A bamboo powder reinforced polypropylene composite fiber for concrete crack resistance and its preparation method

Abstract

This invention relates to the field of concrete reinforcing fiber technology, and particularly to a bamboo powder-reinforced polypropylene composite fiber for concrete crack resistance and its preparation method. Through an integrated process of surface chemical modification (introducing bifunctional groups), melt grafting (forming chemical anchoring), and multi-stage stretching (forming a rough structure in situ), the strong interfacial bond between bamboo powder and polypropylene and the simultaneous construction of fiber surface functions are fundamentally achieved. This method not only solves the global problem of bamboo powder easily detaching from the polymer matrix, but also ensures good rheological properties of the melt due to the minimal damage to the polypropylene backbone caused by chemical grafting. This makes it possible to subsequently prepare fine denier fibers (with diameters as low as 30 μm) through high-ratio multi-stage stretching, overcoming the technical bottleneck of traditional modification methods that struggle to balance strong interfacial bonding and high spinnability.

Description

Technical Field

[0001] This invention relates to the field of concrete reinforcing fiber technology, and in particular to a bamboo powder reinforced polypropylene composite fiber for concrete crack resistance and its preparation method. Background Technology

[0002] Fiber-reinforced concrete, as an important engineering composite material, improves the early crack resistance, toughness, and durability of concrete by incorporating fibers into a cement matrix, and has been widely used in the field of civil engineering. Among many fiber materials, polypropylene fiber has become one of the most widely used synthetic fibers on the market due to its advantages such as low cost, resistance to acid and alkali corrosion, and good processing performance.

[0003] However, traditional polypropylene fibers still have the following prominent problems in concrete applications:

[0004] Poor interfacial adhesion: Polypropylene is a non-polar polymer with low surface energy, resulting in weak affinity with highly polar cement hydration products. This leads to low bond strength between the fiber and the concrete matrix, making it difficult to fully utilize the fiber's reinforcing and toughening effects. Especially in the early shrinkage stage, the fiber is prone to detaching from the matrix, making it difficult to effectively inhibit the generation and propagation of cracks.

[0005] Poor dispersion uniformity: The main production process of polypropylene fiber is currently the membrane cracking method. The fiber bundles produced by this method are prone to agglomeration during concrete mixing due to factors such as static electricity and uneven wetting. This results in uneven distribution of fibers in the concrete, forming local weak areas and affecting the overall performance and reliability of the concrete.

[0006] Limitations of existing modification methods: To improve interfacial properties, existing studies have employed methods such as surface coating treatment or chemical grafting modification (e.g., maleic anhydride grafting). However, these methods still have the following shortcomings:

[0007] (1) The surface coating is mostly physically attached, and it is easy to wear or fall off during long-term service;

[0008] (2) Grafting of polar monomers such as maleic anhydride can improve fiber polarity, but it can easily cause degradation of polypropylene molecular chains or excessive cross-linking, resulting in decreased melt strength and poor spinnability.

[0009] (3) The grafted layer and the fiber matrix are mostly bonded by physical adsorption or weak hydrogen bonds. During subsequent high stretching, the interface is easily peeled off, and the structural integrity and surface function of the fiber cannot be stably maintained.

[0010] In addition, recent studies have attempted to introduce natural fibers (such as bamboo powder) into polypropylene to improve its hydrophilicity and environmental friendliness. However, due to the poor interfacial compatibility between bamboo powder and polypropylene, bamboo powder tends to aggregate and disperse unevenly in the fibers, and is prone to detaching from the matrix during stretching, resulting in decreased fiber strength, insufficient surface roughness, and difficulty in forming effective mechanical interlocking in concrete.

[0011] Therefore, there is an urgent need to develop a bamboo powder modified polypropylene fiber and its preparation method that can achieve strong interfacial bonding between bamboo powder and polypropylene, maintain good spinnability, and be uniformly dispersed in concrete with significantly improved interfacial adhesion performance. Summary of the Invention

[0012] The purpose of this invention is to overcome the shortcomings of existing technologies, such as weak bonding between traditional polypropylene fibers and concrete, poor dispersibility in concrete, and the tendency of existing chemical modification methods (such as maleic anhydride grafting) to degrade polypropylene and weak bonding with fillers, which leads to easy detachment under high tensile strength. This invention provides a bamboo powder reinforced polypropylene composite fiber for concrete crack resistance and its preparation method.

