Modified high-performance polypropylene cable protection pipe and preparation method thereof

Abstract

This invention relates to the field of polymer cable protection pipes, and discloses a modified high-performance polypropylene cable protection pipe and its preparation method. It comprises the following components by weight percentage: 55%~75% polypropylene matrix, 12%~22% phytic acid-chitosan composite microspheres, 6%~14% silane coupling agent modified cellulose nanocrystals, 2%~6% polypropylene grafted maleic anhydride compatibilizer, 0.5%~1.5% composite antioxidant, and 0.3%~1.2% erucic acid amide lubricant. The phytic acid-chitosan composite microspheres are formed by the electrostatic self-assembly of phytic acid and chitosan; the silane coupling agent modified cellulose nanocrystals are obtained by grafting cellulose nanocrystals with a silane coupling agent. This solution solves the problem of the inability to simultaneously achieve flame retardancy and toughening under high filler content, achieving halogen-free flame retardancy, improved elongation at break, avoiding the release of corrosive and toxic gases, and eliminating stress concentration in the matrix.

Description

Technical Field

[0001] This invention relates to the field of polymer cable protection pipes, and discloses a modified high-performance polypropylene cable protection pipe and its preparation method. Background Technology

[0002] In existing technologies, high-performance polypropylene cable protection pipes are widely used in underground cable laying. To meet the flame retardant requirements of underground environments, conventional technical solutions often involve adding halogenated or inorganic flame retardants to the polypropylene matrix. Inorganic flame retardants, such as magnesium hydroxide or aluminum hydroxide, have low inherent flame retardant efficiency and must be added to the polypropylene matrix in large proportions. They rely on thermal decomposition to release water vapor and absorb heat to dilute flammable gases and lower the temperature. Halogenated flame retardants, on the other hand, achieve flame retardancy by releasing halogenated free radicals to capture active free radicals in the combustion chain reaction.

[0003] The aforementioned conventional technical solutions suffer from the technical problem of failing to simultaneously achieve flame retardancy and toughening under high filler content. The introduction of a large number of inorganic flame retardant particles creates numerous stress concentration points within the polypropylene matrix, disrupting the continuity of the polypropylene molecular chains and leading to a precipitous decrease in material toughness. This makes the material highly susceptible to brittle cracking when subjected to external pressure or foundation settlement. While halogenated flame retardants are added in relatively small quantities, the hydrogen halide gas released during combustion is highly corrosive and toxic, failing to meet the fire safety requirements of underground enclosed spaces. Furthermore, they cannot overcome the toughness degradation defect in the polypropylene matrix caused by the introduction of multiphase particles. Summary of the Invention

[0004] To address the shortcomings of existing polypropylene cable protection pipes, such as the reduction in toughness due to the use of a large proportion of inorganic flame retardants, the release of toxic and corrosive gases during combustion of halogenated flame retardants, and the inability to simultaneously achieve both flame retardancy and toughness enhancement, this invention provides a modified high-performance polypropylene cable protection pipe and its preparation method.

[0005] To address the aforementioned technical problems, this invention provides a modified high-performance polypropylene cable protection pipe, comprising the following components by weight percentage: 55%–75% polypropylene matrix, 12%–22% phytic acid-chitosan composite microspheres, 6%–14% silane coupling agent modified cellulose nanocrystals, 2%–6% polypropylene grafted maleic anhydride compatibilizer, 0.5%–1.5% composite antioxidant, and 0.3%–1.2% erucic acid amide lubricant; the phytic acid-chitosan composite microspheres are formed by the electrostatic self-assembly reaction of phytic acid and chitosan; the silane coupling agent modified cellulose nanocrystals are obtained by surface grafting modification of cellulose nanocrystals with a silane coupling agent; the polypropylene matrix is ​​composed of homopolymer polypropylene and block copolymer polypropylene in a weight ratio of 2:1.

[0006] Phytic acid molecules contain multiple phosphate groups, and chitosan molecules contain a large number of free amino groups. In aqueous solution, the two can self-assemble into microsphere structures through electrostatic interactions of positive and negative charges. When heated, phytic acid first decomposes to produce acidic phosphoric acid substances, which catalyze the dehydration and carbonization reaction of chitosan molecules to form a dense, expanded carbon layer. This carbon layer can isolate oxygen and heat from being transferred to the matrix, while diluting the concentration of combustible gases, thus achieving halogen-free flame retardancy. The surface of silane coupling agent-modified cellulose nanocrystals is grafted with organic groups that are compatible with polypropylene. These groups can be uniformly dispersed in the matrix and form a three-dimensional network structure, effectively transferring stress. Homopolymer polypropylene and block copolymer polypropylene are compounded in a 2:1 ratio to balance the rigidity and toughness of the pipe. Polypropylene grafted with maleic anhydride compatibilizer can improve the interfacial bonding between the inorganic and organic phases and prevent phase separation.

[0007] Furthermore, in the above technical solution, the raw materials for preparing the phytic acid-chitosan composite microspheres include, by weight, 10-20 parts of chitosan with a degree of deacetylation greater than 85%, 30-50 parts of a phytic acid aqueous solution with a mass concentration of 50%, 5-10 parts of a glacial acetic acid aqueous solution with a mass concentration of 1%, 2-5 parts of Tween-80 emulsifier, and 1-3 parts of glutaraldehyde crosslinking agent; the volume average particle size distribution of the phytic acid-chitosan composite microspheres is 5μm-20μm; the phytic acid-chitosan composite microspheres have a core-shell structure, with an internal phytic acid-enriched core layer and an external chitosan shell layer crosslinked and cured by the glutaraldehyde crosslinking agent.

