin dilaurate) to obtain modified solution.

[0077] (2) The spun and fully stretched PA66 industrial yarn (linear density 2350dtex) is pre-dried at 90~110℃ for 2~4h to remove surface moisture and activate end groups, thereby improving reaction efficiency.

[0078] (3) The industrial yarn is continuously passed through the modified liquid impregnation tank at a temperature of 135°C, a liquid carryover rate of 16 wt%, and a residence time of 30-40 seconds. This temperature range is based on the activation energy of the isocyanate reaction (approximately 50-70 kJ / mol) to ensure surface wetting without excessive evaporation.

[0079] (4) Immediately enter the 150℃ pre-reaction zone and stay for 50 seconds to promote the rapid addition of the -NCO group of MI to the terminal -NH2 of PA66 to form the alkyl / cycloalkyl urea end-capped structure.

[0080] (5) Then enter the 190℃ heat setting oven for 70 seconds to evaporate the solvent while heat setting. This step utilizes the boiling point and vapor pressure characteristics of the solvent to achieve efficient evaporation. The total heat treatment time should not exceed 3 minutes to avoid thermal degradation.

[0081] (6) After washing with water or blowing with hot air, rewind to ensure that the residual solvent is ≤0.06%.

[0082] Example 2 A surface-sealing modification solution for PA66 industrial yarn has the following composition: 1.0 wt% n-butyl isocyanate (n-BI) and a mixed solvent of amyl acetate / cyclohexanone (mass ratio 6:4, balance).

[0083] The above-mentioned surface end-capping modification liquid was used for surface end-capping modification of PA66 industrial yarn. The only difference from Example 1 was the composition of the modification liquid. All other conditions were the same as in Example 1.

[0084] In this example, the MI concentration is reduced to lower costs, while the mixed solvent optimizes solubility and volatility.

[0085] Test results: terminal amino group 23 mmol / kg, 14-day wet heat intensity retention rate 91%.

[0086] Example 3 A surface-sealing modification liquid for PA66 industrial yarn has the following composition: 3.0 wt% phenyl isocyanate (PI) and DBE (balance).

[0087] The above-mentioned surface end-capping modification liquid was used for surface end-capping modification of PA66 industrial yarn. The only difference from Example 1 was the composition of the modification liquid and the immersion temperature was changed to 145°C; other conditions were the same as in Example 1.

[0088] In this example, a high-boiling-point DBE is used to ensure high-temperature stability, achieving a -NCO retention rate of ≥99.9%.

[0089] Test results: terminal amino group 20 mmol / kg, solvent residue 0.04%, 14-day wet heat strength retention rate 95%.

[0090] Example 4 A surface-sealing modification solution for PA66 industrial yarn has the following composition: 2.5 wt% cyclohexyl isocyanate (CHI) and DMF (balance).

[0091] The above-mentioned surface end-capping modification liquid was used for surface end-capping modification of PA66 industrial yarn. The only difference from Example 1 was the composition of the modification liquid and the immersion temperature was changed to 130°C; other conditions were the same as in Example 1.

[0092] In this example, DMF is used as a polar solvent to enhance solubility, but the temperature needs to be controlled to avoid slight side reactions.

[0093] Test results: Terminal amino group 20 mmol / kg, 14-day wet heat intensity retention rate 93%.

[0094] Comparative Example 1 An unmodified yarn, PA66 industrial yarn (linear density 2350 dtex).

[0095] Comparative Example 2 A surface-capping modification solution for PA66 industrial yarn differs from Example 1 in that cyclohexyl isocyanate is replaced with 1,6-hexamethylene diisocyanate (HDI), while other conditions are the same as in Example 1.

[0096] The above-mentioned surface end-capping modification liquid was used for surface end-capping modification of PA66 industrial yarn. The only difference from Example 1 was the composition of the modification liquid. All other conditions were the same as in Example 1.

[0097] In this example, bifunctional HDI was used for modification to obtain bifunctional HDI modified silk.

