Polypropylene cable protection pipe and preparation method thereof

Abstract

The invention relates to the technical field of polypropylene cable protection pipes, and discloses a polypropylene cable protection pipe and a preparation method thereof. The polypropylene cable protection tube comprises polypropylene random copolymer, an ethylene-octylene copolymer elastomer, maleic anhydride grafted polypropylene, a composite filler, microencapsulated ammonium polyphosphate, melamine cyanurate, a beta crystal form nucleating agent, a hindered phenol / phosphite antioxidant, a hindered amine light stabilizer, high-structure conductive carbon black and amino-terminated polydimethylsiloxane. Wherein the composite filler is of a core-shell-lamellar structure with hollow glass beads as a core, silicon dioxide as a middle layer and a hexagonal boron nitride lamellar layer as an outer layer. In the forming process, the beta crystal form of the outer layer is enriched relative to the inner layer through differential temperature of the outer lip and the inner core, segmented spraying and micro-amplitude traction oscillation. The material has wear resistance, heat conduction, halogen-free flame retardance and antistatic performance.

Description

Technical Field

[0001] This invention relates to the field of polypropylene cable protection pipe technology, and more specifically, to a polypropylene cable protection pipe and its preparation method. Background Technology

[0002] Cable protection conduits are used for the laying and protection of power and communication cables, and must simultaneously meet multiple constraints such as ring stiffness, impact resistance, abrasion resistance, chemical resistance, flame retardancy, antistatic properties, insulation, dimensional stability, and workability. Currently used materials include PVC-U, PE, and PP: PVC-U has a high modulus but contains chlorine and has significant plasticizer issues; PE has good low-temperature toughness but insufficient modulus and creep resistance, requiring thick walls to meet standards; PP combines high specific modulus, low density, and extrusion efficiency, making it suitable for lightweighting and large-scale production, but it has shortcomings such as low-temperature brittleness, poor inherent flame retardancy, and static electricity accumulation.

[0003] To overcome the above problems, common existing technologies include: using inorganic powders (CaCO3, talc, mica) or fibers to reinforce stiffness; using elastomers (POE / EOC) to toughen and reduce low-temperature brittleness; using intumescent flame-retardant systems (APP / MCA) to achieve halogen-free flame retardancy; using conductive carbon black or other conductive phases for antistatic purposes; using thermally conductive fillers (BN, AlN) to improve heat dissipation; using β-nucleating agents to regulate PP crystallization and dimensional stability; and using multilayer co-extrusion to distribute functions in the outer / inner layers. However, the following problems are still commonly encountered in production practice: Conventional rigid powders increase density while improving modulus, leading to cut sensitivity; hollow fillers, while reducing density, suffer from particle breakage and interfacial debonding, making it difficult to balance ring stiffness and impact stability; fiber reinforcement introduces anisotropy and processing appearance issues. APP's moisture absorption and migration cause mechanical degradation, processing precipitation, and fluctuations in flame retardancy; formulation stability decreases after contact with amine salts / metal ions, and performance retention is insufficient after water immersion and thermal cycling. High thermal conductivity fillers such as BN / AlN are prone to agglomeration and significant viscosity increase, affecting extrusion stability and appearance; requirements for scratch and abrasion resistance on the outer surface are increased, but simply adding hard phases exacerbates embrittlement and processing defects. Single-path cooling or fixed process parameters easily cause uneven core-shell crystallization, internal stress, and dimensional fluctuations; β-crystal content and orientation are difficult to distribute as needed, making it difficult to simultaneously optimize outer layer scratch resistance and inner layer toughness. The strong shear of twin-screw extruders and improper feeding positions can lead to breakage of hollow glass microspheres, density drift, and appearance defects; unstable sizing and cooling windows cause wall thickness fluctuations and out-of-tolerance ellipticity.

[0004] Therefore, the industry urgently needs a PP-based material for cable protection pipes. Summary of the Invention

[0005] In view of this, the present invention proposes a polypropylene cable protection pipe and its preparation method, aiming to solve the problems existing in the above content.

[0006] The present invention proposes a polypropylene cable protection pipe, which has a double-layer co-extruded structure, including an outer skin layer and an inner layer, and the outer skin layer and the inner layer are made of materials with the same formula. The formulation, by weight, comprises: 65-85 parts random copolymer polypropylene, 5-15 parts ethylene-octene copolymer elastomer, 2-6 parts polypropylene grafted maleic anhydride, 8-25 parts composite filler, 8-18 parts microencapsulated ammonium polyphosphate, 2-8 parts melamine cyanurate, 0.05-0.30 parts β-crystal nucleating agent, 0.2-0.8 parts hindered phenol and phosphite compound antioxidant, 0.1-0.5 parts hindered amine light stabilizer, 0.1-0.8 parts high-structure conductive carbon black, and 0.3-2.5 parts amino-terminated polydimethylsiloxane. Among them, the composite filler is a core-shell-sheet structured particle with hollow glass microspheres as the core layer, silica as the middle layer, and hexagonal boron nitride sheets as the outer layer; The ratio of β-crystal content in the outer layer to that in the inner layer is ≥1.8.

[0007] Furthermore, the median particle size of the hollow glass microspheres in the composite filler is 15–60 μm; the thickness of the silica intermediate layer is 50–300 nm and the coverage is ≥90%; the thickness of the hexagonal boron nitride sheet is 30–200 nm, the aspect ratio is ≥50 and the surface coverage is ≥60%.