[0008] In practice, chitosan with a deacetylation degree greater than 85% has a higher amino content, which can form more electrostatic interactions with the phosphate groups of phytic acid, thus improving the structural stability of the microspheres. Tween-80 emulsifier can reduce the surface tension of the system, allowing the phytic acid aqueous solution to disperse in the chitosan solution to form uniform microdroplets, controlling the microsphere particle size within the range of 5μm to 20μm, and avoiding stress concentration caused by excessively large particle size. Glutaraldehyde can react with the amino groups of chitosan to form a Schiff base reaction, forming a covalent cross-linked network, solidifying the chitosan shell layer, and preventing premature leakage of phytic acid during melt processing. The core-shell structure allows phytic acid to accumulate inside the microspheres. When heated, phytic acid is slowly released from the core layer, continuously catalyzing the carbonization of chitosan, extending the working time of the expansion insulation layer, and improving long-term flame retardant stability.

[0009] Furthermore, in the above technical solution, the silane coupling agent modified cellulose nanocrystals are prepared by the following method: cellulose nanocrystals are dispersed in a mixed solvent of anhydrous ethanol and deionized water, the pH value is adjusted to 4 / 5, γ-methacryloyloxypropyltrimethoxysilane is added at 8% / 15% of the weight of the cellulose nanocrystals, and the reaction is carried out under reflux at 50℃ / 65℃ for 6h / 10h, filtered and washed with anhydrous ethanol, and then freeze-dried under vacuum to obtain the cellulose nanocrystals; the length of the cellulose nanocrystals is 100nm / 300nm, and the diameter is 10nm / 30nm.

[0010] In practice, pH 4-5 is the optimal condition for the hydrolysis of γ-methacryloxypropyltrimethoxysilane, which allows it to fully hydrolyze to generate silanol groups, which then undergo dehydration condensation with the hydroxyl groups on the surface of cellulose nanocrystals, achieving covalent grafting modification. Cellulose nanocrystals with lengths of 100nm / 300nm and diameters of 10nm / 30nm have a high aspect ratio, which can form a continuous three-dimensional stress transfer network in the polypropylene matrix, significantly improving the toughness of the material. Vacuum freeze-drying allows the solvent to sublimate directly, avoiding the aggregation of cellulose nanocrystals due to surface tension during the drying process, and ensuring their uniform dispersion in the matrix.

[0011] Furthermore, in the above technical solution, the melt flow rate of the polypropylene matrix is ​​5 g / 10 min / 15 g / 10 min; the grafting rate of the polypropylene grafted with maleic anhydride compatibilizer is 0.8% / 1.5%; the composite antioxidant is composed of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a weight ratio of 1:2; the polypropylene grafted with maleic anhydride compatibilizer is enriched in the interfacial region between the polypropylene matrix and the silane coupling agent modified cellulose nanocrystals.

[0012] In practice, a polypropylene matrix with a melt flow rate of 5 g / 10 min / 15 g / 10 min exhibits suitable processing fluidity, ensuring sufficient dispersion of each component during melt blending. A polypropylene grafted with maleic anhydride compatibilizer at grafting rates of 0.8% / 1.5% allows its maleic anhydride groups to chemically react with the active groups on the surface of silane coupling agent-modified cellulose nanocrystals. Simultaneously, its polypropylene segments can entangle with the polypropylene molecular chains of the matrix, spontaneously accumulating at the interface between the two phases, significantly reducing interfacial tension and enhancing interfacial bonding. Hindered phenolic primary antioxidants can capture alkoxy radicals generated by polymer degradation, while phosphite-based auxiliary antioxidants can decompose hydroperoxides. The 1:2 ratio of these two antioxidants provides a synergistic antioxidant effect, effectively inhibiting the thermo-oxidative aging of polypropylene during processing and use.

[0013] Furthermore, in the above technical solution, in the microstructure of the modified high-performance polypropylene cable protection pipe, the phytic acid-chitosan composite microspheres are uniformly dispersed inside the polypropylene matrix as a dispersed phase, and the silane coupling agent modified cellulose nanocrystals are intercalated and coated around the phytic acid-chitosan composite microspheres, forming a core-shell coating structure with the phytic acid-chitosan composite microspheres as the core and the silane coupling agent modified cellulose nanocrystal network as the shell. The polypropylene grafted maleic anhydride compatibilizer is enriched at the interface between the core-shell coating structure and the polypropylene matrix.

[0014] In practice, the silane coupling agent-modified cellulose nanocrystals are intercalated and coated around the phytic acid-chitosan composite microspheres to form a core-shell coating structure, which allows the cellulose nanocrystal network to tightly wrap the composite microspheres. When the pipe is subjected to external force, the stress can be quickly transferred to multiple composite microspheres through the cellulose nanocrystal network. The composite microspheres absorb energy through elastic and plastic deformation, avoiding brittle cracking of the matrix caused by local stress concentration. When combustion occurs, the cellulose nanocrystals first carbonize to form a rigid skeleton, supporting the expansion insulation layer formed by the carbonization of the phytic acid-chitosan composite microspheres, preventing the carbon layer from collapsing and falling off, and significantly improving the heat insulation and flame retardant effect. Polypropylene grafted maleic anhydride compatibilizer is enriched at the interface between the core-shell coating structure and the matrix, further enhancing the bonding force between the core-shell structure and the matrix, and inhibiting the debonding phenomenon of the composite microspheres under stress.

