Polypropylene nanofiber and method for preparing the same

Abstract

The application relates to the technical field of polypropylene fiber manufacturing, and discloses a preparation method of polypropylene nanofiber, which comprises the following steps: dividing a polypropylene melt into a core layer melt and a shell layer melt, making the shell layer melt flow through a reinforced heat transfer channel of an embedded static mixing unit for temperature reduction and homogenization treatment, and feeding back adjusting the strength of the temperature reduction treatment based on a bypass rheological monitoring signal; concentrically coating the homogenized shell layer melt outside the core layer melt to form a composite melt flow; and stretching the composite melt flow and feeding back controlling a cooling and solidification process based on a stretching tension signal, wherein the static mixing unit is used to solve the physical bottleneck of uneven cross section of high-viscosity melt during temperature reduction, ensure the uniformity and concentricity of the constraint shell layer, provide a reliable physical prerequisite for high-multiplying stable stretching of the core layer, and through feed-forward compensation and tension feedback, the industrial constraints of raw material fluctuation and solidification point drift are solved, and stable preparation of nanofiber is realized.

Description

Technical Field

[0001] This invention relates to a polypropylene nanofiber and its preparation method, belonging to the field of polypropylene fiber manufacturing technology. Background Technology

[0002] Currently, the industrial production of polypropylene fibers mainly relies on melt spinning. This method extrudes polypropylene melt from spinnerets with diameters of 200-500 µm (micrometers), and then solidifies it through high-speed airflow cooling and mechanical stretching. This is a mature industrial route with high throughput, low cost, and no solvents, and its products are generally limited to the micrometer diameter. Currently, the demand for nanoscale polypropylene fibers with diameters less than 1000 nm is increasing in fields such as high-efficiency filtration, special protection, and energy storage membranes. However, when using the mature melt spinning process to prepare nanofibers, there is an inherent physical constraint: the contradiction between tensile stability and stretching ratio. The essence of this constraint is that to obtain nanoscale fibers, an extremely high stretching ratio must be applied to the initial micrometer-sized melt flow. The prerequisite for high stretching ratio is that the melt itself must be in a low-viscosity, high-flow-rate state to facilitate deformation. However, the low-viscosity melt flow has extremely low melt strength under high stretching ratio, and is prone to hydrodynamic instabilities such as tensile resonance, causing the melt flow to fluctuate violently or even break before reaching the nanometer scale.

[0003] To address this issue, various attempts have been made within the conventional melt-spinning framework, but all are limited by its inherent physical principles or engineering constraints. One approach is to construct a high-viscosity outer layer to provide stability; however, the extremely low thermal conductivity of polypropylene melt in laminar flow inevitably leads to severe unevenness in the cross-sectional rheological properties due to external cooling and internal heating with any conventional external cooling method. This non-concentric constraint shell is highly susceptible to failure due to stress concentration during high-speed stretching. Another approach is to introduce a complex high-frequency closed-loop control system to actively suppress tensile resonance, but this contradicts the core advantages of melt-spinning—low cost and high throughput—and results in poor process stability in industrial environments. Besides the aforementioned attempts to improve the melt-spinning process itself, other methods have also been explored. Other technical solutions attempt to prepare specific fibers through material composites and chemical post-treatments, but they also have fundamental flaws. For example, Chinese invention patent CN112064143A discloses a polypropylene fiber and its preparation process. This solution involves blending polypropylene with raw materials such as nano-silica and spinning them. Sodium hydroxide solution is used to dissolve the silica in the fiber to form micropores and improve softness. This process path of sacrificing components and chemical dissolution is essentially chemical modification. It has completely deviated from the core industrial advantages of melt spinning, such as solvent-free and low-cost operation. It introduces a complex wet post-treatment process and fails to solve the hydrodynamic stability bottleneck faced by the melt spinning process itself under high-ratio stretching.

[0004] Therefore, the technical problem to be solved by this invention is how to provide a method for preparing polypropylene nanofibers that can decouple the aforementioned conflict between fluidity and stability within a simple and stable process framework, and avoid the problem of non-uniformity in the construction of the stable layer in principle, so as to meet the practical needs of raw material fluctuations in industrial production. Summary of the Invention

[0005] This invention provides a polypropylene nanofiber and its preparation method. Its main purpose is to solve the problems of mutual restriction between stretch flowability and stability in the existing melt spinning process, and the difficulty in overcoming the uneven cross-section of rheological properties and the interference of raw material fluctuations when constructing a stable outer layer.

[0006] To achieve the above objectives, the present invention provides a method for preparing polypropylene nanofibers, characterized in that the method comprises:

[0007] Step 101, melt splitting, dividing the polypropylene melt into core melt and shell melt;

[0008] Step 102, rheological control and feedforward compensation, maintains the core melt at a high temperature and allows the shell melt to flow through an enhanced heat transfer channel for cooling to increase its viscosity. A static mixing unit is fixedly installed inside the enhanced heat transfer channel, and the static mixing unit is used to homogenize the shell melt hydrodynamically. The method also includes: diverting a bypass monitoring stream from the shell melt's transport channel; passing the bypass monitoring stream through a rheological characteristic monitoring device to obtain a monitoring signal characterizing the actual viscosity of the shell melt; and adjusting the intensity of the cooling treatment in a feedforward manner based on the monitoring signal to compensate for the influence of raw material batch fluctuations on the shell melt viscosity.

