High-strength polypropylene fiber and preparation method thereof

Abstract

The invention relates to the technical field of polypropylene fibers, in particular to a high-strength polypropylene fiber and a preparation method thereof. The problems that in the prior art, polypropylene fibers are low in strength and poor in temperature resistance are solved. The preparation method comprises the following steps: respectively prefabricating master batches through silane coupling agent modified nano titanium dioxide and carboxylated multi-walled carbon nanotubes, carrying out melt blending on the master batches, isotactic polypropylene, ultra-high molecular weight polypropylene, a liquid crystal polymer, a beta-nucleating agent and the like, and combining a double-region cooling process under high-speed spinning and two-stage temperature difference drafting to obtain the high-strength polypropylene fiber. According to the method, hierarchical dispersion and interface strengthening of nano components in a polypropylene matrix are achieved, the molecular chain orientation degree and crystallization regularity of the fiber are remarkably improved while good processability is maintained, and finally the performance of the polypropylene fiber is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of polypropylene fiber technology, specifically to a high-strength polypropylene fiber and its preparation method. Background Technology

[0002] Polypropylene (PP) fiber is widely used in textiles, carpets, and industrial fabrics due to its lightweight, low cost, excellent chemical resistance, low-energy production process, and good water repellency and warmth retention. Air-textured yarn, on the other hand, is produced by using air jets to create loops in some monofilaments, giving the fiber a fluffy and linen-like appearance. However, traditional polypropylene fibers often exhibit low breaking strength in post-processing and final applications, severely limiting their use in functional, industrial, or high-performance outdoor textiles where high strength is required. Existing technologies primarily focus on crimp and bulk, often employing a single strategy of increasing the draw ratio to improve strength, which easily leads to fiber breakage and poor air-textured stability. Therefore, there is an urgent need to overcome the strength bottleneck of polypropylene fibers through the synergistic design of multi-component materials and processes.

[0003] Polypropylene fiber's main chain contains tertiary carbon atoms, making it highly susceptible to thermal oxidative degradation when exposed to high temperatures or prolonged exposure to light and heat. This leads to yellowing, embrittlement, and a sharp decline in strength. Polypropylene's low melting point also presents challenges in dyeing, ironing, and some high-temperature finishing processes, limiting its use in certain applications (such as products requiring high-temperature sterilization and heat setting). Traditional solutions involve adding phenolic or phosphate ester heat stabilizers, but these auxiliaries often suffer from migration and precipitation issues and offer limited improvement in the fiber's long-term heat resistance, failing to fundamentally enhance the fiber's thermal structural stability.

[0004] To this end, a high-strength polypropylene fiber and its preparation method are proposed. Summary of the Invention

[0005] The purpose of this invention is to design a high-strength polypropylene fiber and its preparation method. This invention involves pre-forming masterbatches of nano-titanium dioxide modified with a silane coupling agent and carboxylated multi-walled carbon nanotubes, which are then melt-blended with isotactic polypropylene, ultra-high molecular weight polypropylene, liquid crystal polymers, and β-nucleating agents. This is combined with a dual-zone cooling process under high-speed spinning and two-stage temperature difference drawing to obtain high-strength polypropylene fibers. This method achieves hierarchical dispersion and interfacial reinforcement of nano-components in a polypropylene matrix, significantly improving the molecular chain orientation and crystallinity of the fiber while maintaining good processability, ultimately resulting in a substantial improvement in the performance of the polypropylene fiber.

[0006] To achieve the above objectives, the present invention provides the following technical solution: unless otherwise specified, the parts in the solution are all parts by weight.

[0007] This invention provides a method for preparing high-strength polypropylene fiber, comprising the following steps: Nano-titanium dioxide was modified with a silane coupling agent and then combined with isotactic polypropylene to prepare a modified titanium dioxide masterbatch. Carboxylated multi-walled carbon nanotubes were combined with isotactic polypropylene to prepare a carbon nanotube masterbatch. Polypropylene, liquid crystal polymer, modified titanium dioxide masterbatch, carbon nanotube masterbatch, β-nucleating agent and composite antioxidant were melt-mixed and extruded to obtain fiber masterbatch. The fiber masterbatch was melt-spun and subjected to dual-zone cooling treatment to obtain pretreated fiber. The pretreated fiber was stretched and heat-set, and then wound into a finished product to obtain high-strength polypropylene fiber.