[0015] Furthermore, in the above technical solution, the modified high-performance polypropylene cable protection pipe wall comprises an inner layer, a middle layer, and an outer layer. The middle layer is composed of a blend of the polypropylene matrix, the phytic acid-chitosan composite microspheres, and the silane coupling agent modified cellulose nanocrystals. Both the inner layer and the outer layer are composed of pure polypropylene material. The thickness of the middle layer accounts for 60% / 75% of the total wall thickness of the modified high-performance polypropylene cable protection pipe, and the thicknesses of the inner layer and the outer layer each account for 12.5% / 20% of the total wall thickness.

[0016] In practice, the pipe wall adopts a three-layer gradient structure design. The inner and outer layers are made of pure polypropylene, which has excellent chemical corrosion resistance and wear resistance, and can effectively resist acid and alkali corrosion in underground soil and mechanical wear during cable laying. The middle layer is a modified blend, which concentrates the flame retardant and toughening functions. Its thickness accounts for 60% to 75%, which can ensure that the pipe has sufficient flame retardant and impact resistance. The inner and outer layers each account for 12.5% ​​to 20% of the thickness, which ensures basic protection performance while avoiding the overall flame retardant performance degradation caused by excessive thickness of the pure polypropylene layer.

[0017] To address the aforementioned technical problems, this invention also provides a method for preparing a modified high-performance polypropylene cable protection pipe, comprising the following steps: S1, dissolving chitosan in an aqueous glacial acetic acid solution, adding an emulsifier and stirring, then adding an aqueous phytic acid solution for electrostatic self-assembly, adding a crosslinking agent for curing, and filtering, washing, and drying to obtain phytic acid-chitosan composite microspheres; S2, dispersing cellulose nanocrystals in an aqueous ethanol solution, adjusting the pH to acidic, adding a silane coupling agent for reflux reaction, and filtering, washing, and drying to obtain silane coupling agent modified cellulose nanocrystals; S3, placing the polypropylene matrix, the phytic acid-chitosan composite microspheres, the silane coupling agent modified cellulose nanocrystals, a compatibilizer, an antioxidant, and a lubricant in a high-speed mixer and mixing them evenly to obtain a premix; S4, feeding the premix into a twin-screw extruder for melt blending and granulation, extruding the resulting granules through a pipe extruder, cooling and shaping to obtain the modified high-performance polypropylene cable protection pipe.

[0018] Step S1 involves dissolving chitosan to form a homogeneous solution, then slowly adding phytic acid solution. This utilizes the electrostatic interaction between the amino groups of chitosan and the phosphate groups of phytic acid to achieve self-assembly. Adding a crosslinking agent stabilizes the core-shell structure of the microspheres. Step S2 involves surface grafting modification of cellulose nanocrystals using a silane coupling agent, introducing organic groups that are compatible with polypropylene. Step S3 involves high-speed mixing to initially and uniformly disperse the solid components, preventing local agglomeration during melt blending. Step S4 utilizes the strong shearing action of a twin-screw extruder to achieve full melt blending of the components, followed by extrusion molding using a pipe extruder, ensuring the structural uniformity and performance stability of the pipe.

[0019] Further, in the above preparation method, in step S1, the mass concentration of the glacial acetic acid aqueous solution is 1% / 2%, and the concentration of chitosan in the glacial acetic acid aqueous solution is 0.02g / mL / 0.05g / mL; the specific operation of adding the phytic acid aqueous solution is as follows: under mechanical stirring at 800rpm / 1200rpm, the phytic acid aqueous solution is added dropwise to the chitosan solution at a rate of 0.5mL / min / 1mL / min, and after the addition is completed, the temperature is raised to 45℃ / 55℃ and the reaction continues for 2h / 3h; the crosslinking agent is glutaraldehyde, and after its addition, the pH value of the reaction system is maintained between 5.5 and 6.5, and the crosslinking reaction time is 4h / 6h.

[0020] In practice, a 1%–2% glacial acetic acid aqueous solution can fully protonate and dissolve chitosan, forming a uniform and transparent chitosan solution; a chitosan concentration of 0.02 g / mL or 0.05 g / mL ensures that the solution has a suitable viscosity, which is beneficial for the uniform dispersion of phytic acid aqueous solution and the formation of microspheres; a stirring speed of 800 rpm or 1200 rpm and a dropping rate of 0.5 mL / min or 1 mL / min can make the phytic acid aqueous solution form uniformly sized microdroplets in the chitosan solution, precisely controlling the microsphere particle size within the target range; a reaction temperature of 45℃ or 55℃ can promote the full progress of the electrostatic self-assembly reaction; a pH value of 5.5–6.5 is the optimal condition for the Schiff base reaction between glutaraldehyde and chitosan amino groups, which can ensure that the cross-linking reaction is fully carried out and a stable chitosan shell layer is formed.

[0021] Furthermore, in the above preparation method, in step S2, the volume ratio of anhydrous ethanol to deionized water in the ethanol-water solution is 4:1 / 6:1, the dispersion concentration of the cellulose nanocrystals in the ethanol-water solution is 0.03 g / mL / 0.08 g / mL, the acidifying agent used to adjust the pH value to 4-5 is glacial acetic acid, the silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane, and its addition amount is 8% / 15% of the weight of the cellulose nanocrystals, the reflux reaction temperature is 50℃ / 65℃, the time is 6h-10h, the vacuum freeze-drying temperature is -40℃ to -50℃, and the vacuum degree is less than 10Pa.

