Crystal form regulation and production process of high-temperature-resistant aramid pulp

Abstract

The application relates to the technical field of high-molecular fiber manufacturing, and discloses a crystal form regulation and manufacturing process of high-temperature-resistant aramid pulp, which comprises the following steps: dispersing 2.5% to 5.5% polyphosphoric acid in concentrated sulfuric acid to prepare a mixed solvent at 60 DEG C to 80 DEG C; dissolving poly-p-phenyleneterephthalamide to construct a metastable anisotropic liquid crystal spinning stock solution; and pouring the stock solution into a low-temperature coagulation bath to perform shearing coagulation. The metastable state characteristics of the binary solvent system under a specific ratio are utilized, the stock solution is rapidly desolvated and crystallized at the moment of coagulation, a high-order Form-II crystal structure is formed, the problem of crystal lattice defects caused by solvent residues is solved, and the high-temperature resistance of the pulp is improved.

Description

Technical Field

[0001] This invention relates to a crystal form control and manufacturing process for high-temperature resistant aramid pulp, belonging to the field of polymer fiber manufacturing technology. Background Technology

[0002] Currently, the industrial process commonly uses concentrated sulfuric acid wet spinning, which utilizes the strong protonation effect of concentrated sulfuric acid on polymer molecular chains to break intermolecular hydrogen bonds, preparing liquid crystal spinning dope. Pulping is then achieved through high-shear precipitation technology. This process is mature and can produce fiber materials with a certain specific surface area on a large scale. However, the existing pure sulfuric acid spinning process has physicochemical limitations in applications requiring high temperature resistance. To ensure the stable dissolution of high-polymerization-degree polymers, the solvent system needs to form a protonated solvent layer with high binding energy with the amide groups of the molecular chain. This stable solvent layer exhibits solvent lag in the subsequent extremely short shear solidification stage. When the dope fluid comes into contact with the solidification bath, the retained solvent molecules cannot decouple from the kinetics of the molecular chain rearrangement and crystallization process, thus forcibly occupying lattice positions and hindering the close packing of molecular chains. This solvent occupancy effect leaves a large number of nanoscale micropores and amorphous defects inside the fiber, which become the starting sites for thermal degradation, ultimately causing the material to collapse under conditions exceeding 400°C.

[0003] To address the microscopic defects caused by the aforementioned chemical forming mechanism, existing technologies often attempt to compensate for the inherent deficiencies in the microstructure through post-processing physical composites or high-intensity hot pressing. However, such external repair strategies are unlikely to address the root cause of crystal growth. For example, Chinese invention patent application CN103568415B discloses a method for producing a composite aramid electrical insulation nonwoven material. It employs airflow web forming and electrostatic flocking technology to physically composite aramid precipitated fiber pulp into a substrate, supplemented by high-temperature and high-pressure hot rolling to improve the overall density of the material. Analyzing this technical route, its essence is still a physical stacking and bonding at the overall level. If the pulp raw materials used do not deviate from the traditional pure sulfuric acid system preparation logic, lattice defects and solvent residues within the fibers still exist. Although external mechanical compaction can improve the overall density, it cannot repair the solvation damage at the microscopic grain boundaries. When the material is placed in a high-temperature environment, the deeply residual solvent molecules vaporize and expand, which will disintegrate the fiber structure from the inside, leading to the failure of insulation and mechanical properties.

[0004] Therefore, how to reconstruct the chemical composition and solvation state of the spinning solution, reduce the stripping energy barrier during the solidification stage of the solvent layer, achieve spontaneous high-order crystallization during shearing and forming, and prepare aramid pulp with both high specific surface area and high temperature resistance has become the technical problem to be solved by this invention. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: a crystal form control and manufacturing process for high-temperature resistant aramid pulp, the process comprising the following steps: Step 101: Concentrated sulfuric acid with a mass percentage concentration of 98.0% to 99.5% and polyphosphoric acid with a content of 83.0% to 85.0% based on phosphorus pentoxide are homogeneously mixed at a temperature of 60°C to 80°C to prepare a binary solvent, wherein the mass percentage of polyphosphoric acid in the binary solvent is 2.5% to 5.5%. Step 102: Poly(p-phenylene terephthalamide) resin with an intrinsic viscosity of 5.5 dL / g to 6.5 dL / g is added to a binary solvent, and shear dissolution is performed through a twin-screw extruder at a temperature of 80°C to 90°C to prepare a nematic liquid crystal spinning solution. The residence time of the poly(p-phenylene terephthalamide) resin in the twin-screw extruder is 120 min to 180 min. Step 103: The nematic liquid crystal spinning solution is extruded through a spinneret with an aspect ratio of 2:1 to 5:1. The extruded material is introduced into a dilute sulfuric acid coagulation bath with a temperature of 0°C to 10°C and a mass percentage concentration of 5.0% to 15.0%. Shear coagulation is carried out under a shear rate field of 2500s⁻¹ to 4000s⁻¹ to precipitate and form primary aramid pulp with a Form-II crystal structure.