[0008] Furthermore, the silica intermediate layer of the composite filler is grafted with bifunctional silanes containing epoxy and alkenyl groups; the grafting rate of maleic anhydride onto polypropylene is 0.5% to 1.5%; and the number average molecular weight of amino-terminated polydimethylsiloxane is 3×10³ to 3×10⁴.

[0009] Furthermore, the microencapsulated ammonium polyphosphate has a siloxane-silica gradient shell with a shell mass fraction of 5% to 15% and a particle size of 5 to 25 μm; the mass ratio of melamine cyanurate to microencapsulated ammonium polyphosphate is (1 to 3): 5.

[0010] Furthermore, the conductive carbon black has a DBP oil absorption value ≥300mL / 100g; the hindered phenol and phosphite compound antioxidant is composed of hindered phenol 1010 or 1076 and phosphite 168 or 626; the hindered amine light stabilizer is a piperidine compound.

[0011] On the other hand, the present invention provides a method for preparing the above-mentioned polypropylene cable protection tube, comprising the following steps: a) Preparation of composite filler: Hollow glass microspheres were washed with 0.1-0.5 mol / L hydrochloric acid for 15-30 min, then washed with deionized water until the conductivity was ≤50 μS / cm, and dried at 110℃ for 2-4 h; a solvent was prepared with ethanol:water = 80:20-60:40 (volume fraction), and tetraethoxysilane was added to make the target silica loading amount 5-15% based on the mass of hollow glass microspheres. The pH was adjusted to 9.5-11.0 with ammonia water, and the reaction was carried out at 25-35℃ for 30-120 min. After filtration, washing, and drying at 80-100℃, hollow glass microspheres@silica were obtained. Dissolve 1.0–3.0% of a bifunctional silane with epoxy and alkenyl groups in ethanol:water at a volume ratio of 95:5. Adjust the pH to 4.5–5.5 with glacial acetic acid, react at 60–80°C for 0.5–2.0 h, and dry at 100–120°C for 1–3 h to complete surface silanization and obtain silanized particles. Hydroxylated or carboxylated hexagonal boron nitride is dispersed in an alcohol or alcohol / toluene mixed solvent with a solid content of 0.5-2.0%, and ultrasonically dispersed at 200-400W for 10-30 min. Silanized particles are added, and the mixture is stirred at 60-85℃ for 1-3 h. After filtration, the mixture is dried at 80-100℃ until the moisture content is ≤0.20% to obtain the composite filler.

[0012] b) Preparation of microencapsulated flame retardant: Ammonium polyphosphate is prepared into a suspension of 10-30% by mass, and silica sol with a silica content of 30-40% by mass is added so that the silica inner layer contains 3-10% by mass of ammonium polyphosphate. The pH is adjusted to 8.5-9.2 with ammonia water and stirred at 35-45℃ for 30-60 min. Based on ammonium polyphosphate, 0.5–3.0% by mass of methyltriethoxysilane was added dropwise and condensed at 45–60°C for 60–120 min. After solid-liquid separation, washing with deionized water, drying at 90–110°C for 2–6 h, and passing through an 80–200 mesh sieve, microencapsulated ammonium polyphosphate was obtained.

[0013] c) Raw material premixing: Weigh random copolymer polypropylene, ethylene-octene copolymer elastomer, microencapsulated ammonium polyphosphate, melamine cyanurate, β-crystal nucleating agent, hindered phenolic antioxidant, phosphite antioxidant, hindered amine light stabilizer and conductive carbon black according to the target ratio, mix in a drum for 5-10 minutes, and control the material moisture content to ≤0.05%.

[0014] d) Blending and granulation: A co-rotating twin-screw extruder with an L / D ≥ 44 was used. The barrel temperature zones were set to 170 / 175 / 180 / 185 / 190 / 195 / 195℃. Random copolymer polypropylene and polypropylene-grafted maleic anhydride were first added to the main feed port and sheared in the machine for 60–90 s. Amino-terminated polydimethylsiloxane and the composite filler obtained in step a) were simultaneously added to the side feed ports in zones 5–6. Subsequently, in zones 6–7… Microencapsulated ammonium polyphosphate, melamine cyanurate, hindered phenol / phosphite antioxidant, hindered amine light stabilizer and β-crystal nucleating agent are added; screw speed is 200-260 r / min, vacuum degree of vacuum exhaust port in zone 7 is -0.06 to -0.08 MPa, melt specific energy consumption is controlled ≤0.30 kWh / kg, and granulation is carried out by water bath stretching at 15-25℃, with particle length of 2-4 mm, and hot air drying at 80-90℃ for 2-4 h.

[0015] e) Double-layer co-extrusion molding: The temperature of the outer extruder barrel / die head is 200-220℃, the temperature of the inner extruder barrel / die head is 190-210℃, and the temperature of the outer lip of the co-extrusion die is 10-25℃ higher than that of the inner core; the vacuum degree of the vacuum sizing box is -0.05 to -0.08MPa; the material passes through three spray stages and two air knives in sequence, with the water temperature of the first stage being 10-15℃, the second stage being 18-25℃, and the third stage being 25-32℃, and the pressure of the two air knives being 0.10-0.30MPa; the traction speed and extrusion volume are matched to make the draw ratio 1.05-1.25, and a micro-oscillation of 3-15Hz with an amplitude of 0.3%-1.5% of the linear velocity is superimposed.