[0008] Preferably, the preparation method of modified titanium dioxide masterbatch is as follows: 95-105 parts of nano titanium dioxide (average particle size of 20-50 nm) are dispersed in 900 parts of 95% ethanol / water mixed solution, 1-3 parts of silane coupling agent KH550 are added, the pH value is adjusted to 4.5 (acetic acid is added), and the mixture is stirred at 60°C for 2-4 h. After the reaction is completed, the mixture is filtered, repeatedly washed with ethanol, and dried in a vacuum oven at 80°C for 12 h to obtain modified titanium dioxide. 15 parts of modified titanium dioxide and 50 parts of isotactic polypropylene are mixed and fed into a twin-screw extruder, 1 part of polyethylene wax is added, and the mixture is mixed for 12 min. The screw speed is controlled at 300 rpm, the melt temperature is 180-200°C, and the modified titanium dioxide masterbatch is obtained after extrusion, cooling, and pelletizing.

[0009] Preferably, the preparation method of carbon nanotube masterbatch is as follows: 5-15 parts of carboxylated multi-walled carbon nanotubes (CAS: 308068-56-6) are dried, and then mixed with 90 parts of isotactic polypropylene, 4 parts of maleic anhydride-grafted polypropylene and 0.3 parts of antioxidant 1010. The mixture is fed into a high-speed twin-screw extruder for melt blending for 15 minutes. The screw speed is set to 300-600 rpm and the melt temperature is 200-220℃. The mixture is melt-extruded in a screw with an L / D ratio of 40:1 and water-cooled and pelletized to obtain carbon nanotube masterbatch.

[0010] Preferably, the preparation process of the fiber masterbatch is as follows: 90-95 parts of isotactic polypropylene (melt flow rate: 4-8 g / 10 min; melting point ≈ 165℃), 2 parts of ultra-high molecular weight polypropylene (weight average molecular weight ≥ 2 million), 2 parts of liquid crystal polymer (polyester, Tm: 190-200℃), 0.5-1 parts of modified titanium dioxide masterbatch, 0.1-0.5 parts of carbon nanotube masterbatch, 0.1-0.3 parts of β-nucleating agent (TMB-5), and 0.8 parts of composite antioxidant are mixed at high speed. The mixture was dry-mixed for 15 minutes, and then fed into a twin-screw extruder for melt blending and granulation. The extruder L / D ratio was 40:1, the screw speed was 250 rpm, and the melt blending temperature was controlled in zones between 210-230℃. After cooling, the blend was pelletized and sent to a hot air drying oven for 5 hours to reduce the moisture content to below 0.02% to obtain fiber masterbatch. The composite antioxidants included antioxidant 1010 and antioxidant 168, with a weight ratio of antioxidant 1010 to antioxidant 168 of 2:1.

[0011] Preferably, the specific process of melt spinning and dual-zone cooling is as follows: the fiber masterbatch is fed into a single-screw spinning machine, the spinning temperature is controlled in zones at 250-270℃, the melt pressure is 10MPa, a 50-mesh sintered metal multi-stage filter is used to ensure the purity of the melt, and the melt is extruded from the spinneret (orifice diameter of 0.5mm) after the flow rate is precisely controlled by a metering pump; high-speed spinning is adopted, the winding speed is controlled at 3500-4500m / min, and dual-zone cooling is implemented below the spinneret: the first zone (height 15cm) uses low-temperature (10℃) high-speed airflow cooling with a wind speed of 0.6-0.8m / s to quickly solidify the molecular chain structure; the second zone (height 90cm) uses 22℃ mild airflow with a wind speed of 0.1-0.3m / s to eliminate stress and obtain pretreated fibers.