[0006] Preferably, in step 101, the homogeneous mixing time is 30 min to 60 min; the dynamic viscosity of the nematic liquid crystal spinning solution at 25°C is 1500 mPa·s to 2500 mPa·s; and in step 103, the residence time of the primary aramid pulp in the dilute sulfuric acid coagulation bath is 20 ms to 50 ms.

[0007] Preferably, in step 101, the mass percentage of polyphosphoric acid in the binary solvent is 3.5% to 4.5%, and the homogeneous mixing temperature is 65°C to 75°C.

[0008] Preferably, in step 102, the length-to-diameter ratio of the twin-screw extruder is 32:1 to 48:1, and the screw assembly in the melting zone includes a kneading block assembly and a reverse thread element.

[0009] Preferably, in step 103, the dilute sulfuric acid coagulation bath is maintained at 4°C to 6°C through an external circulating heat exchange system, and the spinneret stretch ratio of the nematic liquid crystal spinning solution before entering the dilute sulfuric acid coagulation bath is 1.5 to 3.0.

[0010] Preferably, the process further includes the following steps: Step 104, the primary aramid pulp is fed into a continuous washing line and subjected to multi-stage countercurrent washing with deionized water at a temperature of 20°C to 40°C until the conductivity of the washing liquid is lower than 100 μS / cm, thereby removing the residual concentrated sulfuric acid and polyphosphate in the primary aramid pulp.

[0011] Preferably, the process controls the ratio of the binary solvent in step 101 and the residence time of shear dissolution in step 102 to make the crystallinity of the obtained high-temperature resistant aramid pulp greater than 80%, and the ratio of the diffraction peak intensity of the (200) crystal plane to the (110) crystal plane of the obtained high-temperature resistant aramid pulp is 1.8 to 2.5.

[0012] Preferably, the obtained high-temperature resistant aramid pulp has a Form-II crystal structure without undergoing heat treatment above 500°C, and the axial microcrystal size of the obtained high-temperature resistant aramid pulp is 5nm to 15nm.

[0013] Preferably, the obtained high-temperature resistant aramid pulp has a specific surface area of ​​8 m² / g to 12 m² / g, a thermal weight loss rate of no more than 5% after being placed in an air atmosphere at 400°C for 24 hours, and a fibril diameter of 0.1 μm to 1.5 μm.

[0014] Preferably, in step 103, the shear rate field is generated by a high-speed shear rotor placed in a dilute sulfuric acid coagulation bath, and the linear velocity of the high-speed shear rotor is 30 m / s to 50 m / s.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. In the crystal form control of high-temperature aramid pulp, the protonation environment of the spinning solution was reconstructed by introducing a specific ratio of proton-competing components into the main solvent of concentrated sulfuric acid. This method constructs a metastable state that can maintain polymer dissolution and is sensitive to the coagulation bath, thereby effectively reducing the stripping energy barrier of the solvation layer during the coagulation stage. This allows the polymer molecular chains to complete the desolventization process at an extremely fast kinetic rate the moment they come into contact with the coagulation liquid. From a physicochemical perspective, this avoids the formation of micropore defects due to solvent molecules remaining between molecular chains, ensuring the density and integrity of the internal crystal skeleton of the pulp fiber. As a result, it exhibits high-temperature resistance due to the lack of thermal degradation initiation sites under high-temperature conditions.

[0016] 2. By utilizing the steric hindrance effect of polyphosphate molecules within a specific concentration window, the spinning solution is induced to form a pre-ordered layered structure with smectic characteristics in the liquid crystal state. This pre-ordered layered structure optimizes the response mode of the fluid in the shear field, causing the polymer molecular chains to tend to undergo interlayer cleavage along the axial direction rather than disordered fracture. This mechanism ensures that while obtaining abundant micron-sized fibrils by applying high shear force, the continuity of the molecular backbone lattice can still be maintained. This fundamentally overcomes the technical contradiction between pursuing high specific surface area and sacrificing the integrity of the crystal structure in the traditional wet spinning process.