[0013] In a first aspect, the present invention provides a method for preparing bamboo powder reinforced polypropylene composite fibers for concrete crack resistance, the method comprising the following steps:

[0014] S1, Surface chemical modification of bamboo powder: Carbonized bamboo powder is reacted with a composite coupling agent at 60℃-80℃ for 30-50 minutes to obtain modified bamboo powder with amino and vinyl active functional groups grafted on the surface; the composite coupling agent is composed of aminosilane and vinylsilane.

[0015] S2, Melt Blending and Chemical Grafting: The modified bamboo powder, polypropylene resin with a melt index of 10-20 g / 10min, and peroxide accounting for 0.5%-2% of the mass of the polypropylene resin are melt blended and extruded at 180℃-230℃ to form coarse filaments with a diameter of 0.8-2mm; the peroxide initiates a free radical grafting reaction between the polypropylene and the vinyl functional groups on the surface of the modified bamboo powder to form a chemical anchor.

[0016] S3, stretching and shaping: the coarse filament is stretched in multiple stages at 70℃-100℃ to orient the polypropylene molecular chains and obtain monofilament fibers with a diameter of 30-1000 μm. During the stretching process, some of the modified bamboo powder protrudes from the fiber surface and forms micro-grooves.

[0017] S4, Cutting: Cut the monofilamentous fiber into short fibers with a length of 3-65 mm.

[0018] In the technical solution of this invention, during the mixing reaction in step S1, the silanol groups generated by the hydrolysis of the composite coupling agent and moisture in the air condense with the hydroxyl groups on the surface of bamboo powder to form a strong Si-OC covalent bond, anchoring silane molecules to the surface of bamboo powder, allowing them to carry amino (-NH2) and vinyl (-CH=CH2) active functional groups respectively. During the melt blending process, the peroxide decomposes upon heating to generate free radicals. These free radicals attack the polypropylene molecular chains, generating polypropylene free radicals (·PP). Simultaneously, the polypropylene free radicals undergo a free radical addition reaction with the carbon-carbon double bonds (-CH=CH2) of vinyl silane on the surface of bamboo powder, forming a covalent graft anchoring structure of PP-g-(silane-bamboo powder), thereby achieving chemical bonding between bamboo powder and the polypropylene matrix. Furthermore, the amino functional groups on the surface of bamboo powder can form Si-O-NH- bonds through hydrogen bonding or condensation reactions with the silanol groups in the system, further enhancing interfacial bonding and reducing interfacial defects. Ultimately, the system forms a dual composite structure consisting of bamboo powder-silane-PP covalent bridges and a PP matrix crosslinking network.

[0019] Unlike existing maleic anhydride grafting modifications, the vinylsilane / peroxide radical grafting system used in this invention primarily involves reactions between the bamboo powder-silane interface and the polypropylene molecular chains, significantly reducing the degradation impact on the polypropylene backbone. Simultaneously, the auxiliary effect of aminosilane further stabilizes the interfacial structure. This system achieves strong chemical bonding between bamboo powder and polypropylene while maintaining good melt strength and rheological properties of the polypropylene matrix, laying a material foundation for subsequent high-ratio stretching preparation of fine denier fibers.

[0020] During the stretching and setting process, the polypropylene matrix is ​​thinned, and the molecular chains are oriented along the stretching direction. The bamboo powder particles, firmly anchored by chemical bonds, exhibit minimal dimensional change due to their rigidity and anchoring effect. This causes some bamboo powder particles close to the fiber surface to protrude beyond the fiber surface. These protruding particles further stretch and create micro-grooves, ultimately forming a unique, non-post-modified micro-rough structure on the fiber surface. The composite fiber of this invention can be directly incorporated into concrete. Chemical modification improves the surface properties of the fiber and reduces static electricity, resulting in uniform fiber dispersion without clumping.