[0012] Preferably, the specific process of stretching and heat setting is as follows: the pretreated fiber first undergoes a first-stage stretching: this is carried out on a hot roller with the temperature controlled at 75-85℃ and the stretching ratio controlled at 4-6 times; then it undergoes a second-stage stretching: this is carried out in a hot air chamber with the temperature controlled at 135-145℃ and the stretching ratio controlled at 1.8 times; finally, it is stably wound into a finished product with a tension of 0.3cN / dtex to obtain polypropylene fiber.

[0013] Another aspect of the present invention provides a high-strength polypropylene fiber, the raw materials for which include polypropylene, liquid crystal polymer, modified titanium dioxide masterbatch, carbon nanotube masterbatch, β-nucleating agent and composite antioxidant.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention combines the dotted dispersion reinforcement of modified titanium dioxide with the linear rigid support of multi-walled carbon nanotubes to form a spatial network reinforcement structure within a polypropylene matrix. This synergistic effect of dots and lines, combined with the long-chain molecular entanglement network provided by ultra-high molecular weight polypropylene, significantly improves the stress transfer efficiency between the matrix and the reinforcing phase, thereby enhancing the tensile strength of polypropylene fibers.

[0015] The high-melting-point thermotropic liquid crystal polymer incorporated in this scheme can form highly oriented rigid microfibers in situ under the high-shear flow field of melt spinning. These microfibers are arranged like a skeleton within the polypropylene matrix, and due to their extremely high heat distortion temperature, they significantly restrict the thermal movement of the polypropylene molecular chains at high temperatures. This not only improves the fiber's temperature resistance but also endows the fiber with excellent dimensional stability and mechanical retention at high temperatures.

[0016] By adding a β-nucleating agent and implementing strict temperature gradient control during the drawing process, the transformation of polypropylene from a metastable β-crystalline form to a stable, high-strength α-crystalline form was successfully induced. This crystalline transformation was completed in the secondary drawing stage with the aid of high thermal energy, resulting in a more compact and regular arrangement of molecular chains, eliminating internal residual stress, and stabilizing the high-strength properties of the fiber from a crystallographic perspective.

[0017] This invention employs a unique dual-zone gradient cooling technology. In the first zone, high-speed cold air rapidly freezes the initial orientation induced by high-speed spinning, preventing molecular chain retraction. In the second zone, a gentle airflow acts as an online annealing process, mitigating the temperature difference between the inner and outer layers of the fiber and effectively suppressing core-sheath structure differences caused by uneven cooling. This precise control of the thermal process ensures the uniformity of the pretreated fibers during subsequent high-ratio drawing, significantly reducing the breakage rate.

[0018] Surface modification of nano-titanium dioxide using KH550 and a stepwise masterbatch preparation strategy solved the industry problem of easy agglomeration of nanomaterials. The modified nanoparticles formed a strong interfacial bond with the polypropylene matrix, acting not only as a highly efficient heterogeneous nucleating agent but also avoiding stress concentration points caused by particle agglomeration. This excellent dispersion consistency ensures the continuous stability of the spinning process and imparts excellent stability to the final product. Attached Figure Description

[0019] Figure 1 The mechanical properties of Example 1 and Comparative Examples 10-13 in this invention are shown in the diagram. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] For details, please refer to [link / reference]. Figure 1 This invention provides a high-strength polypropylene fiber and its preparation method, the technical solution of which is as follows: Example 1