[0017] 3. The traditional back-end physical heat treatment conversion process is moved forward and integrated into the front-end chemical forming stage. By precisely controlling the solvent system components, the highly ordered Form-II crystal structure is directly locked during the solidification and solvent removal process. This eliminates the need for the high-temperature thermal relaxation process required in traditional technologies to repair crystal defects, avoids oxidation damage to the fiber surface caused by high-temperature treatment, simplifies the process, and achieves low-energy production. In addition, the unique associative acid structure formed by the binary solvent system within a specific ratio range inhibits the exponential growth of the system viscosity while maintaining a moderate plasticizing effect on the polymer molecular chains. This rheological balance ensures that the high-concentration spinning solution maintains a stable laminar flow state without melt-free fracture when passing through the spinneret and entering the high-shear flow field. This provides a stable process operation window for continuous and uniform industrial spinning production, ensuring the quality consistency between batches of the final product. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the entire process of crystal form control and preparation of high-temperature aramid pulp according to the present invention; Figure 2 This is a logic diagram of the core operation use cases and key control nodes for the production process engineer of this invention. Detailed Implementation

[0019] The present invention will be further described below with reference to specific embodiments. This description is intended to illustrate the technical concept of the present invention through specific examples and should not be regarded as a limitation on the scope of protection of the claims.

[0020] This invention relates to a crystal form control and manufacturing process for high-temperature resistant aramid pulp, comprising constructing a binary proton-competitive solvent system, preparing a metastable liquid crystal stock solution, and shear-induced transient desolventizing crystallization. To reduce the protonated hydrogen bond energy between the poly(p-phenylene terephthalamide) molecular chain and the solvent, and to avoid solvation lag during the solidification stage, a binary proton-competitive solvent system is constructed. Concentrated sulfuric acid with a mass percentage concentration of 98.0% to 99.5% is selected as the main solvent, and polyphosphoric acid with a content of 83.0% to 85.0% (based on phosphorus pentoxide, P2O5) is added as a solvation regulator. The polyphosphoric acid is dispersed in concentrated sulfuric acid at a temperature controlled at 60°C to 80°C in a mixing vessel to prepare a homogeneous mixed acid solvent. The mass percentage of polyphosphate in the mixed acid solvent is 2.5% to 5.5%. When the polyphosphate content is within this range, the Hammett acidity function of the mixed acid shifts, placing the solvent molecules in a critical state where the protonation ability of the polymer amide groups is sufficient to maintain dissolution and is easily replaced by water. If the polyphosphate content is below 2.5%, an effective proton competition environment cannot be formed, making it difficult to eliminate amorphous defects inside the fiber. If the polyphosphate content is above 5.5%, the system viscosity increases, leading to rheological instability. During the preparation of the binary solvent, to determine the protonation capability boundary of the mixed acid system and place it in a metastable window where the polymer can be dissolved and easily replaced, a homogeneous mixed solvent sample is taken and a concentration of 1×10⁻⁶ is added. -4 The protonation ratio of a mol / L p-nitroaniline indicator was determined by measuring absorbance using a spectrophotometer. The proportion of polyphosphate was dynamically adjusted based on the calculated Hammett acidity function H0, where H0 = pK. BH + -lg(C BH + / C B ), where H0 is the Hammett acidity function, pK BH + C is the negative logarithm of the indicator dissociation constant. BH + The equilibrium concentration is in protonated form and the unit is mol / L, C B The equilibrium concentration is in neutral form and the unit is mol / L. When the obtained H0 deviates from the range of -11.5 to -12.0, the amount of polyphosphate added can be increased or decreased to maintain the protonation of the amide group of the poly(p-phenylene terephthalamide) molecular chain by the solvent in a kinetically sensitive state, so as to provide thermodynamic driving force for the transient solvent removal in the subsequent shear solidification stage.

[0021] To reduce random entanglement of molecular chains and improve structural order, a metastable liquid crystal solution was prepared. Poly(p-phenylene terephthalamide) resin with an intrinsic viscosity (IV) of 5.5 dL / g to 6.5 dL / g was added to the aforementioned mixed acid solvent. The solution was shear-dissolved in a twin-screw extruder at 80°C to 90°C, with the resin residence time in the extruder controlled to be 120 min to 180 min, yielding a nematic liquid crystal spinning solution. During this stage, polyphosphate molecules utilize steric hindrance to insert between polymer molecular chains, inducing molecular chain... Exhibiting a pre-similar layered arrangement, physical entanglement is transformed into a topologically ordered structure. Addressing the need for simultaneous improvement in fibrillation and crystallinity during pulp fiber formation, a shear-induced transient desolventizing crystallization process is employed. The nematic liquid crystal spinning solution is extruded through spinnerets with an aspect ratio of 2:1 to 5:1, controlling the spinneret draw ratio to 1.5 to 3.0. The extruded material is then introduced into a dilute sulfuric acid coagulation bath at a temperature of 0°C to 10°C and a mass percentage concentration of 5.0% to 15.0%, and subjected to a high-speed shearing rotor at a rate of 2500 s⁻¹. -1 up to 4000s -1 In a shear rate field, by utilizing a low solvent layer stripping energy barrier, the polymer molecular chains complete desolvation and coalescing crystallization within 20ms to 50ms, directly forming primary aramid pulp with a Form-II crystal structure.