[0021] The technical solution of this invention fundamentally achieves the simultaneous construction of strong interfacial bonding between bamboo powder and polypropylene and fiber surface functions through an integrated process of surface chemical modification (introducing bifunctional groups), melt grafting (forming chemical anchoring), and multi-stage stretching (forming a rough structure in situ). This method not only solves the global problem of bamboo powder easily detaching from the polymer matrix, but also ensures good rheological properties of the melt due to the minimal damage to the polypropylene backbone caused by chemical grafting. This makes it possible to subsequently prepare fine denier fibers (with diameters as low as 30 μm) through high-ratio multi-stage stretching, breaking through the technical bottleneck of traditional modification methods that struggle to balance strong interfacial bonding and high spinnability.

[0022] Preferably, according to the above-mentioned method for preparing bamboo powder reinforced polypropylene composite fiber for concrete crack resistance, in step S1, the carbonized bamboo powder is obtained by pre-treating raw bamboo with an age of 2-5 years. The pre-treatment steps are as follows: after removing the green and yellow parts of the raw bamboo, raw bamboo fiberboard is obtained. Bamboo fiberboard with a tensile strength greater than 300MPa is crushed into bamboo powder with a particle size of 100-2000 mesh. The bamboo powder is then lightly carbonized at 180℃-200℃ for 60-120 minutes.

[0023] The bamboo powder accounts for 5%-55% of the mass of the modified polypropylene fiber.

[0024] By screening the source and pretreatment process of bamboo powder raw materials, high-performance carbonized bamboo powder raw materials were obtained by subjecting raw bamboo of specific ages to a process of "removing green and yellowing, high-intensity screening, crushing, and shallow carbonization." Shallow carbonization effectively removes thermally unstable components such as hemicellulose from bamboo, improving the thermal stability of the bamboo powder. This allows it to withstand subsequent melt processing at temperatures up to 230℃ without significant thermal degradation or discoloration, ensuring the stability of the final fiber color and the reliability of its mechanical properties. Simultaneously, controlling the particle size range of the bamboo powder facilitates its uniform dispersion in the polymer melt and allows for sufficient subsequent surface chemical modification.

[0025] Preferably, the amount of the composite coupling agent is 0.5%-3% of the mass of carbonized bamboo powder, and the mass ratio of aminosilane to vinylsilane is 1:1-1:2.

[0026] This solution further optimizes the dosage and ratio of the composite coupling agent. By controlling the dosage of the composite coupling agent to 0.5%-3% of the bamboo powder mass, it is possible to ensure that the surface of the bamboo powder is effectively and fully modified, covering sufficient active functional groups, while avoiding the side effects caused by excessive coupling agent self-polymerization, which would affect the interfacial properties.

[0027] By limiting the mass ratio of aminosilane to vinylsilane to 1:1-1:2, the synergistic effect of the two silanes is optimized: vinylsilane provides sufficient grafting sites to ensure the density of chemical anchoring; aminosilane further enhances interfacial coupling (such as reaction with residual silanol groups) and significantly improves the overall hydrophilicity of the fiber. This ratio is the key to achieving strong chemical bonding and excellent hydrophilicity.

[0028] Preferably, the peroxide is dicumyl peroxide or 2,5-dimethyl-2,5-di-tert-butylperoxyhexane.

[0029] Dicumyl peroxide (DCP) and 2,5-dimethyl-2,5-di-tert-butylperoxyhexane are both commonly used free radical initiators suitable for polypropylene processing temperatures. Their decomposition rates are well-matched to the melt blending process conditions (180℃~230℃), allowing for stable and effective decomposition within this temperature window to generate free radicals, thereby controllably initiating the grafting reaction between polypropylene and the vinyl groups on the bamboo powder surface. This avoids excessive cross-linking or severe chain breakage of polypropylene due to overly rapid initiator decomposition, ensuring the controllability of the reaction and the stability of the final fiber melt strength.