[0022] 100 parts of nano-titanium dioxide were dispersed in 900 parts of a 95% ethanol / water mixed solution. 2 parts of silane coupling agent KH550 were added, and the pH was adjusted to 4.5 (acetic acid was added). The mixture was stirred at 60℃ for 3 hours. After the reaction was completed, the mixture was filtered, repeatedly washed with ethanol, and dried in a vacuum oven at 80℃ for 12 hours to obtain modified titanium dioxide. 15 parts of modified titanium dioxide and 50 parts of isotactic polypropylene were mixed and fed into a twin-screw extruder. 1 part of polyethylene wax was added, and the mixture was mixed for 12 minutes. The screw speed was controlled at 300 rpm, and the melt temperature was 190℃. After extrusion, cooling, and pelletizing, modified titanium dioxide masterbatch was obtained. Ten parts of carboxylated multi-walled carbon nanotubes were dried and then mixed with 90 parts of isotactic polypropylene, 4 parts of maleic anhydride-grafted polypropylene and 0.3 parts of antioxidant 1010. The mixture was fed into a high-speed twin-screw extruder for melt blending for 15 minutes. The screw speed was set to 450 rpm and the melt temperature was 210℃. The mixture was melt-extruded in a screw with an L / D ratio of 40:1 and water-cooled to obtain carbon nanotube masterbatch. 92 parts of isotactic polypropylene, 2 parts of ultra-high molecular weight polypropylene, 2 parts of liquid crystal polymer, 0.8 parts of modified titanium dioxide masterbatch, 0.3 parts of carbon nanotube masterbatch, 0.2 parts of β-nucleating agent (TMB-5), and 0.8 parts of composite antioxidant were dry-mixed in a high-speed mixer for 15 minutes. The mixture was then fed into a twin-screw extruder for melt blending and granulation. The extruder L / D ratio was 40:1, the screw speed was 250 rpm, and the melt blending temperature was controlled at 220℃ in different zones. After cooling, the blend was pelletized and dried in a hot air drying oven for 5 hours to reduce the moisture content to below 0.02% to obtain fiber masterbatch. The fiber masterbatch is fed into a single-screw spinning machine, with the spinning temperature zoned and controlled at 260℃ and the melt pressure at 10MPa. A 50-mesh sintered metal multi-stage filter is used to ensure melt purity. After the melt flow rate is precisely controlled by a metering pump, it is extruded from the spinneret (0.5mm orifice). High-speed spinning is adopted, with the winding speed controlled at 4000m / min. Dual-zone cooling is implemented below the spinneret: the first zone (15cm high) uses low-temperature (10℃) high-speed airflow cooling at a wind speed of 0.7m / s to rapidly solidify the molecular chain structure; the second zone (90cm high) uses 22℃ mild airflow at a wind speed of 0.2m / s to relieve stress and obtain pretreated fibers. The pretreated fiber first undergoes a first-stage stretching process: this is carried out on a hot roller with the temperature controlled at 80℃ and the stretching ratio controlled at 5 times; then it undergoes a second-stage stretching process: the temperature is controlled at 140℃ and the stretching ratio is controlled at 1.8 times; finally, it is stably wound into a finished product with a tension of 0.3cN / dtex to obtain polypropylene fiber.

[0023] Examples 2-5 refer to the parameter conditions in Example 1, with specific differences as shown in Table 1.

[0024] Table 1 Parameters and conditions for Examples 1-5

[0025] Comparative Example 1 follows the same parameters and conditions as in Example 1, except that the nano-titanium dioxide is not modified.

[0026] Comparative Example 2 follows the same parameters and conditions as in Example 1, except that the modified titanium dioxide is not prepared as a masterbatch and is added directly.

[0027] Comparative Example 3 follows the same parameters and conditions as in Example 1, except that no modified titanium dioxide masterbatch is added.

[0028] Comparative Example 4 follows the same parameters and conditions as in Example 1, except that the carboxylated multi-walled carbon nanotubes are not prepared as masterbatches and are added directly.

[0029] Comparative Example 5 follows the same parameters and conditions as in Example 1, except that no carbon nanotube masterbatch is added.

[0030] Experimental Example 1: Mechanical Property Testing The fracture strength and elongation at break of Examples 1-5 and Comparative Examples 1-5 were tested according to GB / T 14337-2022, and the results are shown in Table 2.