[0009] Step 103, coaxial composite: upstream of the spinneret, the shell melt, which has been cooled and homogenized by fluid dynamics, is concentrically wrapped around the core melt to form a core-shell composite melt flow.

[0010] Step 104, stretching and solidification point anchoring: The core-shell composite melt flow is extruded from the spinneret and stretched by the stretching device. The method further includes: setting a tension sensor downstream of the stretching device to monitor the stretching tension signal of the fiber bundle; processing the stretching tension signal to obtain the average tension signal characterizing the average position of the solidification point and the tension fluctuation signal characterizing the stability of the solidification point; and based on the average tension signal and the tension fluctuation signal, respectively feeding back and adjusting the total cooling amount and airflow stability in the cooling and solidification step.

[0011] Preferably, the static mixing unit in step 102 is a flow channel insert with spiral guide vanes and no moving parts; the static mixing unit is used to perform forced radial mixing of the shell melt, flipping the cooled melt near the wall of the enhanced heat transfer channel to the center of the flow channel, while pushing the hot melt in the center towards the wall, thereby achieving cooling while obtaining a shell melt with uniform rheological properties.

[0012] Preferably, the rheological property monitoring device in step 102 is a gear pump or a capillary rheometer. When the rheological property monitoring device is a gear pump, the monitoring signal is the motor current value driving the gear pump. When the rheological property monitoring device is a capillary rheometer, the monitoring signal is the inlet pressure value of the capillary rheometer. The intensity of the cooling treatment is adjusted by adjusting the speed of the cooling fan facing the enhanced heat transfer channel, or by adjusting the power of the thermoelectric cooling module integrated in the enhanced heat transfer channel.

[0013] Preferably, the processing of the tensile tension signal in step 104 is achieved by: calculating the low-pass filter value of the tensile tension signal to obtain the average tension signal; and calculating the energy or variance of the high-pass filter value of the tensile tension signal to obtain the tension fluctuation signal; the total cooling capacity is achieved by adjusting the total speed of the cooling fan, and the airflow stability is achieved by adjusting the opening or angle of the guide grille in the cooling duct.

[0014] Preferably, in step 103, the core-shell composite melt flow has a core melt with high fluidity and an outer shell melt with high melt strength; the shell melt is used as a flexible support tube in step 104 to passively suppress the hydrodynamic instability of the core melt during stretching.

[0015] Preferably, in step 102, the core melt is maintained at a temperature of 220°C. Up to 240 After the shell melt flows through the enhanced heat transfer channel, its temperature is reduced to 190°C. Up to 215 The viscosity of the core melt is 1.5 to 5.0 times that of the core melt. In step 104, the spinneret orifice has a micron-sized aperture of 200 μm to 500 μm. The stretching device operates at a constant speed of 3000 m / min to 5000 m / min. After stretching, the average diameter of the core melt is less than 1000 nm.

[0016] Preferably, in step 104, the step of processing the tensile tension signal further includes: step 801, calculating a stability index to characterize the curing point drift. Stability Indicators The calculation method is as follows: ,in The first tensile tension signal Each sample value, This is the average tension signal. The number of sampling points and the feedback adjustment of airflow stability are based on the stability index. It is performed by comparing with a preset stability threshold.

[0017] Preferably, the enhanced heat transfer channel in step 102 includes a jacketed pipe for introducing a constant-temperature heat-conducting medium; the intensity of the cooling treatment is adjusted in a feedforward manner through a quasi-static control loop to adjust the set temperature of the constant-temperature heat-conducting medium or to adjust the flow rate of the constant-temperature heat-conducting medium.

[0018] Preferably, in step 102, the logic for adjusting the intensity of the cooling process in a feedforward manner includes: when the monitoring signal is higher than a first preset threshold, it is determined that the viscosity of the shell melt is too high, and the intensity of the cooling process is reduced; when the monitoring signal is lower than a second preset threshold, it is determined that the viscosity of the shell melt is too low, and the intensity of the cooling process is increased.

[0019] A polypropylene nanofiber has a core-shell structure, comprising a polypropylene core layer and a polypropylene shell layer covering the polypropylene core layer. The average diameter of the polypropylene core layer is less than 1000 nm, and the polypropylene shell layer concentrically covers the polypropylene core layer and has a uniform wall thickness.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. By constructing a composite melt flow with layered rheological properties, the two mutually restrictive properties of tensile stability and tensile flowability in conventional melt spinning are allocated to independent melt layers (high-viscosity shell layer and low-viscosity core layer). Based on this, before the shell melt is composited, the method uses a static mixing unit to homogenize the fluid dynamics. This step actively eliminates the non-uniformity of cross-sectional rheological properties that will inevitably occur in the shell melt due to laminar heat transfer during the cooling process by using a purely physical flow channel structure. This ensures that the subsequently formed composite melt flow has a concentric and uniformly thick high-strength constrained shell layer, providing a uniform stress environment and reliable physical premise for the ultra-high ratio stable stretching of the core melt under constraint.