[0022] In practice, a mixed solvent with anhydrous ethanol and deionized water in a volume ratio of 4:1 or 6:1 can simultaneously dissolve the silane coupling agent and effectively disperse cellulose nanocrystals, preventing agglomeration of the cellulose nanocrystals. A cellulose nanocrystal concentration of 0.03 g / mL or 0.08 g / mL ensures uniform dispersion in the solvent, facilitating uniform grafting of the silane coupling agent onto its surface. Glacial acetic acid, as a weak acid, can gently adjust the pH of the system to the optimal range for silane coupling agent hydrolysis. An addition of 8%–15% silane coupling agent ensures sufficient surface modification of the cellulose nanocrystals while preventing self-polymerization from excessive silane coupling agent. Reflux temperatures of 50℃ or 65℃ and reaction times of 6h or 10h ensure sufficient grafting reaction between the silane coupling agent and cellulose nanocrystals. Vacuum freeze-drying conditions of -40℃ to -50℃ and a vacuum degree less than 10 Pa maximize the preservation of the nanoscale size and high aspect ratio of the cellulose nanocrystals.

[0023] Furthermore, in the above-mentioned preparation method, in step S4, the temperatures of each zone of the twin-screw extruder are set as follows: Zone 1 160℃ / 170℃, Zone 2 175℃ / 185℃, Zone 3 185℃ / 195℃, Zone 4 195℃ / 205℃, and the die head zone 200℃ / 210℃; the screw speed of the twin-screw extruder is 250rpm / 350rpm; the pipe extruder uses a three-layer co-extrusion die to extrude pure polypropylene material to form the inner and outer layers, and to extrude the premixed material into granules formed by melt blending and extrusion to form the middle layer; the extrusion temperature of the pipe extruder is set to 190℃ / 210℃, and the traction speed is 0.5m / min / 1.5m / min.

[0024] In practice, the twin-screw extruder employs a gradient heating zone setting, which allows the polypropylene matrix to melt gradually, avoiding thermal degradation of the material caused by localized overheating. The screw speed of 250 rpm / 350 rpm provides sufficient shear force to ensure thorough and uniform mixing of all components, while avoiding the breakage of cellulose nanocrystals and degradation of polypropylene molecular chains caused by excessive shear. The use of a three-layer co-extrusion die allows for the one-time extrusion of a pipe with a three-layer gradient structure, ensuring tight bonding between each layer without delamination. The pipe extrusion temperature of 190℃ / 210℃ and the traction speed of 0.5 m / min~1.5 m / min ensure the dimensional accuracy and surface quality of the pipe, while avoiding pipe deformation or degradation of mechanical properties due to excessively high temperature or excessively high traction speed.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This solution addresses the technical challenge of simultaneously achieving flame retardancy and toughening at high filler volumes by synergistically combining phytic acid-chitosan composite microspheres with silane coupling agent-modified cellulose nanocrystals. The phytic acid-chitosan composite microspheres undergo in-situ carbonization upon heating to form an expanding insulating layer, achieving halogen-free flame retardancy and preventing the release of corrosive and toxic gases. The silane coupling agent-modified cellulose nanocrystals interpenetrate and coat the composite microspheres, forming a core-shell coating structure that constructs a three-dimensional stress transfer network within the polypropylene matrix. This replaces traditional inorganic particles, eliminating stress concentration points and resulting in a significant increase in elongation at break while maintaining flame retardancy.

[0026] 2. In this design, the polypropylene grafted with maleic anhydride compatibilizer is enriched at the interface between the core-shell coating structure and the polypropylene matrix, enhancing the interfacial bonding between the organic modifier and the matrix and inhibiting the debonding of the composite microspheres under stress. The phytic acid-chitosan composite microspheres adopt a structure with a phytic acid-enriched core layer and a cross-linked chitosan shell layer, controlling the heat release rhythm of the flame retardant components and improving the density and long-term flame retardant stability of the insulation layer. The design of using pure polypropylene for the inner and outer layers of the pipe and a modified blend for the middle layer ensures the basic corrosion resistance and wear resistance of the inner and outer sides of the pipe wall. The middle layer concentrates on bearing the flame retardant and toughening functions, realizing the gradient distribution of the radial properties of the pipe. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to embodiments. Those skilled in the art can reproduce the technical solution of the present invention and achieve its claimed technical effects based on the content disclosed in this specification. It should be noted that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention. Any non-substantial improvements and adjustments made based on the core concept of the present invention should fall within the scope of protection of the present invention.

[0028] Example 1: The modified high-performance polypropylene cable protection pipe of this example is composed of the following components by weight percentage: 65% polypropylene matrix, 17% phytic acid-chitosan composite microspheres, 10% silane coupling agent modified cellulose nanocrystals, 4% polypropylene grafted maleic anhydride compatibilizer, 1% composite antioxidant, and 0.8% erucic acid amide lubricant; the polypropylene matrix is ​​composed of homopolymer polypropylene and block copolymer polypropylene in a weight ratio of 2:1, and the melt flow rate is 10g / 10min; the grafting rate of the polypropylene grafted maleic anhydride compatibilizer is 1.2%; the composite antioxidant is composed of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a weight ratio of 1:2.