[0098] Table 2 Performance comparison of Example 1 and Comparative Examples 1-2

[0099] Analysis of Table 2 shows that, regarding the terminal amino content, bifunctional HDI achieves good end-capping effect (due to potential bifunctional reactions), but monofunctional CHI end-capping is sufficient to reduce the terminal amino content to below 23 mmol / kg, avoiding cross-linking caused by excessive reaction. Regarding the 24-hour water absorption rate, bifunctional HDI and monofunctional CHI end-capping have similar water absorption rates, both reducing water absorption by shielding hydrophilic end groups; monofunctional CHI has a slightly better water absorption reduction effect but is still significantly better than unmodified, proving that the end-capping effect can be achieved without cross-linking. Regarding the 14-day wet heat strength retention rate, bifunctional HDI and monofunctional CHI have comparable or slightly better hydrolysis resistance (cross-linking provides an additional barrier), but monofunctional CHI can improve it by 21-25% compared to unmodified yarn, meeting industrial requirements and avoiding side effects caused by embrittlement. Regarding the dry heat aging strength retention rate, monofunctional CHI is better because it maintains thermal stability without cross-linking; HDI cross-linking may cause local stress concentration, resulting in a lower retention rate compared to monofunctional CHI. Regarding fracture strength, bifunctional HDI end-capped and monofunctional CHI end-capped values ​​are similar or slightly higher; HDI may slightly increase rigidity due to crosslinking, but not significantly. Regarding elongation at break, monofunctional CHI shows an 8% increase compared to unmodified filaments, effectively maintaining flexibility; HDI becomes embrittled due to crosslinking, resulting in a 21% decrease in elongation at break, which limits its application in high-strain materials such as tire cord fabric. Regarding the 10,000-cycle fatigue strength retention rate, monofunctional CHI shows a 26% increase compared to unmodified filaments, exhibiting excellent fatigue resistance; HDI crosslinked networks are prone to fatigue fracture, with retention rates close to or lower than unmodified filaments, highlighting the advantage of monofunctionality in avoiding bulky networks. Regarding H-pull-out force, monofunctional CHI improves interfacial compatibility (urea structure compatible with RFL); HDI crosslinking may interfere with surface affinity, resulting in a decrease compared to monofunctional CHI.

[0100] It is evident that, compared to difunctional isocyanate modification, monofunctional isocyanate modification exhibits significantly higher elongation at break and better fatigue resistance, as well as higher 14-day wet heat strength retention rate and H-pull-out force. The terminal amino content, 24-hour water absorption rate, dry heat aging strength retention rate, and fracture strength can also be maintained at a level basically equivalent to those of difunctional isocyanate modification.

[0101] In Examples 1-4 of this invention, monofunctional isocyanates were dissolved in suitable solvents, and the resulting modified solutions were used for surface end-capping modification of PA66 industrial yarns. The resulting surface-capped PA66 industrial yarns exhibited the following characteristics: terminal amino content of 16-23 mmol / kg, 24-hour water absorption rate (23℃ / 65%RH) of 2.2%, 14-day strength retention rate of 91-95% under 85℃ / 95%RH conditions, strength retention rate of 97% under dry heat conditions of 180℃×3h, breaking strength of 8.3-8.5 cN / dtex, elongation at break of 20.8%, strength retention rate of 92% after 10,000 disc fatigue cycles, and solvent residue (GC) ≤0.06%. Furthermore, the total heat treatment time in Examples 1-4 of this invention did not exceed 3 minutes, and the terminal amino content within a 5μm depth of the modified fiber surface was reduced to below 55% of that of the unmodified yarn, without the formation of a three-dimensional cross-linked network.

[0102] In summary, this invention addresses the issues of high hydrolysis sensitivity and poor long-term stability of PA66 in humid environments. It utilizes a monofunctional isocyanate that reacts with the terminal amino groups of the outermost layer of PA66 fibers to form an alkyl / aryl urea-terminated structure. The resulting urea bonds are more resistant to hydrolysis than amide bonds, and the monofunctional design ensures the absence of a three-dimensional cross-linked network, limiting the modification reaction to the surface layer and avoiding performance loss caused by internal modification. The surface-terminated modified PA66 industrial yarn obtained by this invention exhibits high strength retention under high humidity and heat conditions, meeting the long-term durability requirements of industrial applications. Therefore, it shows promising application prospects in the preparation of radial tire cord fabric, automotive seat belts, industrial conveyor belts, or high-pressure rubber hose reinforcement layers.