[0030] Preferably, the polypropylene resin is at least one of homopolymer polypropylene and copolymer polypropylene.

[0031] Homopolymer polypropylene exhibits high crystallinity and rigidity, while copolymer polypropylene typically possesses superior impact toughness. Allowing the use of at least one of these provides flexibility in controlling the final fiber properties: suitable resins can be selected or blended according to different requirements for fiber rigidity or toughness to achieve the comprehensive mechanical properties best suited to concrete reinforcement requirements. Simultaneously, this limitation ensures that the melt index (10-20 g / 10min) of the selected resin meets process requirements, guaranteeing the processing fluidity of the blend system.

[0032] Preferably, in step S3, the number of stretching stages is 2-3; the speed ratio of each stretching stage is 1.5-2.0.

[0033] Limiting the number of stretching stages to 2-3, and controlling the speed ratio of each stage to 1.5-2.0, is a gentle yet efficient orientation method. Multi-stage stretching facilitates the gradual and orderly orientation of polypropylene molecular chains, avoiding stress concentration or breakage within the fiber that may occur with single-stage high-ratio stretching. This parameter setting ensures high tensile strength and modulus of the fiber while creating coordinated differential deformation between the chemically anchored rigid bamboo powder particles and the plastically deformed polypropylene matrix, thus smoothly and stably forming the rough surface structure of "bamboo powder protrusions" and "micro-grooves".

[0034] In a second aspect, the present invention provides a bamboo powder reinforced polypropylene composite fiber for concrete crack resistance, which is prepared according to the above preparation method.

[0035] Preferably, the fiber is based on polypropylene and contains surface-modified bamboo powder, which is chemically bonded to the polypropylene matrix; and some of the bamboo powder protrudes from the fiber surface, giving the fiber surface a micro-groove structure.

[0036] Preferably, the bamboo powder accounts for 5%-55% of the mass of the fiber; the surface roughness Ra of the fiber is 1.5-3.0 μm, and the water contact angle is ≤60°.

[0037] Controlling the bamboo powder content to 5%–55% ensures sufficient modification and improved surface roughness of the fibers while avoiding increased fiber brittleness or processing difficulties caused by excessive bamboo powder content. Limiting the fiber surface roughness Ra value to 1.5–3.0 μm and the water contact angle to ≤60° provides clear and measurable performance indicators. Higher roughness is directly related to mechanical anchoring force with concrete, while a lower contact angle indicates excellent hydrophilicity; both work synergistically to ensure excellent interfacial bonding performance of the fibers in concrete. These quantitative indicators are important standards for judging whether the fibers of this invention achieve the expected technical effects.

[0038] The composite fibers of this invention, when incorporated into concrete, can significantly improve the macroscopic properties of concrete composite materials. Specifically, the fibers can effectively bridge microcracks, greatly improving the early crack resistance of concrete; significantly enhancing the splitting tensile strength and toughness of concrete, and improving its brittle failure mode; and the uniform dispersion of the fibers in the matrix ensures the integrity and reliability of the performance improvement.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] 1. Strong interfacial chemical bonding: Bamboo powder is dually modified with aminosilane and vinylsilane, and a strong covalent bridge, i.e., chemical anchoring, is constructed between bamboo powder and polypropylene matrix through a free radical grafting reaction initiated by peroxide. This reaction has high selectivity and minimal damage to the polypropylene backbone, allowing the modified melt to maintain good fluidity and strength, laying the foundation for subsequent high-strength stretching. This fundamentally solves the problems of weak interfacial bonding of traditional polypropylene fibers and the easy detachment of bamboo powder in physically blended or weakly bonded systems.

[0041] 2. Controllable reaction and stable structure: The selected peroxide has a moderate decomposition rate at the processing temperature, which can effectively initiate the grafting reaction and avoid excessive cross-linking of polypropylene; the silane coupling agent reacts fully on the surface of bamboo powder with few free silanol groups, ensuring the water resistance of the material.