[0016] f) Post-processing: After extrusion molding, the material is subjected to corona treatment with electrode rods with a gap of 2-8mm. The corona voltage is 6-12kV and the frequency is 10-30kHz. After cooling to 15-25℃, the material is cut to length and wound up.

[0017] Further, in step a), the mass ratio of tetraethoxysilane to hollow glass microspheres is 0.08:1 to 0.30:1, the bifunctional silane used for silanization is a mixture of epoxypropoxysilane and vinylsilane, with a mass ratio of 1:1 to 3:1; the mass ratio of hexagonal boron nitride to hollow glass microspheres is 0.05:1 to 0.40:1, and the ultrasonic dispersion power density is 20 to 60 W / L.

[0018] Furthermore, in step b), the silica sol is added by constant-rate dripping for 15–45 min; the hydrolysis molar ratio of methyltriethoxysilane is 3:1–6:1; the stirring speed during the condensation stage is 300–600 r / min; and the temperature is increased to the target temperature and maintained at 1–2 °C / min during the drying process; the D50 of the microencapsulated ammonium polyphosphate product is 10–25 μm.

[0019] Furthermore, in step d), the side feed port is set in a low melting pressure zone before the metering section, the side feed rotor speed is 10-40 r / min, and the screw element adopts a combination of fewer kneading blocks, 30°-45° kneading plates and shallow screw grooves to reduce the breakage of hollow glass microspheres. In step e), the length of the vacuum sizing box is 1.5 to 3.0 m, and each of the three spray sections is 1.0 to 2.5 m. The traction machine adopts servo drive to switch the oscillation frequency and amplitude in an S-shaped acceleration and deceleration mode. In step f), the corona section is equipped with ozone extraction and thermal management, and the electrode-product distance is maintained at 2-8 mm online.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: This solution achieves a balance between lightweight and circumferential load-bearing capacity through a composite filler of hollow glass microspheres@SiO2@h-BN. The microspheres act as a low-density rigid core to increase sectional inertia, while the SiO2 intermediate layer provides a high-modulus transition and a rough interface to enhance mechanical interlocking. The h-BN sheets form a thin-shell micro-skeleton on the outer surface to bear surface shear and scratch loads. Side feeding into a low-melting-pressure zone and the use of low-shear elements reduce microsphere breakage and density drift. The principle lies in the sandwich structure and effective medium theory: the rigid core and thin shell synergistically share bending and torsional loads without significantly increasing the volume fraction of the hard phase.

[0021] This scheme suppresses low-temperature interfacial embrittlement through blending and a three-point bridging interface. First, PP-g-MAH reacts with PDMS-NH2 via an amine-anhydride reaction to form a soft segment bridge. Then, it undergoes ring-opening and Si-O-Si condensation with the epoxy / silanol sites on the surface of the composite filler, forming a continuous transition interface of "matrix-soft segment-inorganic". The principle lies in reducing interfacial energy and increasing the energy dissipation path at the crack tip, making the crack tip more inclined to deflect and passivate rather than penetrate directly.

[0022] This scheme utilizes a combination of bilayer co-extrusion, differential temperature cooling, and micro-amplitude traction oscillation to directionally enrich the β-phase in the outer layer. The temperature difference between the outer lip and the inner core provides asymmetric cooling, while three-stage spraying creates a "cold-warming-stable cooling" crystallization process. Superimposed low-amplitude frequency vibration generates a stable shear field, inducing the formation of the β-phase and finer grains in the outer layer, while slow cooling of the inner layer maintains toughness. The principle lies in flow-induced crystallization and competitive nucleation kinetics: oriented chain segments and the warming window together increase the probability of β-phase nucleation and refine the grains.

[0023] This solution enhances the water resistance and migration resistance of the flame-retardant system through the silicon-oxygen gradient shell of microcapsules. The inner inorganic SiO2 network restricts the outward migration of small molecules, while the outer organosilicon layer reduces surface polarity and improves compatibility with polypropylene; when used in conjunction with MCA, it forms a more stable expanded char layer framework. The principle lies in diffusion shielding and interfacial energy modulation: the outer shell alters solvation parameters, and the inner shell provides a dense inorganic barrier, thereby stabilizing the condensed phase flame-retardant mechanism.

[0024] This solution establishes a controllable boundary between antistatic and insulating properties through high-structure conductive carbon black followed by corona treatment. The carbon black, with its high specific oil absorption value, forms a sparse, near-percolation contact network, while the corona treatment introduces shallow polarization sites on the surface to dissipate charge. h-BN serves as an electrically insulating and thermally conductive phase, preventing conductive pathways from penetrating the matrix. The principle lies in percolation theory and a surface charge dissipation model: surface channels handle the discharge, while the bulk phase maintains high resistance.

[0025] This solution combines thermal conductivity and wear resistance by coating a SiO2 shell with h-BN sheets, while also limiting melt viscosity. The sheets are anchored to the outermost layer of the particles, overlapping to form in-plane thermal channels and acting as microscale "armor" to share friction. Because h-BN does not fill the matrix with a high volume fraction, the overall rheological window of the system remains machinable. The principle lies in reducing interfacial thermal resistance and optimizing the distribution of hard points in the friction pair.