[0031] Table 2 Mechanical properties of Examples 1-5 and Comparative Examples 1-5

[0032] Table 2 shows that in Comparative Example 1, the unmodified nano-titanium dioxide exhibits extremely poor interfacial compatibility with the non-polar polypropylene matrix. The high surface energy of the inorganic particles makes them prone to forming large particle clusters in the melt, and their interaction with the matrix is ​​merely a simple physical accumulation, lacking chemical bonding or strong physical entanglement. Under tensile loads, microcracks easily form at the interface, leading to a significant decrease in fracture strength compared to Example 1. This also restricts molecular chain slippage, resulting in a significant reduction in elongation at break. Comparative Example 2 verifies the importance of the masterbatch method. When modified titanium dioxide powder is directly added, the shear strength of the spinning extruder is insufficient to break the secondary agglomeration of the nanoparticles in a short time, resulting in uneven particle distribution within the fiber. These macroscopic agglomerates become severe stress concentration points during high-speed spinning and subsequent high-ratio drawing, causing fiber breakage or the formation of weak nodes, thus deteriorating the overall mechanical properties of the fiber. Comparing Example 1 and Comparative Example 3, it is evident that modified titanium dioxide not only acts as a filler but also refines the polypropylene grains through its heterogeneous nucleation ability. Without this component, the internal crystal size distribution of the fiber is uneven, leading to a decrease in overall rigidity. The fracture strength drops from 4.35 cN / dtex to 3.73 cN / dtex. This demonstrates the indispensability of modified titanium dioxide as a node in constructing a point-line synergistic reinforcement network. Carbon nanotubes, due to their extremely high aspect ratio, are prone to entanglement. In Comparative Example 4, carbon nanotubes were directly added, resulting in aggregates within the matrix. These aggregates not only fail to provide orientation reinforcement for the nanoskeleton but also disrupt the continuity of the macromolecular chains within the fiber, leading to a strength reduction to the lowest level among the comparative examples. Furthermore, due to severe internal defects, the fiber brittleness increases, and the elongation at break decreases sharply. Comparative Example 5 removed the carbon nanotube masterbatch. Although the fiber still maintained good fiber-forming properties, the elastic modulus and tensile strength of the fiber declined due to the loss of the constraint effect of the high aspect ratio nanoframework on matrix deformation. This further confirms that carbon nanotubes play a backbone role in the whole system and are the core component for achieving high strength performance.

[0033] Examples 6-9 refer to the parameter conditions in Example 1, with specific differences shown in Table 3.

[0034] Table 3 Parameter conditions for Examples 1 and 6-9

[0035] Comparative Example 6 follows the same parameters and conditions as in Example 1, except that ultra-high molecular weight polypropylene is not added.

[0036] Comparative Example 7 follows the same parameters and conditions as in Example 1, except that no liquid crystal polymer is added.

[0037] Comparative Example 8 follows the same parameters and conditions as in Example 1, except that no β-nucleating agent is added.

[0038] Comparative Example 9 follows the same parameters and conditions as in Example 1, except that no composite antioxidant is added.

[0039] Experimental Example 2: Mechanical Properties and Temperature Resistance Tests The mechanical properties of Examples 1, 6-9 and Comparative Examples 6-9 were tested according to the test method of Experimental Example 1; the high temperature resistance of Examples 1, 6-9 and Comparative Examples 6-9 was tested according to GB / T 2423-2016 standard, with a test temperature of 70℃ and a test time of 2h. If there was no damage, fracture or embrittlement in appearance after the test, it was considered qualified; the results are shown in Table 4.

[0040] Table 4 Mechanical properties and temperature resistance of Examples 1, 6-9 and Comparative Examples 6-9