[0022] 2. By introducing a bypass rheological monitoring and feedforward compensation mechanism, the problem of interference from raw material batch fluctuations on core rheological control in melt spinning process is solved. This method draws a bypass monitoring flow from the shell melt channel to obtain a monitoring signal characterizing the actual viscosity of the current raw material batch under specific cooling conditions. This signal is used to adjust the intensity of shell melt cooling treatment in a feedforward quasi-static manner, such as adjusting the heat dissipation airflow speed or thermoelectric cooling power. This approach avoids intrusive closed-loop control of the high-speed main spinning process. Instead, it uses a simple logic of upstream characterization and downstream compensation to pre-digest the differences in raw material rheological properties, ensuring that the key rheological gradient between the core and shell layers remains constant across different raw material batches, greatly improving the width of the process window and the stability of long-term operation.

[0023] 3. This invention also solves the dynamic coordination problem of ultra-high ratio stretching and cooling curing processes. While achieving stable core-shell stretching, this method sets a tension sensor downstream of the stretching device to monitor the stretching tension signal of the fiber bundle. By processing this tension signal, it is decoupled into a tension average signal characterizing the average position of the curing point and a tension fluctuation signal characterizing the stability of the curing point. These two signals are used to feed back and adjust the total cooling amount of the cooling airflow (such as the fan speed) and the airflow stability (such as the opening of the guide grid), respectively. This method translates a curing point drift problem that is difficult to observe optically and difficult to model thermodynamically into a tension characteristic problem that is easy to measure and control electrically. Through non-invasive information reuse, precise anchoring of the stretching and curing processes is achieved, avoiding fiber breakage or roller sticking caused by curing point instability in high-speed spinning. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the process flow and dual control loop of the present invention;

[0025] Figure 2 This is a comparison diagram showing the effect of the static mixing unit of the present invention on the cooling and homogenization of the shell melt;

[0026] Figure 3 This is a comparison diagram of the structure and physical properties of the core-shell structure of the present invention before and after melt flow stretching. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0028] This invention discloses a polypropylene nanofiber and its preparation method. This method can be implemented in existing melt spinning equipment, such as a standard polypropylene extruder, melt delivery pipeline, spinneret assembly, and downstream stretching guide roller system, through a specific functional combination of the melt distribution channel, temperature control system, and cooling system. The process flow mainly includes: performing melt splitting in step 101, dividing the high-temperature polypropylene melt into a core melt and a shell melt; subsequently performing rheological regulation and feedforward compensation in step 102, which maintains the core melt in a high-temperature, low-viscosity state while simultaneously cooling and thickening the shell melt using the enhanced heat transfer channel of the built-in static mixing unit. The process involves homogenization and quasi-static feedforward adjustment of the cooling intensity using bypass rheological monitoring signals. Then, step 103, coaxial compounding, combines the two rheologically different melts upstream of the spinneret into a core-shell composite melt flow. Finally, step 104, stretching and solidification point anchoring, utilizes downstream tensile tension signals to feedback-adjust the airflow parameters during high-speed stretching, thereby synergistically achieving stable nanofiber formation. In one specific embodiment, the initial states of melt splitting in step 101 and rheological control in step 102 are defined, and the polypropylene raw material used has a melt index of 25. Up to 40 Within the range (test condition is 230) , 2.16 The raw material is plasticized in an extruder and then formed at a temperature of 220°C. Up to 240 High-temperature polypropylene melt, a preferred temperature is 230°C. In the melt splitting step, this high-temperature melt is divided into two streams: a core melt and a shell melt. In step 102, the core melt is transported to an insulating flow channel to maintain its original high-temperature state, i.e., 220°C. Up to 240 This maintains its low viscosity and high fluidity; simultaneously, the shell melt is introduced into an enhanced heat transfer channel for cooling, with the goal of reducing the shell melt temperature to 190°C. Up to 215 A preferred temperature is 210°C. At this temperature, the viscosity of the shell melt increases, reaching 1.5 to 5.0 times that of the core melt, thus achieving high melt strength.