[0042] 3. Synergistically enhanced microstructure: During the stretching process, the chemically bonded bamboo powder naturally protrudes from the fiber surface and forms micro-grooves on the fiber surface, which greatly increases the mechanical interlocking force between the fiber and the concrete. At the same time, the exposed hydrophilic groups of bamboo powder enhance the hydrophilicity of the fiber.

[0043] 4. Excellent Comprehensive Performance: The aforementioned chemical and structural improvements result in significantly enhanced bond strength and dispersibility of bamboo powder-modified polypropylene fibers in concrete, thereby synergistically improving the compressive strength, crack resistance, and toughness of fiber-reinforced concrete. These chemical and structural improvements also enhance the splitting tensile strength and toughness of fiber-reinforced concrete without significantly reducing its compressive strength.

[0044] 5. Excellent processability and fine denier potential: Compared with conventional modification methods such as maleic anhydride grafting, the modification system adopted in this invention causes less damage to the molecular chains of the polypropylene matrix and has stable melt properties. In particular, due to the strong chemical bonding, the bamboo powder is not easy to fall off during the stretching process, resulting in a uniform fiber structure. It can successfully prepare fine denier fibers with a diameter of 30~1000 μm through high-ratio stretching, breaking through the technical bottleneck of conventional modified polypropylene's difficulty in stably preparing fine diameter fibers.

[0045] 6. Integrated process: Modification, grafting, and molding are completed in a continuous process flow, without the need for complicated post-processing steps, resulting in high production efficiency and stable performance. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the stretching process according to an embodiment of the present invention.

[0047] Figure 2 This is a microscopic image (I) of the fiber surface microstructure according to an embodiment of the present invention.

[0048] Figure 3 This is a microscopic image II of the fiber surface microstructure according to an embodiment of the present invention.

[0049] Figure 4 This is a comparison chart showing the test results of the mechanical properties of fiber-reinforced concrete with different fiber additions. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to specific embodiments. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention. All quantities mentioned below are parts by weight.

[0051] Example 1

[0052] This embodiment provides a method for preparing bamboo powder modified polypropylene fiber, including the following steps:

[0053] S1. Surface chemical modification of bamboo powder: Carbonized bamboo powder is reacted with a composite coupling agent composed of aminosilane and vinylsilane at 60℃-80℃ for 30-50 minutes to obtain modified bamboo powder with amino and vinyl active functional groups grafted on the surface; wherein, the carbonized bamboo powder is obtained by pretreatment of raw bamboo aged 2-5 years, the pretreatment steps are as follows: after removing the green and yellow from the raw bamboo, raw bamboo fiberboard is obtained, the raw bamboo fiberboard with a tensile strength greater than 300MPa is crushed into bamboo powder with a particle size of 600 mesh, and then the bamboo powder is lightly carbonized at 190℃ for 60 minutes to obtain the product.

[0054] More specifically, the amount of the composite coupling agent is 0.5%-3% of the mass of carbonized bamboo powder, and the mass ratio of aminosilane to vinylsilane is 1:1-1:2.

[0055] Take 300 parts of carbonized bamboo powder, 3 parts of γ-aminopropyltriethoxysilane (KH-550), and 3 parts of γ-vinyltrimethoxysilane (KH-171); add the above-mentioned carbonized bamboo powder, KH-550, and KH-171 to a high-speed mixer and stir at 500 r / min for 40 minutes at 70°C to obtain modified bamboo powder.

[0056] S2. Melt blending and chemical grafting: The modified bamboo powder, polypropylene resin with a melt index of 10-20 g / 10min, and peroxide accounting for 0.5%-2% of the mass of polypropylene resin are melt blended and extruded at 180℃-230℃ to form coarse filaments with a diameter of 0.8-2mm.

[0057] The peroxide initiates a free radical grafting reaction between polypropylene and the vinyl functional groups on the surface of modified bamboo powder to form a chemical anchor; more specifically, the peroxide is dicumyl peroxide or 2,5-dimethyl-2,5-di-tert-butylperoxyhexane.