[0041] Table 4 shows that Comparative Example 6, which removed ultra-high molecular weight polypropylene (UHMWPP), demonstrates that in polypropylene fibers, the long molecular chains of UHMWPP can interweave with ordinary isotactic polypropylene molecular chains to form physical cross-linking points, constructing a robust entangled network. Without this component, when the fiber is subjected to tensile force, the molecular chains are more prone to relative slippage rather than effective orientation, leading to a significant decrease in breaking strength. Although the heat resistance is barely acceptable, the mechanical load-bearing capacity is significantly weakened due to the lack of macromolecular constraint. Comparative Example 7 verifies the core role of liquid crystal polymer as a heat-resistant skeleton. The microfibers formed in situ by the liquid crystal polymer during spinning have extremely high heat distortion temperatures and can effectively anchor the polypropylene molecular chains. After removing the liquid crystal polymer, the fiber exhibits significant surface damage during the 70°C high-temperature test due to the violent thermal motion of the polypropylene matrix and the lack of rigid support, resulting in a failed test. Comparative Example 8, by removing the β-nucleating agent, resulted in the nascent fiber primarily consisting of the α-crystalline form common in isotactic polypropylene. Lacking the β-crystalline form as a buffer during the stretching process, the molecular chains lacked flexibility for adjustment during forced orientation, leading to microscopic breakage before reaching high stretching ratios. Comparative Example 9, by removing the composite antioxidant, showed the most severe deterioration. Polypropylene is extremely prone to thermal degradation during high-temperature spinning. Without antioxidant protection, the melt viscosity fluctuated drastically, generating numerous microscopic defects within the fiber caused by thermal oxidative degradation. This not only caused a precipitous drop in breaking strength and elongation but also resulted in significant yellowing and brittle fracture during a 70°C temperature resistance test, rendering the fiber unusable.

[0042] Examples 10-13 refer to the parameter conditions in Example 1, with specific differences shown in Table 5.

[0043] Table 5 Parameter conditions for Examples 1 and 10-13

[0044] Comparative Example 10 follows the same parameters and conditions as in Example 1, except that only high-speed airflow cooling is performed during the spinning cooling process.

[0045] Comparative Example 11 follows the same parameters and conditions as in Example 1, except that only warm airflow cooling is used during the spinning cooling process.

[0046] Comparative Example 12 follows the same parameters as in Example 1, except that the stretching and heat setting process continues until the first stage of stretching.

[0047] Comparative Example 13 follows the same parameters as in Example 1, except that the stretching and heat setting process includes a second stage of stretching.

[0048] Experiment Example 3 Mechanical Property Testing The mechanical properties of Examples 1, 10-13, and Comparative Examples 10-13 were tested according to the test method of Experimental Example 1. The results are shown in Table 6. The mechanical properties of Examples 1 and Comparative Examples 10-13 are as follows: Figure 1 As shown.

[0049] Table 6 Mechanical properties of Examples 1, 10-13 and Comparative Examples 10-13

[0050] From Table 6 and Figure 1It can be observed that Comparative Example 10 only uses high-speed airflow cooling in the first zone. Although the high-speed airflow can quickly freeze the molecular chain orientation, the lack of a gentle annealing process in the second zone results in extremely uneven cooling rates between the inner and outer layers of the fiber, generating huge residual internal stress. This leads to excessive differences in the core-sheath structure of the nascent fiber, making it prone to microcracks during subsequent stretching, manifested as decreased breaking strength and a significant reduction in breaking elongation, resulting in obvious brittleness of the fiber. Comparative Example 11 only uses gentle airflow cooling. Due to the excessively slow cooling rate, the polypropylene melt stays in the crystallization-sensitive zone for too long, causing severe relaxation of the orientation of the molecular chains under spinning tension. At the same time, slow cooling promotes the growth of large spherulites, resulting in coarse grain size inside the fiber. This microstructure prevents the fiber from achieving efficient structural transformation during stretching, reducing breaking strength to a minimum. Although the elongation increases due to low crystallinity, it completely loses the characteristics of a high-strength fiber. Comparative Example 12 only undergoes the first stage of stretching. Although the fiber has established a certain molecular orientation at this point, the movement of molecular chain segments is restricted due to the low drawing temperature. The crystalline regions have not undergone complete reorganization, and a large number of unstable amorphous regions exist internally. The lack of a second-stage high-temperature drawing prevents the complete transformation of the β-crystalline form to the high-strength α-crystalline form and fails to effectively eliminate drawing stress, resulting in a final fiber strength far lower than the standard of the example. Comparative Example 13 skips low-temperature drawing and directly performs high-temperature drawing, which is a major taboo in spinning processes. At high temperatures, the fiber is in a near-molten state with extremely high molecular chain flexibility. If the structure is not solidified by the first-stage low-temperature pre-orientation, direct high-temperature drawing will cause violent slippage of the molecular chains instead of ordered orientation, easily leading to severe thinning of the fiber bundle or even melting, severely damaging the internal structure of the fiber and generating numerous physical defects.