[0029] To address the engineering challenge of severely uneven cross-sectional rheological properties, resulting from external cooling and internal heating due to laminar heat transfer characteristics, when high-viscosity polypropylene melt is externally cooled in conventional pipelines, this invention incorporates a static mixing unit fixedly installed inside the enhanced heat transfer channel in step 102. This static mixing unit is a flow channel insert without moving parts, such as a Kenics-type or Sulzer-type mixing element with helical guide vanes. When the high-viscosity shell melt flows through this static mixing unit, the guide vanes inside forcefully mix the melt radially. The mechanism involves continuously flipping the cooled, high-viscosity melt near the enhanced heat transfer channel wall into the flow channel. At the center, the hot melt with higher temperature and lower viscosity at the center of the flow channel is pushed towards the pipe wall; this fluid dynamic homogenization process improves heat transfer efficiency and also obtains a shell melt with highly uniform rheological properties in cross-section, providing a physical prerequisite for the subsequent formation of a concentric core-shell structure with uniform wall thickness; to solve the problem that the static cooling treatment in step 102 cannot adapt to the actual working conditions of upstream raw material batch fluctuations (i.e., melt index fluctuations), the method of the present invention also includes a feedforward compensation step; this step diverts a bypass monitoring flow from the shell melt conveying channel, i.e., before or in the enhanced heat transfer channel; and the bypass monitoring flow passes through a rheological property monitoring device, which can be selected from the art. A mature gear pump or capillary rheometer is used. When a gear pump is selected, the monitoring signal is the motor current required to drive the pump at a constant speed. When a capillary rheometer is selected, the monitoring signal is the inlet pressure of the rheometer at a constant extrusion rate. Both signals can indirectly characterize the actual viscosity of the current batch of shell melt under specific cooling conditions. This monitoring signal is input to a quasi-static controller (such as a PID or PLC), which adjusts the intensity of the cooling process feedforward based on the monitoring signal. When the monitoring signal is higher than a first preset threshold, it is determined that the actual viscosity of the melt is too high (possibly due to low MI batch raw materials), and the intensity of the cooling process is reduced. When the monitoring signal is lower than a certain threshold, the intensity of the cooling process is reduced. When the preset threshold is reached, if the actual viscosity of the melt is determined to be too low (possibly due to high MI batch raw materials), the intensity of the cooling process is increased. The method of adjusting the cooling intensity can be achieved by adjusting the speed of the cooling fan facing the enhanced heat transfer channel or adjusting the power of the thermoelectric cooling module (Peltier module) integrated in the enhanced heat transfer channel. In another preferred embodiment, the enhanced heat transfer channel includes a jacketed pipe for introducing a constant temperature heat transfer medium (such as heat transfer oil). In this case, the feedforward adjustment is achieved by adjusting the set temperature of the constant temperature heat transfer medium or adjusting its flow rate through a quasi-static control loop. This method helps to achieve a smooth response and high control accuracy.

[0030] Installing a static mixing unit within the enhanced heat transfer channel in step 102 increases the flow resistance of the shell melt. To match the output flow rate of the upstream melt delivery system, such as an extruder or gear pump, with this flow resistance, a feasible commissioning procedure includes: at the initial stage of system startup, measuring the inlet pressure required for the shell melt to reach the target cooling viscosity after flowing through the enhanced heat transfer channel containing the static mixing unit; this pressure value is used as a control parameter for the upstream melt delivery system, for example, by adjusting the drive motor speed of the gear pump or the screw speed of the extruder to match its outlet pressure with the inlet pressure of the channel, thereby overcoming the flow resistance. Simultaneously, a constant mass flow rate conforming to the core-shell ratio is ensured for the shell melt. In step 103, coaxial compositing, the shell melt obtained in step 102, after cooling and hydrodynamic homogenization, and with uniform viscosity, is combined with the core melt, which maintains a high temperature and low viscosity, upstream of the spinneret through a coaxial compositing channel structure known in the art. The shell melt is concentrically encased outside the core melt, forming a core-shell composite melt flow with a clear rheological gradient. In step 104, stretching and solidification point anchoring, the core-shell composite melt flow is extruded from the spinneret, which has a micron-sized pore size. to The extruded composite melt stream is then subjected to high-speed stretching by a downstream stretching device (such as the first guide roller). to The process operates at a constant speed. During this stretching process, the outer shell melt, due to its high viscosity and high melt strength, acts as a flexible support tube, passively suppressing hydrodynamic instabilities such as tensile resonance in the inner core melt during high-speed stretching. Meanwhile, the inner core melt, due to its high fluidity, can withstand ultra-high stretching ratios under stable constraint, ultimately being stretched to a nanometer-scale diameter, i.e., an average diameter less than [missing information]. .

[0031] To address the secondary technical challenge of filament breakage or roller sticking caused by curing point displacement due to stretching heating or cooling airflow disturbance during ultra-high ratio stretching, the method of this invention further includes a curing point anchoring step based on tensile tension characteristics. This step involves installing a tension sensor downstream of the stretching device to monitor the tensile tension signal of the fiber bundle in real time. After acquiring the signal, a signal processor processes the tensile tension signal to decouple it into two control signals that characterize the state of the curing point. It then calculates the low-pass filter value of the tensile tension signal to obtain the average tension signal characterizing the average position of the curing point. Furthermore, the energy or variance of the high-pass filter value of the tensile tension signal is calculated to obtain the tension fluctuation signal characterizing the stability of the curing point. This provides a concrete numerical example of calculating this tension fluctuation signal, namely, calculating a stability index. The calculation method is as follows: ,in The first tensile tension signal Each sample value, This is the average tension signal. This is the number of sampling points; The value characterizes the severity of tension fluctuations, i.e., the stability of the curing point; finally, the system is based on the average tension signal. With tension fluctuation signal (or ), respectively providing feedback to adjust two different parameters in the cooling and curing steps: based on The deviation between the (representative position) and the set value is used to adjust the total cooling capacity, and its actuator is to adjust the total speed of the cooling fan; simultaneously, based on (or The deviation between the (representing stability) and the set threshold is used to adjust the airflow smoothness. The actuator adjusts the opening or angle of the guide grille within the cooling duct, thereby dynamically anchoring the solidification point at a preset stable position. This invention also provides a polypropylene nanofiber, prepared by any of the aforementioned methods. This fiber has a core-shell structure in its microstructure, comprising a polypropylene core layer and a polypropylene shell layer covering the polypropylene core layer; wherein the average diameter of the polypropylene core layer is less than... Furthermore, due to the hydrodynamic homogenization treatment of the shell melt during the preparation process (step 102), the polypropylene shell concentrically covers the outside of the polypropylene core layer and has a uniform wall thickness.