[0029] The raw materials for preparing the phytic acid-chitosan composite microspheres include, by weight: 15 parts of chitosan with a degree of deacetylation of 90%, 40 parts of phytic acid aqueous solution with a mass concentration of 50%, 7.5 parts of glacial acetic acid aqueous solution with a mass concentration of 1%, 3.5 parts of Tween-80 emulsifier, and 2 parts of glutaraldehyde crosslinking agent; the phytic acid-chitosan composite microspheres have a volume average particle size of 12 μm, have a core-shell structure, with an inner phytic acid-enriched core layer and an outer chitosan shell layer that has been crosslinked and cured with glutaraldehyde.

[0030] The silane coupling agent modified cellulose nanocrystals were prepared by the following method: cellulose nanocrystals with a length of 200 nm and a diameter of 20 nm were dispersed in a mixed solvent of anhydrous ethanol and deionized water in a volume ratio of 5:1, with a dispersion concentration of 0.05 g / mL. The pH value was adjusted to 4.5 with glacial acetic acid, and γ-methacryloyloxypropyltrimethoxysilane accounting for 11.5% of the weight of the cellulose nanocrystals was added. The mixture was refluxed at 57.5 °C for 8 h, filtered, and washed three times with anhydrous ethanol. The mixture was then freeze-dried under vacuum at -45 °C and a vacuum degree of 8 Pa for 24 h to obtain the final product.

[0031] The modified high-performance polypropylene cable protection pipe has an inner layer, a middle layer, and an outer layer. The middle layer is composed of a blend of the aforementioned polypropylene matrix, phytic acid-chitosan composite microspheres, and silane coupling agent modified cellulose nanocrystals. Both the inner and outer layers are composed of pure polypropylene material. The thickness of the middle layer accounts for 67.5% of the total thickness of the pipe wall, and the thicknesses of the inner and outer layers each account for 16.25% of the total thickness of the pipe wall.

[0032] Its preparation method includes the following steps: S1. Dissolve 15 parts of chitosan with a degree of deacetylation of 90% in 7.5 parts of 1% glacial acetic acid aqueous solution, with a chitosan concentration of 0.035 g / mL. Add 3.5 parts of Tween-80 emulsifier and stir at 1000 rpm for 30 min until homogeneous. Then, add 40 parts of 50% phytic acid aqueous solution dropwise at a rate of 0.75 mL / min. After the addition is complete, raise the temperature to 50℃ and continue the reaction for 2.5 h. Add 2 parts of glutaraldehyde crosslinking agent and adjust the pH of the system to 6.0 with dilute sodium hydroxide solution. The crosslinking reaction is carried out for 5 h. After the reaction is completed, filter the solution and wash it with deionized water until the filtrate is neutral. Dry the solution under vacuum at 60℃ for 12 h to obtain phytic acid-chitosan composite microspheres. S2. Cellulose nanocrystals with a length of 200 nm and a diameter of 20 nm were dispersed in a mixed solvent of anhydrous ethanol and deionized water at a volume ratio of 5:1, with a dispersion concentration of 0.05 g / mL. The pH was adjusted to 4.5 with glacial acetic acid, and γ-methacryloyloxypropyltrimethoxysilane (11.5% by weight of cellulose nanocrystals) was added. The mixture was refluxed at 57.5 °C for 8 h. After the reaction was completed, the mixture was filtered, washed three times with anhydrous ethanol, and then freeze-dried under vacuum at -45 °C and 8 Pa for 24 h to obtain silane coupling agent modified cellulose nanocrystals. S3. Weigh the polypropylene matrix, phytic acid-chitosan composite microspheres, silane coupling agent modified cellulose nanocrystals, polypropylene grafted maleic anhydride compatibilizer, composite antioxidant and erucic acid amide lubricant according to the above weight percentages, place them in a high-speed mixer, and mix at 1200 rpm for 10 min to obtain the premix. S4. The premixed material is fed into a twin-screw extruder for melt blending and extrusion granulation. The temperature zones of the twin-screw extruder are set as follows: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 200℃, and Die Head Zone 205℃. The screw speed is 300 rpm. The resulting granules are extruded into pipes using a three-layer co-extrusion die. Pure polypropylene material is extruded to form the inner and outer layers, and the granules formed by melt blending and extrusion of the premixed material are extruded to form the middle layer. The extrusion temperature of the pipe extruder is set to 200℃, and the traction speed is 1 m / min. After extrusion, the material is cooled and shaped with 20℃ cooling water and then cut to obtain the modified high-performance polypropylene cable protection pipe.

[0033] Example 2: The only difference between this example and Example 1 is that the components and weight percentages of the modified high-performance polypropylene cable protection pipe are: polypropylene matrix 55%, phytic acid-chitosan composite microspheres 22%, silane coupling agent modified cellulose nanocrystals 14%, polypropylene grafted maleic anhydride compatibilizer 6%, composite antioxidant 1.5%, erucic acid amide lubricant 1.2%; the other conditions are the same as in Example 1.

[0034] Example 3: The only difference between this example and Example 1 is that the components and weight percentages of the modified high-performance polypropylene cable protection pipe are: 75% polypropylene matrix, 12% phytic acid-chitosan composite microspheres, 6% silane coupling agent modified cellulose nanocrystals, 2% polypropylene grafted maleic anhydride compatibilizer, 0.5% composite antioxidant, and 0.3% erucic acid amide lubricant; the other conditions are the same as in Example 1.