[0022] During shear dissolution in a twin-screw extruder, the complex viscosity η* of the nematic liquid crystal spinning solution at a frequency of 1 rad / s is monitored by an online rheological detection system. Based on the stability feedback of η*, the opening of the die head back pressure valve is adjusted to control the material residence time t. res This allows polyphosphate molecules to insert into the poly(p-phenylene terephthalamide) molecular chains using steric hindrance, inducing the development of liquid crystal texture. Combined with the velocity field generated by the high-speed shear rotor with a linear velocity of 30 m / s to 50 m / s during the shear solidification step, the polymer molecular chains instantaneously lose solvent and crystallize axially within 20 ms to 50 ms of contact with the dilute sulfuric acid solidification bath, locking in the formation of a Form-II crystal structure with an intensity ratio (RI) of 1.8 to 2.5 between the (200) and (110) crystal plane diffraction peaks. This structure, without undergoing 500... Under heat treatment conditions above ℃, the resulting high-temperature resistant aramid pulp exhibits a dense framework with axial crystallite sizes ranging from 5 nm to 15 nm, ensuring that the final high-temperature resistant aramid pulp has a thermal weight loss rate of no more than 5.0% after being placed in an air atmosphere at 400℃ for 24 hours. To guarantee the temperature resistance of the pulp, after fiber formation, the primary aramid pulp is subjected to multi-stage countercurrent washing with deionized water at 20℃ to 40℃ until the liquid conductivity is below 100 μS / cm. The resulting high-temperature resistant aramid pulp has a crystallinity greater than 80%, an axial crystallite size of 5 nm to 15 nm, and a microcrystalline diffraction intensity ratio R. I The calculation is as follows: R I =I (200) / I(110) Among them, I (200) I is the diffraction peak intensity of the (200) crystal plane. (110) The diffraction peak intensity of the (110) crystal plane; R of the obtained product I Within the range of 1.8 to 2.5, this structure ensures that the thermal weight loss of the pulp after being placed in air at 400°C for 24 hours is no more than 5%, and the specific surface area is maintained at 8 m². 2 / g to 12m 2 / g.

[0023] Example 1: In the preparation of high-performance friction materials for aerospace brake pads, aramid pulp needs to maintain the stability of its crystal framework under instantaneous high-temperature friction at 450°C. Traditional wet spinning processes use concentrated sulfuric acid (98.0% by mass) as a single solvent to prepare poly(p-phenylene terephthalamide) pulp. Due to the high binding energy of the protonated bonding layer between concentrated sulfuric acid and the polymer amide groups, a solvent desolvation lag occurs within milliseconds of fiber solidification, causing retained solvent molecules to occupy lattice positions and form structural defects. Under extreme conditions, the pulp fiber experiences framework collapse and fibrillation structure failure. To address these challenges, the present invention prepares a binary proton-competitive solvent system in the aforementioned operating environment, using pentoxide... Polyphosphoric acid with a content of 84.0% by weight was dispersed in concentrated sulfuric acid with a concentration of 98.0% by weight at a mass percentage of 4.0%. The mixture was stirred at 70°C for 45 min to form a homogeneous and transparent mixed acid solvent. Poly(p-phenylene terephthalamide) resin with an intrinsic viscosity of 6.0 dL / g was added and sheared and dissolved at 85°C using a twin-screw extruder. The polyphosphoric acid molecules were used as proton competing components to adjust the Hammett acidity function of the mixed acid solvent system, so that the combination of the solvent layer and the polymer amide groups was in a critical metastable state that was easy to peel off. At the same time, the steric hindrance effect of the polyphosphoric acid molecules pre-opened the physical entanglement points between the polymer molecular chains, inducing the original solution to exhibit a nematic liquid crystal texture and form an ordered pre-smectic layered arrangement.

[0024] When the aforementioned nematic liquid crystal spinning solution enters a dilute sulfuric acid coagulation bath at 5°C and a mass percentage concentration of 10.0% through a spinneret with a length-to-diameter ratio of 3:1, the high-speed shear rotor generates a 3500s... -1 Under the action of a shear rate field, due to the reduction of the solvation layer peeling energy barrier, the polymer molecular chains complete the transient desolventization process within 30 ms of contact with the coagulation bath; simultaneously with solvent removal, the polymer molecular chains aggregate under the induction of the shear force field and directly lock into a highly ordered Form-II crystal structure, resulting in a crystal diffraction intensity ratio R of the high-temperature resistant aramid pulp. I The value is 2.1, obtained through formula R. I =I (200) / I (110) Calculated; where RI I represents the ratio of diffraction intensities of the microcrystalline structure. (200) I is the diffraction peak intensity of the (200) crystal plane. (110) The diffraction peak intensity of the (110) crystal plane; the final high-temperature resistant aramid pulp had a thermal weight loss rate of 3.8% after being placed in an air atmosphere at 400℃ for 24 hours, and the fibril diameter was distributed in the range of 0.5μm to 1.0μm, achieving a simultaneous improvement in specific surface area and thermal stability. By adjusting the binding energy of the solvation layer by polyphosphoric acid and directional induced crystallization under a high shear field, the physical conflict between dissolution stability and desolventization kinetics was resolved, and a crystal form control method using solvent molecular dynamics regulation to replace the traditional high-temperature heat treatment process was established.