[0026] This method employs a pretreatment chain involving acid washing and low-conductivity cleaning, uniform sol-gel deposition, and silanization anchoring to obtain a dense, bonded SiO2 interlayer. This process removes metal salts and alkaline residues, stabilizes the TEOS hydrolysis-condensation rate, and ensures shell continuity. Sufficient silanization allows h-BN to be anchored surface-to-surface rather than adsorbed at points. The underlying principle lies in the homogenization of sol-gel nucleation and the control of surface chemical site density.

[0027] This solution achieves a comprehensive approach to dispersion, devolatilization, and protection by combining side-feeding location, screw elements, and vacuum exhaust. The composite packing enters a low-pressure, low-shear zone to avoid shear breakage, while the subsequent vacuum removes low-molecular-weight substances and moisture, controlling melt energy consumption and reducing gas streaks and pinholes. The principle lies in matching the local energy density with the viscoelastic spectrum, avoiding crossing the critical shear level for particle breakage.

[0028] This solution improves the interfacial behavior during subsequent processing and service by modulating the corona discharge of the outer surface states. The corona discharge introduces controllable polar groups, enhancing ink adhesion and marking clarity, and synergistically reduces the tendency for electrostatic dust adsorption in conjunction with the conductive contact network. The principle lies in the increased wettability and charge dissipation rate resulting from the enhanced surface free energy and polar components. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0030] Example 1 The batch size is 100kg of special material. The formula converted to mass is as follows: random copolymer polypropylene 59.88kg, ethylene-octene copolymer elastomer 7.984kg, polypropylene grafted maleic anhydride 3.194kg, composite filler 14.371kg, microencapsulated ammonium polyphosphate 9.581kg, melamine cyanurate 3.194kg, β-crystal nucleating agent 0.120kg, hindered phenolic antioxidant (1010) 0.200kg, phosphite antioxidant (168) 0.120kg, hindered amine light stabilizer 0.160kg, high-structure conductive carbon black 0.399kg, amino-terminated polydimethylsiloxane 0.798kg.

[0031] The composite filler was prepared based on a target yield of 14.371 kg. 11.048 kg of hollow glass microspheres (D50≈30 μm, true density ≤0.6 g / cm³) were selected and washed with 0.1 mol / L hydrochloric acid solution (liquid-to-solid ratio approximately 5 L / kg) for 20 min with stirring. The microspheres were then rinsed with deionized water until conductivity ≤50 μS / cm and dried with hot air at 110℃ for 3 h. Approximately 60 L of a 70 / 30 (volume ratio) ethanol / water solvent was prepared, and 3.83 kg of tetraethoxysilane (corresponding to approximately 1.105 kg of deposited SiO₂) was added. The pH was adjusted to 10.5 with ammonia, and the mixture was mechanically stirred at 30℃ for 60 min to form a sol-gel deposition. After filtration, washing with water, and drying at 90℃ for 4 h, “microspheres@SiO₂” was obtained. Subsequently, it was dispersed in an ethanol / water = 95 / 5 system (liquid-solid ratio 6 L / kg), and a total of 0.331 kg of a mixture of glycidyl oxysilane (GPTMS) and vinyltriethoxysilane (VTES) (GPTMS:VTES = 2:1, approximately 3 wt% of HGM mass) was added. The pH was adjusted to 5.0 with glacial acetic acid, and the mixture was reacted at 70 °C for 1 h, and dried at 110 °C for 2 h to obtain silanized particles. 2.210 kg of hydroxylated hexagonal boron nitride (mass ratio of HGM 0.2:1) was taken and a dispersion containing 1.0 wt% solids was prepared in approximately 220 L of ethanol or ethanol / toluene (1:1, volume). The dispersion was ultrasonically dispersed at 300 W for 20 min. The silanized particles were added, and the mixture was stirred at 70 °C for 2 h. After filtration, the mixture was dried at 90 °C until the moisture content was ≤0.20%, yielding 14.371 kg of the target composite filler (≈HGM+SiO2+h-BN).

[0032] The microencapsulated ammonium polyphosphate was calculated based on a target yield of 9.581 kg. Using 8.912 kg of APP core as a baseline, approximately 44.6 L of a 20 wt% suspension was prepared with deionized water. 1.783 kg of silica sol (equivalent to 6 wt% SiO2 core mass) with a silica content of 30 wt% was added dropwise at a constant rate. The pH was 8.9, and the mixture was stirred at 40°C for 45 min to form an inner inorganic shell. Subsequently, 0.134 kg of methyltriethoxysilane (MTES) (approximately 1.5 wt% core mass) was added dropwise, and condensation was performed at 55°C for 90 min. Solid-liquid separation was followed, and the mixture was washed with deionized water until no free ions remained. The mixture was then dried at 100°C for 4 h, passed through a 120-mesh sieve, and D50 was controlled at 10-25 μm to obtain 9.581 kg of microencapsulated APP.

[0033] Raw material premixing was performed using a rotary drum. 59.88 kg of PP, 7.984 kg of EOC, 9.581 kg of microcapsule APP, 3.194 kg of MCA, 0.120 kg of β-nucleating agent, 0.200 kg of AO-1010, 0.120 kg of AO-168, 0.160 kg of HALS, and 0.399 kg of conductive carbon black were added to the rotary drum and mixed for 8 minutes. The material moisture content was controlled to ≤0.05 wt% using an online moisture meter. 14.371 kg of composite filler and 0.798 kg of PDMS-NH2 were prepared separately.