[0032] Example 1: This example illustrates a specific application of the technical solution in a particular industrial operation scenario. This scenario involves the continuous production of polypropylene nanofibers, where the production line needs to handle raw material batch switching and dynamic process disturbances. On a continuously operating melt spinning production line, polypropylene nanofibers are used to prepare high-efficiency filter materials. This production line is constructed according to the method of this invention, and the constant speed of the stretching device is set to... At a certain moment, the upstream feeding system automatically switches to a new batch of polypropylene raw material, the melt flow index of which is [missing information]. Higher than the previous batch This means the melt viscosity of the new raw material is low. When this high-MI raw material enters the rheological control and feedforward compensation system in step 102, the bypass monitoring flow is diverted to flow through the rheological characteristic monitoring device. The gear pump in this device, operating at a constant speed, experiences a decrease in its drive motor current. This monitoring signal is interpreted by the system as indicating that the raw material viscosity is below a preset benchmark. In response, the feedforward compensation logic is triggered, and the quasi-static controller adjusts the cooling intensity of the enhanced heat transfer channel. In this specific application, this is achieved by adjusting the set temperature of the constant-temperature heat-conducting medium in the jacketed pipe from... Downgraded to This is achieved by [the process described]. The high-MI, low-viscosity shell melt, when flowing through this enhanced heat transfer channel, is subjected to a set temperature. The cooling process is performed, but the built-in static mixing unit, through its forced radial mixing action, ensures that the melt is uniformly cooled to the new target viscosity across the cross section, avoiding localized overcooling or cross-sectional unevenness that may occur due to increased cooling intensity. Therefore, the shell melt entering step 103 for coaxial compounding maintains its viscosity and melt strength at the high levels required by the process. The low-viscosity core-layer melt is combined, and the rheological differences of the core-shell structure composite melt flow are maintained.

[0033] In step 104, during stretching and solidification point anchoring, the composite melt flow is... The high-speed stretching device utilizes a high-viscosity outer shell layer that acts as a flexible support tube, suppressing tensile resonance that is prone to occur due to the low viscosity of the core layer. During this process, airflow disturbance in the workshop reduces the airflow stability in the cooling and curing zone, causing high-frequency spatial drift at the curing points. Downstream tension sensors monitor the tensile tension signal of the fiber bundles in real time. When fluctuations occur, the signal processor calculates the tension fluctuation signal. The preset stability threshold was exceeded; the solidification point anchoring system then intervened, based on this... The signal deviation prompted feedback to adjust the angle of the airflow guide grilles within the cooling duct, restoring the smoothness of the cooling airflow; simultaneously, the system continuously monitored the average tension signal. The average position of the solidification point is compared with the set value, and the total speed of the cooling fan is adjusted accordingly to keep the average position of the solidification point anchored in the stable range before the stretching endpoint. In this scenario of multiple superimposed working conditions, the rheological control and feedforward compensation mechanism in step 102 eliminates the viscosity interference caused by raw material batch switching in advance. Its built-in static mixing unit ensures the spatial uniformity of the shell constraint while cooling down. The two work together to ensure that the core-shell composite melt flow entering the stretching process always meets the prerequisites of rheological difference and structural concentricity. On this basis, the solidification point anchoring mechanism in step 104 further eliminates the dynamic solidification disturbance in high-speed stretching in real time. This method, through upstream material property feedforward control, midstream structural homogenization guarantee, and downstream dynamic process feedback adjustment, enables the production line to continue to operate stably when facing raw material fluctuations and environmental interference, without fiber breakage or roller sticking, and continuously producing core layer average diameters smaller than 100 mm. Polypropylene nanofibers with uniform structure.

[0034] Example 2: This example verifies, through a comparative experiment, the combination of key technical features in the method of the present invention, particularly the static mixing unit and feedforward compensation in step 102, and the solidification point anchoring feedback in step 104, for achieving high-speed and stable spinning of polypropylene nanofibers. The experiment used the same melt spinning industrial test platform, which can enable or disable specific control modules as needed. The base raw material used in the experiment had a melt index (MI) of 30.1. ( , 2.16 The polypropylene (PP-1) has a melt flow index of 38.2. Polypropylene (PP-2); the spinneret orifice diameter of all test groups was [missing information]. The core melt temperature remains constant. The speed of the tensioning device is set to .

[0035] For comparison, three experimental groups were set up: Control group 1 used conventional homogeneous melt spinning process, in which PP-1 homogeneous melt was... Direct spinning is performed without splitting or special control; control group 2 uses a core-shell structure, where the shell melt flows through a simple jacketed pipe without a static mixing unit for cooling to the target temperature. Furthermore, the feedforward compensation in step 102 and the solidification point anchoring feedback loop in step 104 are not enabled; the experimental group adopts the complete method of the present invention, in which the enhanced heat transfer channel in step 102 has a built-in static mixing unit, and simultaneously enables bypass rheological monitoring and feedforward compensation, as well as stretching and solidification point anchoring feedback loop in step 104.