[0025] Example 2: In a test environment used to verify the performance of high-temperature resistant friction sealing materials, quantitative data were used to confirm the regulatory effect of the binary proton-competitive solvent system on the desolventization kinetics of poly(p-phenylene terephthalamide) molecular chains, and the influence of this effect on the final crystal order. The test platform included a wet spinning machine equipped with a variable frequency high-shear rotor and an online conductivity monitoring system. The data came from real-time signals collected by physical sensors on the physical experimental platform. The temperature control accuracy of the measuring instruments was 0.1℃, and the shear rate adjustment resolution was 50s. -1 The core process parameter is the mass percentage concentration of polyphosphate, i.e., C. PPA The setting follows a decision logic that balances dissolution stability and transient desolvation rate, when C PPA When the value is at the lower limit of the range, it is limited by the insufficient proton competition intensity to destroy the high-energy solvation layer. When it is at the upper limit of the range, it is limited by the rheological instability caused by the increase in the dynamic viscosity of the system. In order to simulate the objective working conditions of the concentration fluctuation of the coagulation bath in real industrial production, 0.5% concentration deviation noise was actively introduced into the coagulation bath during the experiment.

[0026] Multiple gradient comparison experiments were performed, and the ratio R of the diffraction intensity of the microcrystals in each sample group was recorded. I The experimental process, with a control group using pure concentrated sulfuric acid as the solvent system, gradually adjusted the polyphosphate ratio to the required window and boundary states while maintaining a consistent poly(p-phenylene terephthalamide) resin concentration of 6.0 dL / g intrinsic viscosity. See Table 1 for a summary of the comparative test data on the performance of high-temperature aramid pulp. The experimental group at C... PPA At 4.0%, it exhibits a significant performance mutation, with its R... I The value increased from 1.12 in the control group to 2.15, demonstrating that the molecular chain achieves a rapid transformation from the nematic liquid crystal state to the Form-II crystal form under the induction of a shear force field. Furthermore, when C... PPAWhen the deviation from the control group reached 8.0%, the dynamic viscosity of the sample group increased to over 5230 mPa⋅s due to the excessive formation of polyacid structure, resulting in abnormal fluctuations in spinneret pressure and discontinuity of fibrillated structure. This nonlinear response confirms that the 2.5% to 5.5% mass percentage range is the optimal working window for achieving transient peeling of metastable solvation layer.

[0027] Table 1: Comparative Test Data of High-Temperature Resistant Aramid Pulp

[0028] Experimental data show that the thermal stability of the sample group treated by the method of the present invention is enhanced. In particular, the thermal weight loss rate of sample group 4 at 400℃ is reduced to 3.78%, which is consistent with the high crystallinity measured by X-ray diffraction. The above results confirm the effect of the binary proton competition system on the reduction of the solvation layer stripping energy barrier. Without performing high-temperature heat treatment at the back end, the technical problem of maintaining the continuity of the crystal skeleton of aramid pulp while increasing the specific surface area is solved by regulating the micro-lattice integrity through front-end chemical components.

[0029] Example 3: This example combines Figures 1 to 2 This document describes the crystal form control and manufacturing process of a high-temperature resistant aramid pulp, such as... Figure 1 As shown, the process route begins in the raw material preparation stage by selecting concentrated sulfuric acid with a concentration of 98.0% to 99.5% and polyphosphate (PPA) with a P2O5 content of 83.0% to 85.0% as solvent components. Homogeneous mixing is carried out at a temperature of 60℃ to 80℃ and with the polyphosphate content controlled at 2.5% to 5.5%. Poly(p-phenylene terephthalamide) resin with an intrinsic viscosity of 5.5 dL / g to 6.5 dL / g is added to the system. Nematic liquid crystal spinning solution is constructed by shear dissolution using a twin-screw extruder at a temperature of 80℃ to 90℃ and a material residence time of 120 min to 180 min. This solution is then extruded through spinnerets with an aspect ratio of 2:1 to 5:1 and enters a coagulation bath containing 5% to 15% dilute sulfuric acid at a temperature controlled at 0℃ to 10℃. The solution is then subjected to shearing at a shear rate of 2500 s⁻¹. -1 up to 4000s -1 Under the shearing and solidification action, the phase transformation is completed, and finally the primary aramid pulp with the Form-II crystal structure is precipitated.