[0035] Example 4: The only difference between this example and Example 1 is that the raw materials for preparing the phytic acid-chitosan composite microspheres include, by weight, 10 parts of chitosan with a degree of deacetylation of 86%, 30 parts of phytic acid aqueous solution with a mass concentration of 50%, 5 parts of glacial acetic acid aqueous solution with a mass concentration of 1%, 2 parts of Tween-80 emulsifier, and 1 part of glutaraldehyde crosslinking agent; the volume average particle size of the phytic acid-chitosan composite microspheres is 5 μm; the other conditions are the same as in Example 1.

[0036] Example 5: The only difference between this example and Example 1 is that the raw materials for preparing the phytic acid-chitosan composite microspheres include, by weight, 20 parts of chitosan with a degree of deacetylation of 92%, 50 parts of phytic acid aqueous solution with a mass concentration of 50%, 10 parts of glacial acetic acid aqueous solution with a mass concentration of 1%, 5 parts of Tween-80 emulsifier, and 3 parts of glutaraldehyde crosslinking agent; the volume average particle size of the phytic acid-chitosan composite microspheres is 20 μm; the other conditions are the same as in Example 1.

[0037] Example 6: The only difference between this example and Example 1 is that the length of the cellulose nanocrystals is 100 nm and the diameter is 10 nm; the amount of silane coupling agent γ-methacryloxypropyltrimethoxysilane added is 8% of the weight of the cellulose nanocrystals; the other conditions are the same as in Example 1.

[0038] Example 7: The only difference between this example and Example 1 is that the length of the cellulose nanocrystals is 300 nm and the diameter is 30 nm; the amount of silane coupling agent γ-methacryloxypropyltrimethoxysilane added is 15% of the weight of the cellulose nanocrystals; the other conditions are the same as in Example 1.

[0039] Example 8: The only difference between this example and Example 1 is that the grafting rate of the polypropylene grafted with maleic anhydride compatibilizer is 0.8%; the other conditions are the same as in Example 1.

[0040] Example 9: The only difference between this example and Example 1 is that the grafting rate of the polypropylene grafted with maleic anhydride compatibilizer is 1.5%; the other conditions are the same as in Example 1.

[0041] Example 10: The only difference between this example and Example 1 is that the screw speed of the twin-screw extruder in step S4 is 250 rpm; the other conditions are the same as in Example 1.

[0042] Example 11: The only difference between this example and Example 1 is that the traction speed of the pipe extruder in step S4 is 0.5 m / min; the other conditions are the same as in Example 1.

[0043] Example 12: The only difference between this example and Example 1 is that the phytic acid aqueous solution is added at a rate of 1 mL / min in step S1; the other conditions are the same as in Example 1.

[0044] Comparative Example 1: The only difference between this comparative example and Example 1 is that the phytic acid-chitosan composite microsphere component is removed, and its weight percentage is made up by the polypropylene matrix, i.e., the polypropylene matrix is ​​82%. The remaining components, weight percentages, and preparation methods are the same as in Example 1.

[0045] Comparative Example 2: This comparative example uses conventional inorganic flame retardant modified polypropylene cable protection pipes. Its components and weight percentages are as follows: 70% polypropylene matrix, 25% magnesium hydroxide flame retardant, 3% polypropylene grafted maleic anhydride compatibilizer, 1% composite antioxidant, and 1% erucic acid amide lubricant. The polypropylene matrix is ​​the same as in Example 1. The preparation method is as follows: after the components are mixed evenly in proportion, they are melt-blended and extruded into granules by a twin-screw extruder, and then extruded into single-layer pipes by a pipe extruder. The extrusion process parameters are the same as in Example 1.

[0046] Comparative Example 3: The only difference between this comparative example and Example 1 is that the dropping rate of the phytic acid aqueous solution in step S1 is 2 mL / min, which exceeds the limit range; the other conditions are the same as in Example 1.

[0047] Comparative Example 4: The only difference between this comparative example and Example 1 is that the silane coupling agent modification step is omitted, and unmodified cellulose nanocrystals are used directly instead of silane coupling agent modified cellulose nanocrystals; the other conditions are the same as in Example 1.

[0048] Test method: Limiting oxygen index test: The test shall be conducted in accordance with GB / T2406.2-2009 standard, and the sample size shall be 150mm×6.5mm×3mm.

[0049] Vertical burning test: The test shall be conducted in accordance with the standard GB / T2408-2008, and the sample size shall be 125mm×13mm×3mm.

[0050] Tensile property test: The test was conducted in accordance with GB / T1040.2-2006 standard, with a tensile rate of 50 mm / min, and the elongation at break and tensile strength were tested.

[0051] Impact strength test: The notched impact strength test of the cantilever beam was carried out in accordance with GB / T1843-2008 standard. The sample size was 80mm×10mm×4mm and the notch depth was 2mm.

[0052] Carbon layer performance test: The sample was calcined in a muffle furnace at 800℃ for 10 min, and the carbon layer thickness and carbon layer residue rate were tested.