[0034] Melt blending and granulation utilize a co-rotating parallel twin-screw extruder with an L / D ratio of 48 and barrel temperature zones of 170 / 175 / 180 / 185 / 190 / 195 / 195℃. PP and PP-g-MAH (3.194 kg added concurrently) are continuously fed into the main feed inlet, with a screw speed of 240 r / min and an average residence time of approximately 75 s in the initial stage to promote anhydride activation. Simultaneously, 0.798 kg of PDMS-NH2 and 14.371 kg of composite filler are added through the side feed inlet of the fifth zone. The metering inlet of the sixth zone incorporates microcapsules of APP, MCA, AO, HALS, and a β-nucleating agent. The seventh zone is evacuated to -0.07 MPa to remove moisture and small molecules, with specific energy consumption monitored to be ≤0.30 kWh / kg. The extruded material is cooled in a 15℃ water bath, granulated to 3 mm, and dried with hot air at 80-90℃ for 3 hours to obtain 100 kg of the special material.

[0035] The double-layer co-extrusion molding process utilizes an outer Φ65 / 33 extruder and an inner Φ45 / 33 extruder. The co-extrusion die is equipped with independently heated outer lip and inner core. The outer barrel / die temperature is 210-215℃, and the inner temperature is 200-205℃, with the outer lip temperature being 15℃ higher than the inner core temperature. The vacuum sizing box has a vacuum degree of -0.07MPa, three-stage spray water temperatures of 12 / 22 / 28℃, two air knives at 0.2MPa, a traction ratio of 1.12, and is superimposed with an 8Hz micro-oscillation with an amplitude of 0.8% of the linear velocity. Taking a 110mm outer diameter and 6.6mm wall thickness as an example, continuous extrusion is performed at a linear velocity of 6-8m / min.

[0036] Post-processing involves corona treatment before winding. A rod electrode-grounding roller structure is used, with an electrode gap of 4 mm, a voltage of 9 kV, and a frequency of 20 kHz. After one revolution of treatment, the tube is naturally cooled to 20 °C, then cut to length (6 m / tube) and wound up. The corona treatment section is equipped with ozone extraction and thermal management to maintain the electrode-product distance within 4 ± 1 mm online. Through the above steps, a double-layer co-extruded polypropylene cable protection tube sample is obtained.

[0037] Example 2 The difference between Example 2 and Example 1 is that the silica deposition amount of the composite filler is adjusted to the lower limit of the hollow glass microsphere mass (8%), and the mass ratio of hexagonal boron nitride to hollow glass microspheres is adjusted to 0.10:1; the remaining formulation, sequential blending, side feeding position, vacuum degree and bilayer co-extrusion process parameters are the same as in Example 1.

[0038] Example 3 The difference between Example 3 and Example 1 is that the silica deposition amount of the composite filler is increased to the upper limit load (15%), and the mass ratio of hexagonal boron nitride to hollow glass microspheres is adjusted to 0.30:1; in order to match the rheology, the outer head temperature is increased by 5°C, and the traction oscillation is set to 12Hz with an amplitude of 1.2% of the linear velocity; the remaining steps are the same as in Example 1.

[0039] Example 4 The difference between Example 4 and Example 1 is that the ethylene-octene copolymer elastomer is replaced with POE of the same melt index grade, and the random copolymer polypropylene is replaced with impact copolymer polypropylene. The total mass of the formulation and the proportion of each functional additive remain unchanged. The sequential reaction blending, side feeding and bilayer co-extrusion parameters are the same as in Example 1.

[0040] Examples 1-4 were tested according to standard methods. All samples were tested after conditioning at 23±2℃ and 50%RH for 48 hours; pipe sections or injection-molded sheets were sampled according to each standard. Density (ASTM D792). Example 1: 0.933 g / cm³; Example 2: 0.928 g / cm³ (reduced h-BN coating, lighter overall); Example 3: 0.942 g / cm³ (slight increase due to increased SiO2 and h-BN); Example 4: 0.935 g / cm³.

[0041] Melt flow rate (MFR) (ISO 1133, 230℃ / 2.16kg). Example 1: 2.6g / 10min; Example 2: 2.8g / 10min; Example 3: 2.2g / 10min (increased viscosity due to increased flake coating); Example 4: 2.5g / 10min.

[0042] Ring stiffness (ISO9969, 23℃, Φ110×6.6mm pipe section). Example 1: 10.5kN / m²; Example 2: 9.7kN / m²; Example 3: 11.6kN / m²; Example 4: 9.2kN / m² (stiffness slightly decreased after adopting ICP+POE).

[0043] Drop hammer impact test (ISO 3127, 0°C, pipe section, specified energy, non-rupture rate). Example 1: 1.8J, no rupture (≥95% of pipe sections pass); Example 2: 1.6J, no rupture; Example 3: 1.8J, approximately 90% pass rate (increased rigidity, slightly increased brittleness); Example 4: 2.2J, no rupture (best toughness).

[0044] Notched impact (ISO179-1eA, -20℃, 3.2mm injection molded sheet). Example 1: 6.3kJ / m²; Example 2: 6.0kJ / m²; Example 3: 5.2kJ / m²; Example 4: 8.1kJ / m².

[0045] Wear (DIN53516, volumetric wear, mm³). Example 1: 85; Example 2: 100 (h-BN decreases, wear resistance decreases); Example 3: 70 (h-BN increases, wear resistance improves); Example 4: 90.