[0030] like Figure 2As shown, the production process engineer, as the core executor of the system, leads and executes four core use cases: configuring a binary proton-competitive solvent, preparing a metastable liquid crystal stock solution, performing shear solidification and forming, and multi-stage countercurrent washing. In the solvent preparation stage, the polyphosphate ratio must be controlled within the range of 2.5% to 5.5%, and the Hammett acidity function must be monitored to ensure solvent activity. In the stock solution preparation stage, homogenization is achieved using twin-screw shear dissolution technology. In the shear solidification and forming stage, a 2500s... -1 up to 4000s -1 The high-speed rotor shearing action directly locks the Form-II crystal structure, and the subsequent multi-stage countercurrent washing process includes a step to monitor the quality indicators of the final product.

[0031] Example 4: In the production process of sealing gaskets where the batch stability requirements of aramid pulp are extremely high, the intrinsic viscosity (IV value) of the raw material poly(p-phenylene terephthalamide) resin fluctuates within the range of 5.5 dL / g to 6.5 dL / g. The system needs to dynamically calibrate the chemical activity of the binary proton-competing solvent system to maintain the equilibrium of desolventizing kinetics. The experiment followed the acidity calibration procedure for the mixed acid solvent, selecting 98.5% concentrated sulfuric acid as the main solvent. Polyphosphoric acid (containing 84.5% phosphorus pentoxide) was added at a mass percentage of 4.5%. The mixture was continuously stirred at 75°C for 60 min in a mixing vessel. During this process, the system was monitored using a UV-Vis spectrophotometer. The specific measurement procedure is as follows: The indicator overlap method was used, selecting p-nitroaniline indicators (including 2-nitroaniline, 4-nitroaniline, and 2,4-dinitroaniline) with gradient basicity. The indicator concentration was prepared to be 1.0 × 10⁻⁶. -4 mol / L to 5.0×10 -4 mol / L; using a UV-Vis spectrophotometer, the wavelength range of 200nm-500nm was scanned at a constant temperature of 25℃, and the characteristic absorption peak (λ) of the neutral form of the indicator was recorded. max The absorbance at (pK) was measured. By comparing the absorbance changes in the pure solvent and the binary solvent to be tested, the protonation ratio was calculated to ensure that the measured values ​​under different acidity gradients were logically consistent and repeatable. The absorption spectrum under the indicator was obtained, and the Hammett acidity function H0 of the mixed acid system was calculated. The calculation formula is as follows: H0 = pK BH + -lg(C BH + / C B ); where H0 is the Hammett acidity function, pK BH + C is the negative logarithm of the dissociation constant of the indicator. BH + The equilibrium concentration of the protonated form of the indicator, in mol / L, C BThe equilibrium concentration of the indicator in neutral form is expressed in mol / L. The measurement results show that the H0 value is stable between -11.5 and -12.0. This value represents the metastable state window in which the solvent system is in a state where the polymer is dissolved and easily replaced in the coagulation bath. Here, metastable state means that by precisely controlling H0 within the range of -11.5 to -12.0, the degree of protonation of the polymer amide groups by the solvent system is at the critical kinetic equilibrium point between saturated dissolution and spontaneous precipitation. In this state, the binding energy barrier between the solvation layer and the molecular chain is reduced to the minimum (more than 30% lower than that of the pure concentrated sulfuric acid system), thereby providing the thermodynamic driving force for instantaneous desolventization in the subsequent shearing and shaping step 103.

[0032] To prevent thermal degradation of the resin during high-viscosity dissolution, the material residence time (tre) of the twin-screw extruder was set according to the resin's IV value. When a sample with an IV value of 6.5 dL / g entered the extruder with a screw speed of 200 rpm, the tre was adjusted by changing the opening of the end back pressure valve. res The time was set to 180 min to ensure that polyphosphate molecules had sufficient time to insert into the highly entangled molecular chains. During this time, the time was monitored using an online rheometer at a shear rate of 10 s. -1 The complex viscosity η* is determined to be 2200 mPa⋅s. When η* stabilizes at 2200 mPa⋅s, the nematic liquid crystal texture is considered to have developed and the molecular chains have completed pre-smectic layered arrangement. If t res If the coagulation time is less than 120 min, the proportion of residual amorphous defect regions in the obtained primary aramid pulp will increase due to insufficient molecular combing; when the metastable liquid crystal stock solution is placed in a dilute sulfuric acid coagulation bath at a temperature of 4℃ and a mass percentage concentration of 8.0%, the coagulation time is 4000 s. -1 The solvent removal process was performed under the action of a shear rate field. The solvent concentration gradient at the solidification interface was monitored online using a laser Raman spectrometer, and the solvent removal rate constant k was obtained. d Compared to the pure sulfuric acid system, the yield is 3.5 times higher. The resulting high-temperature resistant aramid pulp maintains a crystallinity of 85.5% and a crystal diffraction intensity ratio R without undergoing heat treatment above 500℃. I The value is 2.42, and this ratio is calculated according to the formula R. I =I (200) / I (110) Calculated, where I (200) For the (200) crystal plane diffraction intensity, I (110) The diffraction intensity of the (110) crystal plane was 3.2% after the product was continuously operated at 400℃ for 24 hours. This achieved the bearing capacity of the high crystallinity skeleton for thermal stress. By adjusting the Hammett acidity function and controlling the time scale of the extrusion rheological process, the physical boundary of the ordered stacking of molecular chains was established, and the uncertainty brought about by the fluctuation of raw materials was converted into a controlled change in chemical potential energy.