[0058] 15 parts of dicumyl peroxide (DCP), 2500 parts of homopolymer polypropylene granules with a melt index of 10~20 g / 10 min, and 500 parts of copolymer polypropylene granules with a melt index of 10~14 g / 10 min. All the above raw materials are in parts by weight.

[0059] Specifically, modified bamboo powder, all polypropylene granules, and DCP are added to a twin-screw extruder for melt blending. The temperatures in each zone are controlled as follows: feeding section 120℃, plasticizing section 200℃, homogenizing section 190℃, and die head 190℃. After melt blending, coarse filaments with a diameter of 1.2mm are extruded.

[0060] S3. Stretching and Shaping: The coarse filament is stretched in multiple stages at 70℃-100℃ to orient the polypropylene molecular chains and obtain monofilament fibers with a diameter of 30-1000 μm. During the stretching process, some of the modified bamboo powder protrudes from the fiber surface and forms micro-grooves. The number of stretching stages is 2-3. The speed ratio of each stretching stage is 1.5-2.0.

[0061] Specifically, a seven-roller traction device is used to heat the coarse filaments at 90°C, followed by two-stage traction stretching at traction speeds of 0.5 m / s and 1.0 m / s to obtain monofilament bamboo powder modified polypropylene fibers with a diameter of approximately 100 μm.

[0062] S4, Cutting: The monofilamentous fibers are cut into short fibers with a length of 3-65 mm. Specifically, the stretched and shaped bamboo powder modified polypropylene fibers are wound up and cut into 15 mm short fibers.

[0063] In this embodiment, the amount of each material used is summarized in Table 1:

[0064] Table 1: Material Usage Table for Examples (Parts by Weight)

[0065]

[0066] Example 2

[0067] S1. Take bamboo aged 2-5 years and remove the green and yellow parts to obtain bamboo fiberboard. Then, take the bamboo with a tensile strength greater than 300MPa and crush it into 300-mesh bamboo powder. The bamboo powder is then lightly carbonized at 180℃ for 60 minutes to obtain carbonized bamboo powder.

[0068] Take 785 parts of carbonized bamboo powder, 7.85 parts of γ-aminopropyltriethoxysilane (KH-550), 15.7 parts of γ-vinyltrimethoxysilane (KH-171), 30 parts of 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, 2500 parts of homopolymer polypropylene particles with a melt index of 10~20 g / 10 min, and 500 parts of copolymer polypropylene particles with a melt index of 10~20 g / 10 min. All the above raw materials are in parts by weight.

[0069] The carbonized bamboo powder, KH-550, and KH-171 in the above-mentioned weight proportions were added to a high-speed mixer and stirred at 500 r / min for 50 minutes at 75°C to obtain modified bamboo powder.

[0070] S2: Modified bamboo powder, all polypropylene granules, and 2,5-dimethyl-2,5-di-tert-butylperoxyhexane are added to a twin-screw extruder for melt blending. The temperatures in each zone are controlled as follows: feeding section 180℃, plasticizing section 200℃, homogenizing section 210℃, and die head 230℃. After melt blending, coarse filaments with a diameter of 1.5mm are extruded.

[0071] S3 involves heating the coarse filament at 70°C, followed by three stages of traction stretching at traction speeds of 0.5 m / s, 1.0 m / s, and 1.5 m / s to obtain monofilamentous bamboo powder modified polypropylene fibers with a diameter of approximately 1000 μm.

[0072] S4. The stretched and shaped bamboo powder modified polypropylene fiber is wound up and cut into 25mm short fibers.

[0073] Example 3

[0074] This invention provides a bamboo powder-modified polypropylene fiber, which is a modified polypropylene fiber prepared using the preparation method described in Example 1. The matrix of the modified polypropylene fiber is polypropylene, and surface-modified bamboo powder is incorporated into the matrix of the modified polypropylene fiber. The modified bamboo powder is chemically bonded to the polypropylene matrix.