[0036] The experiment was conducted under two conditions: Condition 1, the stable condition, in which all test groups used PP-1 raw material and ran continuously for 30 minutes without external disturbance; Condition 2, the disturbed condition, in which the material supply was switched to PP-2 raw material, and lateral airflow disturbances were introduced into the cooling and curing zone at 10-second intervals for 1 second each time, and this was run continuously for 30 minutes; during the experiment, the tensile tension signal was monitored in real time and its fluctuation value was calculated. The number of fiber breaks within 30 minutes was recorded, and samples of the formed fibers were taken after the experiment. The average diameter and distribution of the core layer were measured and statistically analyzed by scanning electron microscopy (SEM).

[0037] Table 1: Comparison of operational stability and fiber formation results for different test groups under two working conditions.

[0038]

[0039] Referring to the experimental data in Table 1, control group 1 exhibited high tension fluctuations even under operating condition 1. The frequent breakage (9 times) indicates that conventional homogeneous melt spinning cannot withstand the effects. High-speed stretching, under the disturbance of condition two, cannot be spun; in condition one, the stability of control group 2 is improved after the introduction of the core-shell structure, but the tension fluctuation value ( The concentration remains high, and it is accompanied by fiber breakage (twice), with a wide product diameter distribution. The result was a 310nm diameter, attributed to the lack of a static mixing unit, leading to uneven cooling of the shell melt and resulting in non-concentric constraints. Under the second perturbation condition, the control group 2, lacking feedforward compensation and feedback adjustment capabilities, experienced a dramatic increase in tension fluctuation to 15.8 and 11 fiber breakages, resulting in process failure. In contrast, the experimental group, under the stable condition of the first condition, achieved a tension fluctuation of only 1.1 and zero fiber breakages due to the static mixing unit ensuring shell uniformity, and the fiber core diameter was uniform (815nm). 65nm); After entering the second disturbance condition, the feedforward compensation mechanism of the experimental group responded to the viscosity change of the PP-2 raw material and adjusted the cooling intensity. At the same time, the curing point anchoring feedback loop suppressed the tension fluctuation caused by airflow disturbance, so that the tension fluctuation value of the system only increased to 1.7, the number of fiber breakages remained at 0, and uniform nanofibers (830) were continuously and stably prepared. (72nm); The results of this comparative experiment show that the core-shell structure alone is insufficient to cope with cross-sectional inhomogeneity and dynamic disturbances in industrial production; The technical combination of static mixing unit, rheological feedforward compensation and tension feedback control loop proposed in this invention ensures the spatial uniformity of shell constraint and the temporal consistency of the process. This combination provides effective process conditions for achieving high-speed, continuous and stable preparation of uniform polypropylene nanofibers.

[0040] To further analyze the specific contribution of different combinations of technical features in the method of the present invention to the process stability, the following comparative examples are added based on the test platform and operating conditions of Example 2.

[0041] Example 3: This example uses the same experimental platform, basic raw material (PP-1), disturbing raw material (PP-2), basic process parameters, and operating conditions one (stable) and two (disturbed) as Example 2. Comparative Example 3 and Comparative Example 4 are added. Comparative Example 3 uses a core-shell structure and enables the enhanced heat transfer channel (built-in static mixing unit) in step 102 to ensure shell homogenization. However, the bypass rheological monitoring and feedforward compensation in step 102 are disabled, as are the stretching forming and curing point anchoring feedback loop in step 104. The temperature of the jacketed pipe for shell cooling is kept constant at [temperature value missing]. This is the set value for PP-1 raw material; Comparative Example 4, based on Comparative Example 3, enabled the enhanced heat transfer channel (built-in static mixing unit) and bypass rheological monitoring and feedforward compensation in step 102, but continued to disable the stretching molding and curing point anchoring feedback loop in step 104. The test groups and control group 2 in Comparative Example 3, Comparative Example 4 and Example 2 were tested under operating conditions 1 and 2, and the results are summarized in Table 2.

[0042] Table 2: Comparison of Stability of Different Combinations of Technical Features under Disturbance Conditions

[0043]

[0044] Referring to the data in Table 2, in Comparative Example 3 under operating condition 1 (stable), because the static mixing unit solves the problem of shell inhomogeneity, its stability ( ) and fiber uniformity ( 70nm) is superior to control group 2 ( , 310nm), which demonstrates the fundamental role of hydrodynamic homogenization in step 102 in constructing a high-quality core-shell structure; however, upon entering condition two (perturbation), the stability of Comparative Example 3 deteriorates sharply ( Comparative Example 4 (10 fiber breaks) performed similarly to Control Group 2, indicating that the static homogenization structure itself could not cope with fluctuations in the raw material (PP-2) and cooling airflow disturbances. Comparative Example 4 performed well in Condition 1, but in Condition 2, due to the activation of feedforward compensation, the system compensated for changes in the viscosity of the PP-2 raw material, thus improving its stability. ) is better than comparative example 3 ( However, the process still suffered from tension fluctuations due to the inability to cope with cooling airflow disturbances, resulting in multiple filament breaks (7 times), indicating that the process remained unstable. The experimental results of Comparative Examples 3 and 4, compared with the experimental results in Example 2, show that there is a synergistic effect among the three technical features of the present invention: static mixing unit, bypass rheological feedforward compensation, and tensile tension solidification point anchoring. The absence of any one of these elements makes it impossible to achieve high-speed melt spinning. In production, it is impossible to achieve continuous and stable preparation of polypropylene nanofibers by simultaneously overcoming the triple constraints of uneven cross-section, raw material fluctuations, and dynamic disturbances.