[0033] Example 5: When the phosphorus pentoxide content in the polyphosphate feedstock fluctuates within the range of 83.0% to 85.0%, the acidity standard calibration procedure for a binary proton-competitive solvent system is performed. Polyphosphate samples are collected and dispersed in concentrated sulfuric acid (98.0% concentration) at a mass percentage of 2.5% to 5.5% to obtain a homogeneous mixed acid solvent. An alkaline indicator is introduced, and the protonation ratio is measured using a spectrophotometer. The result is then calculated according to the formula H0 = pK. BH + -lg(C BH + / C B ), calculate the Hammett acidity function of the mixed acid solvent; where H0 is the Hammett acidity function, pK BH + C is the negative logarithm of the indicator dissociation constant. BH + The equilibrium concentration of the protonated form indicator is given in mol / L, C B The concentration of the neutral indicator is expressed in mol / L. When the calculated H0 deviates from the calibration value of -11.5, the mass percentage concentration of polyphosphoric acid is adjusted to return it to the range where the polymer is dissolved and easily replaced by water. This calibration procedure provides a constant thermodynamic driving force for the subsequent desolvation kinetics of the polymer molecular chains in the shear field.

[0034] Under the condition of continuous operation of high-speed shear rotor leading to the accumulation of heat generated by mechanical friction, the coagulation bath temperature baseline compensation maintenance procedure is implemented. Temperature data of dilute sulfuric acid coagulation bath is collected using temperature sensors arranged at the outlet of the heat exchange system of shear disperser. When the coagulation bath is found to deviate from the calibration range of 4℃ to 6℃ due to the heat carried by the raw liquid and the dissipation of mechanical energy, the pumping rate of the heat exchange circuit is adjusted and the refrigerant flow rate at the heat exchanger end is changed, so as to maintain the solvent stripping environment in the preset kinetic steady state during the coagulation and forming stage. The experimental data show that the ratio of the diffraction peak intensity R of the (200) crystal plane to the (110) crystal plane of the primary aramid pulp obtained under a pressure with a mechanical heat generation power of 450W is I It remains around 2.2, and this ratio is obtained through the formula R. I =I (200) / I (110) Calculated; where R I I represents the ratio of diffraction peak intensities. (200) I is the diffraction peak intensity of the (200) crystal plane. (110) The diffraction peak intensity of the (110) crystal plane; the final high-temperature resistant aramid pulp had a thermal weight loss rate of 3.2% after being placed in an air atmosphere at 400℃ for 24 hours. This state maintained the consistency of the micro-lattice order of the high-temperature resistant aramid pulp during the production process.

[0035] Example 6: In the engineering deployment of wet spinning components or under the condition of periodic replacement of spinnerets, a calibration procedure is performed to match the spinneret flow channel morphology with the rheology of the feed solution. A spinneret with a length-to-diameter ratio of 3:1 is selected, and the back pressure P at the outlet of the spinning component is monitored using a pressure sensor. ext When a nematic liquid crystal spinning solution with an intrinsic viscosity of 6.0 dL / g passes through the spinneret at a speed of 10 m / min, the speed of the metering pump in the extruder is adjusted to make P ext The pressure is stabilized between 4.5 MPa and 5.5 MPa. This pressure range establishes the laminar flow state of the feed solution inside the spinneret. Due to the steric lubrication effect of polyphosphate molecules on the poly(p-phenylene terephthalamide) molecular chains in the binary proton-competitive solvent system, the shear stress of the fluid at the orifice wall decreases. This physical state limits the occurrence of melt fracture and provides an ordered physical starting point for subsequent fibrillation splitting.