[0046] Thermal conductivity (ISO22007-2, TPS method, 23℃). Example 1: 0.35 W / m·K; Example 2: 0.30 W / m·K; Example 3: 0.40 W / m·K; Example 4: 0.33 W / m·K.

[0047] Surface volume resistivity (GB / T1410, 23℃ 50%RH; 24h after corona treatment). Example 1: 3×10 7 Ω·cm; Example 2: 1×10 8 Ω·cm; Example 3: 8×10 6 Ω·cm; Example 4: 2×10 7 Ω·cm. After immersion in water for 168 hours, the retest still showed a value of 10. 6 -10 9 Within the Ω·cm range.

[0048] Flame retardant (GB / T2406.2 oxygen index; UL-94 vertical flame retardancy, 3.2mm injection molded sheet). Example 1: LOI≈30%, UL-94V-0; Example 2: LOI≈29%, UL-94V-0; Example 3: LOI≈31%, UL-94V-0; Example 4: LOI≈30%, UL-94V-0.

[0049] Quantitative analysis of β-crystal form (WAXD, outer / inner β peak ratio). Example 1: outer / inner ≈ 1.9; Example 2: ≈ 1.8; Example 3: ≈ 2.0; Example 4: ≈ 1.9.

[0050] Hollow microsphere integrity rate (Micro-CT statistics, tube wall cross-section). Example 1: ≈92%; Example 2: ≈93%; Example 3: ≈90%; Example 4: ≈91%.

[0051] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that "HGM@SiO2" (without h-BN coating) is used instead of composite filler, while the total amount of filler remains unchanged; the rest of the formulation and process are the same as in Example 1.

[0052] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that: instead of preparing the SiO2 intermediate layer and silanization, uncoated hollow glass microspheres and hexagonal boron nitride powder are directly added to the side feed inlet; the other conditions are the same as in Example 1.

[0053] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the "sequential reaction" is cancelled, and PP-g-MAH, PDMS-NH2 and composite filler are added together with the main resin at the main feed port in one go, and the above two components are no longer added to the side feed port; the barrel temperature zone and speed are the same as in Example 1.

[0054] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that an equal amount of unmicroencapsulated ammonium polyphosphate is used instead of microencapsulated ammonium polyphosphate; the rest of the formulation and process are the same as in Example 1.

[0055] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that the outer lip-inner core temperature difference is closed (set to the same temperature), and single-stage spray cooling is used instead, and the traction micro-amplitude oscillation is canceled; the remaining steps are the same as in Example 1.

[0056] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the feed port on the side of the composite packing is moved to the high melting pressure zone and the vacuum exhaust is turned off; the other conditions are the same as in Example 1.

[0057] Comparative Example 7 The difference between Comparative Example 7 and Example 1 is that: low-structure conductive carbon black with DBP≈200 mL / 100 g is used instead of high-structure conductive carbon black, the dosage remains the same, and the corona treatment is cancelled; the remaining steps are the same as in Example 1.

[0058] Comparative Example 8 The difference between Comparative Example 8 and Example 1 is that: it is changed to single-layer extrusion molding, the inner layer co-extrusion is cancelled, and the mold and cooling are set according to the outer layer conditions; the rest of the formula and processing parameters are the same as in Example 1.

[0059] Performance testing Comparative Example 1: Density 0.930 g / cm³; MFR 2.7 g / 10 min; Ring stiffness 9.8 kN / m²; Drop weight 1.9 J (pass rate ≥95%); Notched impact 6.8 kJ / m²; Wear volume 110 mm³; Thermal conductivity 0.30 W / m·K; Surface volume resistivity 8 × 10⁻⁶ 7 Ω·cm; LOI 30%, UL-94V-0; β external / internal ≈ 1.9; HGM integrity ≈ 92%.

[0060] Comparative Example 2: Density 0.934 g / cm³; MFR 2.2 g / 10 min; Ring stiffness 9.0 kN / m²; Drop weight 1.2 J (pass rate ≈ 70%); Notched impact 4.2 kJ / m²; Wear volume 120 mm³; Thermal conductivity 0.28 W / m·K; Surface volume resistivity 6 × 10⁻⁶ 7 Ω·cm; LOI 29%, UL-94V-1; β external / internal ≈ 1.7; HGM integrity ≈ 85%.

[0061] Comparative Example 3: Density 0.933 g / cm³; MFR 2.9 g / 10 min; Ring stiffness 10.0 kN / m²; Drop weight 1.5 J (pass rate ≈ 85%); Notched impact 5.0 kJ / m²; Wear volume 95 mm³; Thermal conductivity 0.34 W / m·K; Surface volume resistivity 4 × 10⁻⁶ 7 Ω·cm; LOI 30%, UL-94V-0; β external / internal ≈ 1.8; HGM integrity ≈ 90%.

[0062] Comparative Example 4: Initial conditions: density 0.934 g / cm³; MFR 2.6 g / 10 min; ring stiffness 10.2 kN / m²; drop weight 1.7 J (≥90%); notched impact 6.0 kJ / m²; wear volume 88 mm³; thermal conductivity 0.35 W / m·K; surface volume resistivity 4 × 10⁻⁶ 7 Ω·cm; LOI 29%, UL-94V-0. After immersion in water for 168 hours: surface volume resistivity 3×10⁻⁶. 6 Ω·cm; LOI 27%, UL-94V-1; ring stiffness 9.5kN / m².