Claims

1. Application of a monofunctional isocyanate in the surface end-capping modification of PA66 industrial yarn.

2. A surface-sealing modification liquid for PA66 industrial yarn, characterized in that, The surface-capping modification solution includes isocyanate; the isocyanate is a monofunctional isocyanate.

3. The surface-sealing modified liquid according to claim 2, characterized in that, The content of monofunctional isocyanate in the surface-capping modification solution is 0.3~4 wt%; And / or, the monofunctional isocyanate includes at least one of cyclohexyl isocyanate, phenyl isocyanate, n-butyl isocyanate, isopropyl isocyanate, methyl isocyanate, ethyl isocyanate or octadecyl isocyanate.

4. The surface-capping modified liquid according to claim 2, characterized in that, The surface-capping modification liquid further includes a solvent; the solvent satisfies at least one of the following characteristics: 1) The solvent has a normal pressure boiling point of 120~230℃; 2) The saturated vapor pressure of the solvent at 145℃ is ≤10kPa; 3) The retention rate of -NCO groups after the solvent and monofunctional isocyanate are mixed at 140°C under sealed conditions for 48 hours is ≥98.5%; 4) The solvent has a solubility of ≥80g / 100g for monofunctional isocyanates at 25°C; And / or, the solvent includes at least one of butyl acetate, amyl acetate, cyclohexanone, mesitylene, aromatic solvent oil S-150, a mixture of dicarboxylic acid esters, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, or dimethylacetamide; And / or, the surface-capping modification solution further includes a catalyst; the catalyst includes organotin catalysts, amine catalysts, or combinations thereof; the catalyst content in the surface-capping modification solution is 0.005~0.08 wt%; And / or, the linear density of the PA66 industrial filament is 800~3000 dtex; And / or, the terminal amino content of the PA66 industrial filament is 40~50 mmol / kg.

5. A method for surface end-capping modification of PA66 industrial yarn, characterized in that, The surface end-capping modification liquid as described in any one of claims 2 to 4 is used to modify the surface end-capping of PA66 industrial yarn.

6. The surface end-capping modification method according to claim 5, characterized in that, The surface end-capping modification method includes the following steps: immersing the PA66 industrial yarn in the surface end-capping modification solution to obtain an impregnated yarn; subjecting the impregnated yarn to pre-reaction and heat setting sequentially to obtain a surface-end-capped modified PA66 industrial yarn.

7. The surface end-capping modification method according to claim 6, characterized in that, The impregnation temperature is 100~135℃; And / or, the immersion time is 8~50s; And / or, the liquid content of the impregnated yarn is 8~25wt%; And / or, the temperature of the pre-reaction is 120~160℃; And / or, the pre-reaction time is 15~90s; And / or, the heat setting temperature is 160~200℃; And / or, the heat setting time is 30~120s.

8. A surface-end-modified PA66 industrial yarn, characterized in that, The surface-end-modified PA66 industrial yarn includes PA66 industrial yarn and a surface-end-modified layer; the surface-end-modified layer covers at least a portion of the surface of the PA66 industrial yarn; the surface-end-modified layer is formed by a surface-end-modifying liquid comprising any one of claims 2 to 4, or is prepared by a surface-end-modifying method comprising any one of claims 5 to 7.

9. The surface-end-capped modified PA66 industrial yarn according to claim 8, characterized in that, The thickness of the surface end-capping modification layer is ≤5μm; And / or, the terminal amino content of the surface-capped modified PA66 industrial filament is ≤23mmol / k; And / or, the surface-end-modified PA66 industrial yarn retains ≥91% of its strength after 14 days at 85°C / 95%RH.

10. The application of the surface-end-capped modified PA66 industrial yarn as described in claim 8 or 9 in the preparation of radial tire cord fabric, automotive seat belts, industrial conveyor belts or high-pressure rubber hose reinforcement layers.

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