[0045] Example 4: This example combines Figures 1 to 3 This section describes a type of polypropylene nanofiber and its preparation method, such as... Figure 1 As shown, after the polypropylene melt raw material is input, step 101, melt splitting, is performed to separate it into a core melt and a shell melt. The core melt enters a core melt holding step to maintain a high temperature and low viscosity state. The shell melt enters step 102 for rheological control, undergoing cooling, viscosity enhancement, and static mixing unit homogenization treatment. Simultaneously, a bypass monitoring stream is diverted from the shell melt to a bypass rheological monitoring device to obtain the actual viscosity signal of the shell melt. This signal is used for feedforward compensation to adjust the cooling intensity in step 102. The processed core melt... In step 103, the core and shell melts are coaxially combined to form a core-shell composite melt flow. This composite melt flow enters step 104 for stretching and solidification point anchoring. Through high-speed stretching and cooling solidification, polypropylene nanofibers with a core diameter of less than 1000 nm are obtained. In step 104, the tensile tension of the fiber bundle is monitored by a tension sensor. The tensile tension signal is decoupled into a tension average value and a tension fluctuation signal after tension signal processing. This signal is used to provide feedback to adjust the total cooling amount and airflow stability in step 104.

[0046] like Figure 2 As shown, the vertical axis of this graph represents temperature, with units of... The horizontal axis represents the channel location in cm. In a channel without a static mixing unit, the outer wall temperature (shown by the dashed line) drops sharply, while the center temperature (shown by the dotted line) drops slowly, resulting in a huge temperature difference across the cross section. In contrast, in a channel with a static mixing unit, the average temperature (shown by the solid line) achieves uniform and effective cooling.

[0047] like Figure 3As shown, before stretching, at a spinneret diameter of 200-500μm, the interior consists of a polypropylene core layer with high temperature (220-240℃), low viscosity, and high fluidity. The exterior is coated with a polypropylene shell layer with high viscosity and high melt strength, cooled to 190-215℃, and with a viscosity 1.5-5.0 times that of the core layer. After the composite melt flow is stretched at a high speed of 3000-5000m / min, it forms the finished fiber. The core layer diameter is less than 1000nm, and the shell layer achieves concentric coating and uniform wall thickness.

[0048] Example 5: This example discloses a specific procedure for offline calibration of the control parameters for rheological control and feedforward compensation in step 102 and for stretching and curing point anchoring in step 104 before applying the method of the present invention to a melt spinning production line to produce polypropylene nanofibers of specific specifications. This procedure is used to determine the system's working reference and threshold; to determine the processing method for the tension signal in step 104; and to set the signal processor to acquire the stretching tension signal at a frequency of 100Hz. For tension mean signal The low-pass filter is obtained using the Exponential Moving Average (EMA) algorithm, and its calculation formula is as follows: ,in At the current sampling time, This represents the tension signal value acquired at the current moment. This is the average tension signal from the previous moment. This is a smoothing coefficient, set to 0.05 in this calibration; for tension fluctuation signals... The acquisition is achieved by using a method containing Number of sampling points and set Using a sliding window, calculate the stability index. The calculation method is as follows ,in, For the first in this sliding window Each tension signal sample value This is the tension mean signal obtained by the aforementioned EMA algorithm at the corresponding time. This represents the total number of sampling points within the window.

[0049] For the baseline calibration of the curing point anchoring (step 104), the operator uses the PP-1 standard raw material from Example 2, starts spinning in an open-loop state with all automatic feedback disabled, and stabilizes the stretching speed at... The operator manually adjusts the total speed of the cooling fan and the opening of the guide grille while observing the spinning process until the stretching process is stable, the solidification point is at the appropriate position, and there is no adhesion or fuzz on the fiber surface. Under this optimal stable condition, the system runs continuously for 5 minutes, and the result calculated by the above algorithm is recorded. and Value; within those 5 minutes The average value over time was measured to be 25.2 cN. This value was set as the target setpoint for the average tension signal in step 104, within 5 minutes. The average value was measured to be 1.15. This value was multiplied by a safety factor of 1.5, resulting in 1.73. This result was set as the control threshold for the tension fluctuation signal used to determine the stability of the curing point. Finally, the reference calibration for rheological regulation (step 102) was performed. While maintaining the above optimal stable operation, the system activated the bypass rheological monitoring device of step 102, which in this example is a gear pump, and recorded the motor current value of the corresponding bypass monitoring flow, which was measured to be 1.85A. This current value was set as the reference viscosity set point for the feedforward compensation system. The control logic of this step was set as follows: if the current value monitored in subsequent production deviates from this reference point by more than a preset value... When the current is within the tolerance range, i.e., higher than the first preset threshold of 1.94A or lower than the second preset threshold of 1.76A, feedforward adjustment is initiated, such as adjusting the jacket pipeline temperature, so that the monitored current value returns to the vicinity of the reference point. By executing the above offline calibration procedure, the core algorithm parameters, feedback control setpoints, and feedforward control references required by the method of the present invention are all determined. The system controller completes parameter loading, and the production line has the ability to perform closed-loop, adaptive, and stable production as in Examples 1 to 3.

[0050] 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.