[0036] When performing standardized post-processing on the obtained primary aramid pulp, a multi-stage countercurrent washing efficiency calibration and residual monitoring procedure was implemented. The system selected deionized water as the washing medium and maintained its temperature at 30℃. An online conductivity meter was used to measure the conductivity σ of the outlet liquid from the final washing tank in real time, and a conductivity threshold σ was set. limit The value is 100 μS / cm, and when σ remains below σ for 15 min... limit It was determined that the residual concentrated sulfuric acid and polyphosphate inside the pulp fiber had been removed. At this time, the thermal weight loss of the high-temperature aramid pulp after being placed in an air atmosphere at 400℃ for 24 hours was 3.2%, and the axial microcrystal size reached 12nm. The ratio of the diffraction peak intensity of the (200) crystal plane to the (110) crystal plane of the obtained product was R. I It stabilized at 2.42.

[0037] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A process for controlling the crystal form and manufacturing a high-temperature resistant aramid pulp, characterized in that, Includes the following steps: Step 101: Concentrated sulfuric acid with a mass percentage concentration of 98.0% to 99.5% and polyphosphoric acid with a content of 83.0% to 85.0% based on phosphorus pentoxide are homogeneously mixed at a temperature of 60°C to 80°C to prepare a binary solvent, wherein the mass percentage of polyphosphoric acid in the binary solvent is 2.5% to 5.5%. Step 102: Poly(p-phenylene terephthalamide) resin with an intrinsic viscosity of 5.5 dL / g to 6.5 dL / g is added to a binary solvent, and shear dissolution is performed through a twin-screw extruder at a temperature of 80°C to 90°C to prepare a nematic liquid crystal spinning solution. The residence time of the poly(p-phenylene terephthalamide) resin in the twin-screw extruder is 120 min to 180 min. Step 103: The nematic liquid crystal spinning solution is extruded through a spinneret with an aspect ratio of 2:1 to 5:

1. The extruded material is introduced into a dilute sulfuric acid coagulation bath with a temperature of 0°C to 10°C and a mass percentage concentration of 5.0% to 15.0%. Under the action of a shear rate field of 2500s⁻¹ to 4000s⁻¹, shear coagulation is carried out to precipitate and form a primary aramid pulp with a Form-II crystal structure, so as to obtain a high-temperature resistant aramid pulp. Step 103 specifically involves causing the polymer molecular chains to instantly lose solvent and crystallize axially within 20ms to 50ms of contact with the dilute sulfuric acid coagulation bath, locking in the formation of a Form-II crystal structure with the ratio of the diffraction peak intensity RI of the (200) crystal plane to the (110) crystal plane in the range of 1.8 to 2.

5. This structure exhibits a dense skeleton with an axial microcrystal size of 5nm to 15nm when not subjected to heat treatment above 500℃. This ensures that the final high-temperature resistant aramid pulp has a thermal weight loss rate of no more than 5.0% after being placed in an air atmosphere at 400℃ for 24 hours, with a fibril diameter of 0.1μm to 1.5μm and a specific surface area maintained at 8m². 2 / g to 12m 2 / g; The resulting high-temperature resistant aramid pulp has a crystallinity greater than 80%, and its microcrystalline diffraction intensity ratio R I The calculation is as follows: R I =I (200) / I (110) Among them, I (200) I is the diffraction peak intensity of the (200) crystal plane. (110) The intensity of the diffraction peak of the (110) crystal plane.

2. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, In step 101, the homogeneous mixing time is 30 min to 60 min; the dynamic viscosity of the nematic liquid crystal spinning solution at 25°C is 1500 mPa·s to 2500 mPa·s.

3. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, In step 101, the mass percentage of polyphosphoric acid in the binary solvent is 3.5% to 4.5%, and the homogeneous mixing temperature is 65°C to 75°C.

4. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, In step 102, the length-to-diameter ratio of the twin-screw extruder is 32:1 to 48:1, and the screw assembly in the melting zone includes a kneading block assembly and a reverse thread element.

5. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, In step 103, the dilute sulfuric acid coagulation bath is maintained at 4°C to 6°C through an external circulating heat exchange system, and the spinneret stretch ratio of the nematic liquid crystal spinning solution before entering the dilute sulfuric acid coagulation bath is 1.5 to 3.

0.

6. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, The process also includes the following steps: Step 104, the primary aramid pulp is fed into a continuous washing line and subjected to multi-stage countercurrent washing with deionized water at a temperature of 20°C to 40°C until the conductivity of the washing liquid is lower than 100 μS / cm, thereby removing the residual concentrated sulfuric acid and polyphosphate in the primary aramid pulp.

7. The crystal form control and manufacturing process of a high-temperature resistant aramid pulp according to claim 1, characterized in that, In step 103, the shear rate field is generated by a high-speed shear rotor placed in a dilute sulfuric acid coagulation bath, and the linear velocity of the high-speed shear rotor is 30 m / s to 50 m / s.

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