[0053] Test results: Table 1 Performance test results of each embodiment and comparative example ; Results analysis: Examples 1-12 all exhibited excellent halogen-free flame retardant properties, with limiting oxygen indices ranging from 29.8% to 33.5%, vertical burning ratings reaching V-0, char layer thicknesses between 2.8 mm and 3.5 mm, and char layer residue rates between 19.7% and 23.2%. Comparative Example 1, lacking phytic acid-chitosan composite microspheres, failed to form an effective expanded char layer, resulting in a limiting oxygen index of only 21.7%, a vertical burning rating of only V-2, and a char layer residue rate of only 3.2%. Comparative Example 2, using a traditional magnesium hydroxide flame retardant, achieved a limiting oxygen index of 28.3%, but its vertical burning rating was only V-1, and the char layer was loose and easily detached. This indicates that the mechanism by which phytic acid-chitosan composite microspheres form a dense, expanded insulating layer through phytic acid-catalyzed in-situ carbonization of chitosan is significantly superior to the endothermic dilution mechanism of traditional inorganic flame retardants.

[0054] The elongation at break of Examples 1-12 were all between 329% and 365%, and the impact strength was all between 11.4 kJ / m. 2 ~12.7kJ / m 2 In comparison, while maintaining good flame retardant properties, excellent toughening effects were achieved. Comparative Example 2, using a high proportion of magnesium hydroxide filler, resulted in numerous stress concentration points within the material, leading to an elongation at break of only 78% and an impact strength of only 4.2 kJ / m². 2 The toughness decreased drastically upon fracture. Comparative Example 4 used unmodified cellulose nanocrystals, which, due to poor interfacial bonding with the polypropylene matrix, were prone to aggregation and debonding, resulting in an elongation at break of only 148% and an impact strength of only 5.8 kJ / m². 2 This indicates that silane coupling agent-modified cellulose nanocrystals effectively eliminate stress concentration points by forming a three-dimensional stress transfer network in the matrix and forming a core-shell coating structure with phytic acid-chitosan composite microspheres, achieving a synergistic balance between flame retardancy and toughening.

[0055] Examples 10-12, with adjustments made to the key parameters of the preparation method within the defined limits, all performance characteristics remained at a high level, indicating that the preparation method of the present invention has good robustness and universality. Comparative Example 3 increased the dropping rate of the phytic acid aqueous solution to 2 mL / min, exceeding the defined range, resulting in excessively large and unevenly distributed phytic acid-chitosan composite microspheres, leading to localized stress concentration, a decrease in elongation at break to 221%, and a decrease in impact strength to 7.5 kJ / m². 2 Meanwhile, the flame retardant properties also decreased, with the vertical flammability rating dropping to V-1. This indicates that strictly controlling the dripping rate of the phytic acid aqueous solution is crucial to ensuring the uniformity of the microspheres and the performance of the final product.

[0056] This invention achieves a dual enhancement of flame retardancy and toughening through the synergistic effect of phytic acid-chitosan composite microspheres and silane coupling agent-modified cellulose nanocrystals. The phytic acid-chitosan composite microspheres provide the primary flame retardant function, while the silane coupling agent-modified cellulose nanocrystals not only provide toughening but also form a rigid framework to support the expanding char layer during combustion, preventing char layer collapse and further improving the flame retardant effect. Polypropylene grafted with maleic anhydride compatibilizer is enriched at the interface between the core-shell coating structure and the matrix, enhancing interfacial bonding and inhibiting the debonding of the composite microspheres under stress, further improving the material's mechanical properties. The three-layer gradient structure design, while ensuring the corrosion and wear resistance of the pipe base, concentrates the flame retardant and toughening functions of the modified blend, achieving optimized distribution of the pipe's radial properties.

Claims

1. A modified high-performance polypropylene cable protection pipe, characterized in that, It consists of the following components by weight percentage: Polypropylene matrix 55%~75%, phytic acid-chitosan composite microspheres 12%~22%, silane coupling agent modified cellulose nanocrystals 6%~14%, polypropylene grafted maleic anhydride compatibilizer 2%~6%, composite antioxidant 0.5%~1.5%, erucic acid amide lubricant 0.3%~1.2%; The phytic acid-chitosan composite microspheres are formed by the electrostatic self-assembly reaction of phytic acid and chitosan. The silane coupling agent modified cellulose nanocrystals are prepared by surface grafting modification of cellulose nanocrystals with a silane coupling agent. The polypropylene matrix is ​​composed of homopolymer polypropylene and block copolymer polypropylene in a weight ratio of 2:

1.

2. The modified high-performance polypropylene cable protection pipe according to claim 1, characterized in that, The raw materials for preparing the phytic acid-chitosan composite microspheres include, by weight: 10-20 parts of chitosan with a degree of deacetylation greater than 85%, 30-50 parts of phytic acid aqueous solution with a mass concentration of 50%, 5-10 parts of glacial acetic acid aqueous solution with a mass concentration of 1%, 2-5 parts of Tween-80 emulsifier, and 1-3 parts of glutaraldehyde crosslinking agent. The volume average particle size distribution of the phytic acid-chitosan composite microspheres is 5 μm to 20 μm; The phytic acid-chitosan composite microspheres have a core-shell structure, with an internal phytic acid-rich core layer and an external chitosan shell layer that has been cross-linked and cured by the glutaraldehyde cross-linking agent.

3. The modified high-performance polypropylene cable protection pipe according to claim 1, characterized in that, The silane coupling agent modified cellulose nanocrystals were prepared by the following method: Cellulose nanocrystals were dispersed in a mixed solvent of anhydrous ethanol and deionized water, and the pH was adjusted to 4-5. γ-methacryloyloxypropyltrimethoxysilane, accounting for 8%-15% of the weight of the cellulose nanocrystals, was added. The mixture was refluxed at 50℃-65℃ for 6-10 hours. The mixture was then filtered, washed with anhydrous ethanol, and freeze-dried under vacuum to obtain the final product. The cellulose nanocrystals have a length of 100nm~300nm and a diameter of 10nm~30nm.