[0075] Figure 2 The image shows a fiber micrograph from Example 3. Figure 3 for Figure 2 A magnified view of a portion, such as Figure 2 , 3 As shown, some modified bamboo powder protrudes from the fiber surface, and the fiber surface also has micro-grooves, which makes the fiber surface form a unique, non-post-modified micro-rough structure, enhancing the surface roughness. When it is added to concrete to form fiber concrete, it will greatly increase the mechanical interlocking force between the fiber and the concrete. At the same time, the exposed hydrophilic groups of bamboo powder enhance the hydrophilicity of the fiber.

[0076] Example 4

[0077] This invention provides a bamboo powder modified polypropylene fiber, wherein the fiber is a bamboo powder modified polypropylene fiber prepared by the preparation method described in Example 2.

[0078] Example 5

[0079] This invention also provides a fiber-reinforced concrete, comprising concrete and fibers incorporated into the concrete. The concrete is C30 concrete, and the fiber is a bamboo powder-modified polypropylene fiber provided in Example 3, with a fiber volume fraction of 1.2‰. During mixing, the fibers are uniformly dispersed without clumping.

[0080] Example 6

[0081] This invention also provides a fiber-reinforced concrete, comprising concrete and fibers incorporated into the concrete. The concrete is C30 concrete, and the fiber is a bamboo-powder-modified polypropylene fiber provided in Example 4, with a fiber volume fraction of 1.2‰. During mixing, the fibers are uniformly dispersed without clumping.

[0082] Using ordinary polypropylene fibers with a diameter of 300 μm and a length of 15 mm as Comparative Example 1, Comparative Example 1, Example 3, and Example 4 were tested, and the test results are shown in Table 2. As can be seen from Table 2, compared with ordinary polypropylene fibers, the tensile strength, tensile modulus, and surface roughness coefficient of Examples 3 and 4 were all improved, while the water contact angle was reduced. This shows that bamboo powder modified polypropylene fibers have better interfacial bonding potential, and the higher the bamboo powder content, the higher the tensile strength, tensile modulus, and surface roughness coefficient, and the lower the water contact angle.

[0083] Table 2: Fiber Performance Testing Table

[0084]

[0085] Note: The water contact angle was measured using the fine fiber capillary water absorption method and a contact angle measuring instrument.

[0086] Comparative Example 2 uses conventional C30 concrete; Comparative Example 3 uses fiber-reinforced concrete formed by adding ordinary polypropylene fibers with a diameter of 300 μm and a length of 15 mm to C30 concrete at the same volume ratio as in Example 5. Following GB / T21120-2018 Synthetic Fibers for Cement Concrete and Mortar and GB / T 50081-2019 Standard for Test Methods of Physical and Mechanical Properties of Concrete, 10 sets of samples each for Comparative Example 2, Comparative Example 3, Example 5, and Example 6 were prepared and tested for splitting tensile strength after 24 days of curing. Simultaneously, to verify the necessity of the synergistic modification of aminosilane (KH-550) and vinylsilane (KH-171) in the composite coupling agent, comparative fiber-reinforced concrete samples were also prepared, including those made with bamboo powder without any coupling agent treatment (Comparative Example 4), treated only with KH-550 (Comparative Example 5), and treated only with KH-171 (Comparative Example 6). The splitting tensile strength test results of these samples are summarized together with those of Examples 5 and 6, and the specific data are shown in Table 3.

[0087] From Table 3 and Figure 4It can be seen that the ordinary C30 concrete without any added fibers (Comparative Example 2) has the lowest splitting strength (average approximately 3.03 MPa); the splitting strength of Comparative Example 3, which incorporates ordinary polypropylene fibers, is improved (average approximately 3.45 MPa); while the splitting strength of Examples 5 and 6, which incorporate bamboo powder modified polypropylene fibers prepared by the method of this invention, is significantly improved, with average values ​​reaching approximately 3.86 MPa and 3.83 MPa, respectively. In contrast, the fiber-reinforced concrete with bamboo powder not treated with a coupling agent (Comparative Example 4) and treated with only a single coupling agent (Comparative Examples 5 and 6), although their average splitting strengths (approximately 2.65 MPa, 2.90 MPa, and 2.82 MPa, respectively) are slightly higher than those of ordinary polypropylene fiber-reinforced concrete, but far lower than those of Examples 5 and 6, which are synergistically modified with a composite coupling agent. This indicates that the synergistic use of aminosilane and vinylsilane is crucial for achieving a strong interfacial bond between bamboo powder and polypropylene, thereby maximizing the mechanical properties of fiber-reinforced concrete.