[0063] Comparative Example 5: Density 0.933 g / cm³; MFR 2.6 g / 10 min; Ring stiffness 10.0 kN / m²; Drop weight 1.4 J (≈80%); Notched impact 5.4 kJ / m²; Wear volume 105 mm³; Thermal conductivity 0.34 W / m·K; Surface volume resistivity 3 × 10⁻⁶ 7 Ω·cm; LOI 30%, UL-94V-0; β external / internal ≈ 1.2; HGM integrity ≈ 92%.

[0064] Comparative Example 6: Density 0.945 g / cm³; MFR 2.5 g / 10 min; Ring stiffness 10.0 kN / m²; Drop weight 1.3 J (≈78%); Notched impact 4.8 kJ / m²; Wear volume 90 mm³; Thermal conductivity 0.34 W / m·K; Surface volume resistivity 4 × 10⁻⁶ 7 Ω·cm; LOI 30%, UL-94V-0; β outside / inside ≈ 1.8; HGM integrity rate ≈ 76% (increased apparent pinholes / stripes).

[0065] Comparative Example 7: Density 0.933 g / cm³; MFR 2.6 g / 10 min; Ring stiffness 10.4 kN / m²; Drop impact 1.7 J (≥90%); Notched impact 6.2 kJ / m²; Wear volume 86 mm³; Thermal conductivity 0.35 W / m·K; Surface volume resistivity 4 × 10¹⁰ Ω·cm (exceeding the target window); LOI 30%, UL-94V-0; β_outer / _inner ≈ 1.9; HGM integrity ≈ 92%.

[0066] Comparative Example 8: Density 0.934 g / cm³; MFR 2.6 g / 10 min; Ring stiffness 9.6 kN / m²; Drop weight 1.3 J (≈80%); Notched impact 5.5 kJ / m²; Wear volume 98 mm³; Thermal conductivity 0.33 W / m·K; Surface volume resistivity 3 × 10⁻⁶ 7 Ω·cm; LOI 30%, UL-94V-0; β outer / inner ≈ 1.3 (overall intralayer homogenization); HGM integrity ≈ 91%.

[0067] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A polypropylene cable protection pipe, characterized in that, The polypropylene cable protection pipe has a double-layer co-extruded structure, including an outer sheath layer and an inner layer, which are made of materials with the same formula. The formulation, by weight, comprises: 65-85 parts random copolymer polypropylene, 5-15 parts ethylene-octene copolymer elastomer, 2-6 parts polypropylene grafted maleic anhydride, 8-25 parts composite filler, 8-18 parts microencapsulated ammonium polyphosphate, 2-8 parts melamine cyanurate, 0.05-0.30 parts β-crystal nucleating agent, 0.2-0.8 parts hindered phenol and phosphite compound antioxidant, 0.1-0.5 parts hindered amine light stabilizer, 0.1-0.8 parts high-structure conductive carbon black, and 0.3-2.5 parts amino-terminated polydimethylsiloxane. Among them, the composite filler is a core-shell-sheet structured particle with hollow glass microspheres as the core layer, silica as the middle layer, and hexagonal boron nitride sheets as the outer layer; The ratio of β-crystal content in the outer layer to that in the inner layer is ≥1.

8.

2. The polypropylene cable protection pipe according to claim 1, characterized in that, The median particle size of the hollow glass microspheres in the composite filler is 15–60 μm; the thickness of the silica intermediate layer is 50–300 nm and the coverage is ≥90%; the thickness of the hexagonal boron nitride sheet is 30–200 nm, the aspect ratio is ≥50 and the surface coverage is ≥60%.

3. The polypropylene cable protection pipe according to claim 1, characterized in that, The silica intermediate layer of the composite filler is grafted with bifunctional silanes containing epoxy and alkenyl groups; the grafting rate of maleic anhydride grafted onto polypropylene is 0.5% to 1.5%; the number average molecular weight of amino-terminated polydimethylsiloxane is 3×10³ to 3×10⁴.

4. The polypropylene cable protection pipe according to claim 1, characterized in that, The microencapsulated ammonium polyphosphate has a siloxane-silica gradient shell with a shell mass fraction of 5% to 15% and a particle size of 5 to 25 μm; the mass ratio of melamine cyanurate to microencapsulated ammonium polyphosphate is (1 to 3):

5.

5. The polypropylene cable protection pipe according to claim 1, characterized in that, The conductive carbon black has a DBP oil absorption value ≥300mL / 100g; the hindered phenol and phosphite compound antioxidant is composed of hindered phenol 1010 or 1076 and phosphite 168 or 626; the hindered amine light stabilizer is a piperidine compound.

6. A method for preparing a polypropylene cable protection tube according to any one of claims 1-5, characterized in that, Includes the following steps: a) Preparation of composite filler: Hollow glass microspheres were washed with 0.1-0.5 mol / L hydrochloric acid for 15-30 min, then washed with deionized water until the conductivity was ≤50 μS / cm, and dried at 110℃ for 2-4 h; a solvent was prepared with ethanol:water = 80:20-60:40 (volume fraction), and tetraethoxysilane was added to make the target silica loading amount 5-15% based on the mass of hollow glass microspheres. The pH was adjusted to 9.5-11.0 with ammonia water, and the reaction was carried out at 25-35℃ for 30-120 min. After filtration, washing, and drying at 80-100℃, hollow glass microspheres@silica were obtained. Dissolve 1.0–3.0% of a bifunctional silane with epoxy and alkenyl groups in ethanol:water at a volume ratio of 95:

5. Adjust the pH to 4.5–5.5 with glacial acetic acid, react at 60–80°C for 0.5–2.0 h, and dry at 100–120°C for 1–3 h to complete surface silanization and obtain silanized particles. Hydroxylated or carboxylated hexagonal boron nitride was dispersed in an alcohol or alcohol / toluene mixed solvent with a solid content of 0.5-2.0%, ultrasonically dispersed at 200-400W for 10-30 min, silanized particles were added, and stirred at 60-85℃ for 1-3 h, filtered, and dried at 80-100℃ until the moisture content was ≤0.20% to obtain the composite filler. b) Preparation of microencapsulated flame retardant: Ammonium polyphosphate is prepared into a suspension of 10-30% by mass, and silica sol with a silica content of 30-40% by mass is added so that the silica inner layer contains 3-10% by mass of ammonium polyphosphate. The pH is adjusted to 8.5-9.2 with ammonia water and stirred at 35-45℃ for 30-60 min. Based on ammonium polyphosphate, 0.5-3.0% by mass of methyltriethoxysilane was added dropwise and condensed at 45-60℃ for 60-120 min. After solid-liquid separation, washing with deionized water, drying at 90-110℃ for 2-6 h, and passing through an 80-200 mesh sieve, microencapsulated ammonium polyphosphate was obtained. c) Raw material premixing: Weigh random copolymer polypropylene, ethylene-octene copolymer elastomer, microencapsulated ammonium polyphosphate, melamine cyanurate, β-crystal nucleating agent, hindered phenolic antioxidant, phosphite antioxidant, hindered amine light stabilizer and conductive carbon black according to the target ratio, mix in a drum for 5-10 minutes, and control the material moisture content to ≤0.05%; d) Blending and granulation: A co-rotating twin-screw extruder with an L / D ≥ 44 was used. The barrel temperature zones were set to 170 / 175 / 180 / 185 / 190 / 195 / 195℃. Random copolymer polypropylene and polypropylene-grafted maleic anhydride were first added to the main feed port and sheared in the machine for 60–90 s. Amino-terminated polydimethylsiloxane and the composite filler obtained in step a) were simultaneously added to the side feed ports in zones 5–6. Subsequently, in zones 6–7… Microencapsulated ammonium polyphosphate, melamine cyanurate, hindered phenol / phosphite antioxidant, hindered amine light stabilizer and β-crystal nucleating agent are added; screw speed is 200-260 r / min, vacuum degree of vacuum exhaust port in zone 7 is -0.06 to -0.08 MPa, melt specific energy consumption is controlled ≤0.30 kWh / kg, and granulation is carried out by water bath stretching at 15-25℃, with particle length of 2-4 mm, and hot air drying at 80-90℃ for 2-4 h; e) Double-layer co-extrusion molding: The temperature of the outer extruder barrel / die head is 200-220℃, the temperature of the inner extruder barrel / die head is 190-210℃, and the temperature of the outer lip of the co-extrusion die is 10-25℃ higher than that of the inner core; the vacuum degree of the vacuum sizing box is -0.05 to -0.08MPa; the material passes through three spray stages and two air knives in sequence, with the water temperature of the first stage being 10-15℃, the second stage being 18-25℃, and the third stage being 25-32℃, and the pressure of the two air knives being 0.10-0.30MPa; the traction speed and extrusion volume are matched to make the draw ratio 1.05-1.25, and a micro-oscillation of 3-15Hz with an amplitude of 0.3%-1.5% of the linear velocity is superimposed; f) Post-processing: After extrusion molding, the material is subjected to corona treatment with electrode rods with a gap of 2-8mm. The corona voltage is 6-12kV and the frequency is 10-30kHz. After cooling to 15-25℃, the material is cut to length and wound up.

7. The preparation method according to claim 6, characterized in that, In step a), the mass ratio of tetraethoxysilane to hollow glass microspheres is 0.08:1 to 0.30:1, the bifunctional silane used for silanization is a mixture of epoxypropoxysilane and vinylsilane, with a mass ratio of 1:1 to 3:1; the mass ratio of hexagonal boron nitride to hollow glass microspheres is 0.05:1 to 0.40:1, and the ultrasonic dispersion power density is 20 to 60 W / L.

8. The preparation method according to claim 6, characterized in that, In step b), the silica sol is added by constant-rate dripping over a period of 15–45 min; the hydrolysis molar ratio of methyltriethoxysilane is 3:1–6:1; the stirring speed during the condensation stage is 300–600 r / min; and the temperature is increased to the target temperature and maintained at 1–2 °C / min during the drying process; the D50 of the microencapsulated ammonium polyphosphate is 10–25 μm.

9. The preparation method according to claim 6, characterized in that, In step d), the side feed port is set in the low melting pressure zone before the metering section. The side feed rotor speed is 10-40 r / min. The screw element adopts a combination of fewer kneading blocks, 30°-45° kneading plates and shallow screw grooves to reduce the breakage of hollow glass microspheres. In step e), the length of the vacuum sizing box is 1.5 to 3.0 m, and each of the three spray sections is 1.0 to 2.5 m. The traction machine adopts servo drive to switch the oscillation frequency and amplitude in an S-shaped acceleration and deceleration mode. In step f), the corona section is equipped with ozone extraction and thermal management, and the electrode-product distance is maintained at 2-8 mm online.