4. The modified high-performance polypropylene cable protection pipe according to claim 1, characterized in that, The melt flow rate of the polypropylene matrix is ​​5 g / 10 min to 15 g / 10 min; The grafting rate of the polypropylene grafted with maleic anhydride compatibilizer is 0.8%~1.5%; The composite antioxidant is composed of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a weight ratio of 1:2; The polypropylene grafted maleic anhydride compatibilizer is enriched in the interfacial region between the polypropylene matrix and the silane coupling agent modified cellulose nanocrystals.

5. The modified high-performance polypropylene cable protection pipe according to claim 1, characterized in that, In the microstructure of the modified high-performance polypropylene cable protection pipe, the phytic acid-chitosan composite microspheres are uniformly dispersed inside the polypropylene matrix as a dispersed phase, and the silane coupling agent modified cellulose nanocrystals are interpenetrated and coated around the phytic acid-chitosan composite microspheres, forming a core-shell coating structure with the phytic acid-chitosan composite microspheres as the core and the silane coupling agent modified cellulose nanocrystal network as the shell. The polypropylene grafted maleic anhydride compatibilizer is enriched at the interface between the core-shell coating structure and the polypropylene matrix.

6. The modified high-performance polypropylene cable protection pipe according to claim 1, characterized in that, The modified high-performance polypropylene cable protection pipe has an inner layer, a middle layer and an outer layer. The middle layer is composed of a blend of the polypropylene matrix, the phytic acid-chitosan composite microspheres and the silane coupling agent modified cellulose nanocrystals. Both the inner layer and the outer layer are made of pure polypropylene material; The thickness of the intermediate layer accounts for 60% to 75% of the total wall thickness of the modified high-performance polypropylene cable protection pipe, and the thickness of the inner layer and the outer layer each account for 12.5% ​​to 20% of the total wall thickness.

7. A method for preparing a modified high-performance polypropylene cable protection pipe, characterized in that, Includes the following steps: S1. Chitosan was dissolved in glacial acetic acid aqueous solution, emulsifier was added and stirred, then phytic acid aqueous solution was added dropwise for electrostatic self-assembly, crosslinking agent was added for curing, and phytic acid-chitosan composite microspheres were obtained by filtration, washing and drying. S2. Disperse cellulose nanocrystals in an aqueous ethanol solution, adjust the pH to acidic, add silane coupling agent and reflux the reaction, filter, wash and dry to obtain silane coupling agent modified cellulose nanocrystals. S3. The polypropylene matrix, the phytic acid-chitosan composite microspheres, the silane coupling agent modified cellulose nanocrystals, compatibilizer, antioxidant and lubricant are placed in a high-speed mixer and mixed evenly to obtain a premix. S4. The premixed material is fed into a twin-screw extruder for melt blending and granulation. The resulting granules are extruded and formed by a pipe extruder, cooled and shaped to obtain the modified high-performance polypropylene cable protection pipe.

8. The method for preparing a modified high-performance polypropylene cable protection pipe according to claim 7, characterized in that, In step S1, the mass concentration of the glacial acetic acid aqueous solution is 1%~2%, and the concentration of chitosan in the glacial acetic acid aqueous solution is 0.02g / mL~0.05g / mL; The specific procedure for adding the phytic acid aqueous solution is as follows: Under mechanical stirring at 800 rpm to 1200 rpm, the phytic acid aqueous solution was added dropwise to the chitosan solution at a rate of 0.5 mL / min to 1 mL / min. After the addition was completed, the temperature was raised to 45℃ to 55℃ and the reaction continued for 2 to 3 hours. The crosslinking agent is glutaraldehyde, and after its addition, the pH value of the reaction system is maintained between 5.5 and 6.5, and the crosslinking reaction time is 4h to 6h.

9. The method for preparing a modified high-performance polypropylene cable protection pipe according to claim 7, characterized in that, In step S2, the volume ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 4:1 to 6:1, and the dispersion concentration of the cellulose nanocrystals in the ethanol aqueous solution is 0.03 g / mL to 0.08 g / mL. The acidifying agent used to adjust the pH value to 4-5 is glacial acetic acid. The silane coupling agent is γ-methacryloxypropyltrimethoxysilane, and its addition amount is 8% to 15% of the weight of the cellulose nanocrystals. The reflux reaction is carried out at a temperature of 50℃~65℃ for 6h~10h, and the vacuum freeze-drying temperature is -40℃~-50℃ with a vacuum degree of less than 10Pa.

10. The method for preparing a modified high-performance polypropylene cable protection pipe according to claim 7, characterized in that, In step S4, the temperatures of each temperature zone of the twin-screw extruder are set as follows: Zone 1: 160℃~170℃, Zone 2: 175℃~185℃, Zone 3: 185℃~195℃, Zone 4: 195℃~205℃, and the head zone: 200℃~210℃. The screw speed of the twin-screw extruder is 250 rpm to 350 rpm; The pipe extruder uses a three-layer co-extrusion die to extrude pure polypropylene material to form the inner and outer layers, and to extrude the premixed material into granules through melt blending and extrusion to form the middle layer. The extrusion temperature of the pipe extruder is set to 190℃~210℃, and the traction speed is 0.5m / min~1.5m / min.