[0088] Table 3: 24-day splitting tensile strength test results of fiber-reinforced concrete (unit: MPa)

[0089]

[0090] The above data fully demonstrate that the bamboo powder-reinforced polypropylene composite fiber prepared by the present invention through surface chemical modification of bamboo powder using a composite coupling agent, combined with melt grafting and multi-stage stretching processes, can significantly improve the splitting tensile strength of fiber-reinforced concrete. Its reinforcing effect is significantly superior to ordinary polypropylene fibers and modified fibers using single or untreated bamboo powder. This verifies the effectiveness and innovation of the method of the present invention in achieving strong interfacial bonding and excellent concrete reinforcement performance.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing bamboo powder reinforced polypropylene composite fibers for concrete crack resistance, characterized in that, Includes the following steps: S1, Surface chemical modification of bamboo powder: Carbonized bamboo powder is reacted with a composite coupling agent at 60℃-80℃ for 30-50 minutes to obtain modified bamboo powder with amino and vinyl active functional groups grafted on the surface; the amount of the composite coupling agent is 0.5%-3% of the mass of the carbonized bamboo powder; the composite coupling agent is composed of aminosilane and vinylsilane, and the mass ratio of aminosilane to vinylsilane is 1:1-1:2; S2, Melt Blending and Chemical Grafting: The modified bamboo powder, polypropylene resin with a melt index of 10-20 g / 10min, and peroxide accounting for 0.5%-2% of the mass of the polypropylene resin are melt blended and extruded at 180℃-230℃ to form coarse filaments with a diameter of 0.8-2mm; the peroxide initiates a free radical grafting reaction between the polypropylene and the vinyl functional groups on the surface of the modified bamboo powder to form a chemical anchor. S3, stretching and shaping: the coarse filament is stretched in multiple stages at 70℃-100℃ to orient the polypropylene molecular chains and obtain monofilament fibers with a diameter of 30-1000 μm. S4, Cutting: Cut the monofilamentous fiber into short fibers with a length of 3-65 mm.

2. The method for preparing bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 1, characterized in that, In S1, the carbonized bamboo powder is obtained by pre-treating raw bamboo aged 2-5 years. The pre-treatment steps are as follows: after removing the green and yellow parts of the raw bamboo, raw bamboo fiberboard is obtained. Raw bamboo fiberboard with a tensile strength greater than 300MPa is crushed into bamboo powder with a particle size of 100-2000 mesh. The bamboo powder is then carbonized at 180℃-200℃ for 60-120 minutes.

3. The method for preparing bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 1, characterized in that, The peroxide is dicumyl peroxide or 2,5-dimethyl-2,5-di-tert-butylperoxyhexane.

4. The method for preparing bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 1, characterized in that, The polypropylene resin is at least one of homopolymer polypropylene and copolymer polypropylene.

5. The method for preparing bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 1, characterized in that, In step S3, the number of stretching stages is 2-3; the speed ratio of each stretching stage is 1.5-2.

0.

6. A bamboo powder-reinforced polypropylene composite fiber for concrete crack resistance, characterized in that, It is prepared by any one of claims 1-5.

7. The bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 6, characterized in that, The fiber uses polypropylene as a matrix and is incorporating surface-modified bamboo powder. The bamboo powder is chemically bonded to the polypropylene matrix, and some of the bamboo powder protrudes from the fiber surface, giving the fiber surface a micro-groove structure.

8. The bamboo powder reinforced polypropylene composite fiber for concrete crack resistance according to claim 7, characterized in that, The bamboo powder accounts for 5%-55% of the mass of the fiber; the surface roughness Ra of the fiber is 1.5-3.0 μm, and the water contact angle is ≤60°.

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