[0051] 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 method for preparing polypropylene nanofibers, characterized in that, The method includes: Step 101, melt splitting, dividing the polypropylene melt into core melt and shell melt; Step 102, rheological control and feedforward compensation, maintains the core melt at a high temperature and allows the shell melt to flow through an enhanced heat transfer channel for cooling to increase its viscosity. A static mixing unit is fixedly installed inside the enhanced heat transfer channel, and the static mixing unit is used to homogenize the shell melt hydrodynamically. The method also includes: diverting a bypass monitoring stream from the shell melt's transport channel; passing the bypass monitoring stream through a rheological characteristic monitoring device to obtain a monitoring signal characterizing the actual viscosity of the shell melt; and adjusting the intensity of the cooling treatment in a feedforward manner based on the monitoring signal to compensate for the influence of raw material batch fluctuations on the shell melt viscosity. Step 103, coaxial composite: upstream of the spinneret, the shell melt, which has been cooled and homogenized by fluid dynamics, is concentrically wrapped around the core melt to form a core-shell composite melt flow. Step 104, stretching and solidification point anchoring: The core-shell composite melt flow is extruded from the spinneret and stretched by the stretching device. The method further includes: setting a tension sensor downstream of the stretching device to monitor the stretching tension signal of the fiber bundle; processing the stretching tension signal to obtain the average tension signal characterizing the average position of the solidification point and the tension fluctuation signal characterizing the stability of the solidification point; and based on the average tension signal and the tension fluctuation signal, respectively feeding back and adjusting the total cooling amount and airflow stability in the cooling and solidification step.

2. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, In step 102, the static mixing unit is a flow channel insert with spiral guide vanes and no moving parts. The static mixing unit is used to perform forced radial mixing of the shell melt, flipping the cooled melt near the wall of the enhanced heat transfer channel to the center of the flow channel, while pushing the hot melt in the center towards the wall, thereby achieving cooling and obtaining a shell melt with uniform rheological properties.

3. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, In step 102, the rheological property monitoring device is either a gear pump or a capillary rheometer. When the rheological property monitoring device is a gear pump, the monitoring signal is the motor current value driving the gear pump. When the rheological property monitoring device is a capillary rheometer, the monitoring signal is the inlet pressure value of the capillary rheometer. The intensity of the cooling treatment is adjusted by adjusting the speed of the cooling fan facing the enhanced heat transfer channel, or by adjusting the power of the thermoelectric cooling module integrated in the enhanced heat transfer channel.

4. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, The tensile tension signal is processed in step 104 by the following method: calculating the low-pass filter value of the tensile tension signal to obtain the average tension signal; and calculating the energy or variance of the high-pass filter value of the tensile tension signal to obtain the tension fluctuation signal. Total cooling capacity is achieved by adjusting the total speed of the cooling fan, while airflow stability is achieved by adjusting the opening or angle of the guide grilles in the cooling duct.

5. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, In step 103, the core-shell composite melt flow has a core melt with high fluidity and an outer shell melt with high melt strength. In step 104, the shell melt is used as a flexible support tube to passively suppress the hydrodynamic instability of the core melt during the stretching process.

6. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, In step 102, the core melt is maintained at a temperature of 220°C. Up to 240 After the shell melt flows through the enhanced heat transfer channel, its temperature is reduced to 190°C. Up to 215 The viscosity of the core melt is 1.5 to 5.0 times that of the core melt. In step 104, the spinneret orifice has a micron-sized aperture of 200 μm to 500 μm. The stretching device operates at a constant speed of 3000 m / min to 5000 m / min. After stretching, the average diameter of the core melt is less than 1000 nm.

7. The method for preparing polypropylene nanofibers according to claim 4, characterized in that, In step 104, the step of processing the tensile tension signal further includes: step 801, calculating a stability index to characterize the curing point drift. Stability Indicators The calculation method is as follows: ,in The first tensile tension signal Each sample value, This is the average tension signal. The number of sampling points and the feedback adjustment of airflow stability are based on the stability index. It is performed by comparing with a preset stability threshold.

8. The method for preparing polypropylene nanofibers according to claim 1, characterized in that, The enhanced heat transfer channel in step 102 includes a jacketed pipe for introducing a constant-temperature heat-conducting medium; the intensity of the cooling treatment is adjusted by a quasi-static control loop to adjust the set temperature of the constant-temperature heat-conducting medium or to adjust the flow rate of the constant-temperature heat-conducting medium.

9. The method for preparing polypropylene nanofibers according to claim 3, characterized in that, In step 102, the logic for adjusting the intensity of the cooling process in a feedforward manner includes: when the monitoring signal is higher than the first preset threshold, it is determined that the viscosity of the shell melt is too high, and the intensity of the cooling process is reduced; when the monitoring signal is lower than the second preset threshold, it is determined that the viscosity of the shell melt is too low, and the intensity of the cooling process is increased.

10. A polypropylene nanofiber, prepared by the method for preparing polypropylene nanofiber according to claim 1, characterized in that, Polypropylene nanofibers have a core-shell structure, which includes a polypropylene core layer and a polypropylene shell layer covering the outside of the polypropylene core layer. The average diameter of the polypropylene core layer is less than 1000 nm, and the polypropylene shell layer concentrically covers the outside of the polypropylene core layer and has a uniform wall thickness.

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