[0051] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-strength polypropylene fiber, characterized in that, Includes the following steps: Nano-titanium dioxide is modified with a silane coupling agent and then combined with polypropylene to prepare a modified titanium dioxide masterbatch; carboxylated multi-walled carbon nanotubes are combined with the polypropylene to prepare a carbon nanotube masterbatch; the polypropylene, liquid crystal polymer, the modified titanium dioxide masterbatch, the carbon nanotube masterbatch, a β-nucleating agent, and a composite antioxidant are melt-mixed, extruded, and granulated to obtain a fiber masterbatch; the fiber masterbatch is melt-spun and subjected to dual-zone cooling treatment to obtain pretreated fibers; the pretreated fibers are stretched and heat-set, and then wound into a finished product to obtain the high-strength polypropylene fiber.

2. The method for preparing high-strength polypropylene fiber according to claim 1, characterized in that, The modified titanium dioxide masterbatch is prepared as follows: the nano-titanium dioxide is dispersed in an ethanol / water mixed solution, silane coupling agent KH550 is added, the pH value is adjusted and the reaction is stirred. After the reaction is completed, the mixture is filtered, washed and dried to obtain modified titanium dioxide. The modified titanium dioxide and the polypropylene are fed into a twin-screw extruder, extruded, cooled and pelletized to obtain the modified titanium dioxide masterbatch.

3. The method for preparing high-strength polypropylene fiber according to claim 1, characterized in that, The carbon nanotube masterbatch is prepared by drying the carboxylated multi-walled carbon nanotubes and feeding them, along with polypropylene, maleic anhydride-grafted polypropylene, and antioxidant 1010, into a twin-screw extruder, followed by melt extrusion and water cooling pelletizing to obtain the carbon nanotube masterbatch.

4. The method for preparing high-strength polypropylene fiber according to claim 1, characterized in that, The fiber masterbatch is prepared by mixing the polypropylene, the liquid crystal polymer, the modified titanium dioxide masterbatch, the carbon nanotube masterbatch, the β-nucleating agent and the composite antioxidant to obtain a mixture; the mixture is fed into a twin-screw extruder for melting, the melting temperature is controlled at 210-230℃, and after extrusion and cooling, it is pelletized and dried to obtain the fiber masterbatch.

5. The method for preparing high-strength polypropylene fiber according to claim 1, characterized in that, The pretreated fiber is prepared by feeding the fiber masterbatch into a single-screw spinning machine for spinning. The spinning temperature is controlled at 250-270℃. After extrusion from the spinneret, high-speed spinning is used, with the winding speed controlled at 3500-4500m / min. After spinning, the fiber masterbatch is subjected to dual-zone cooling below the spinneret: the wind speed in the first zone is 0.6-0.8m / s, and the wind speed in the second zone is 0.1-0.3m / s, thus obtaining the pretreated fiber.

6. The method for preparing high-strength polypropylene fiber according to claim 1, characterized in that, The high-strength polypropylene fiber is prepared as follows: the pretreated fiber is subjected to a first-stage stretching on a hot roller, the temperature of which is controlled at 75-85℃ and the stretching ratio is controlled at 4-6 times, to obtain a first-stage stretched fiber; then the first-stage stretched fiber is subjected to a second-stage stretching at a temperature controlled at 135-145℃, to obtain a second-stage stretched fiber; the second-stage stretched fiber is then wound to obtain the high-strength polypropylene fiber.

7. A high-strength polypropylene fiber, characterized in that, The high-strength polypropylene fiber is prepared by the preparation method described in any one of claims 1-6; the raw materials for preparing the high-strength polypropylene fiber include polypropylene, liquid crystal polymer, modified titanium dioxide masterbatch, carbon nanotube masterbatch, β-nucleating agent and composite antioxidant.

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