High-strength MPP cable protection pipe and preparation method thereof

Abstract

The application discloses a high-strength MPP cable protection pipe and a preparation method thereof, relates to the technical fields of modified polypropylene pipes and power cable protection pipes, and the protection pipe comprises a reinforced load-bearing section, a gradient transition section and a hot-melt connecting section which are continuously integrally formed in the axial direction, and each section penetrates the pipe wall thickness. The reinforced load-bearing section contains a reinforcing phase and a nucleating agent, the hot-melt connecting section has a lower content of the reinforcing phase and the nucleating agent and contains a propylene-ethylene random copolymer; the content of the reinforcing phase and the nucleating agent in the gradient transition section decreases towards the hot-melt connecting section, and the content of the random copolymer increases. An axial continuous gradient structure is formed through two extruders, an adjustable ratio melt distributor and a partition cooling device; the reinforcing phase and the nucleating agent are mainly concentrated in the reinforced load-bearing section, which can maintain a relatively high axial tensile strength and ring stiffness of the pipe body; the content of the reinforcing phase and the nucleating agent in the hot-melt connecting section is relatively low, which is beneficial to the mutual diffusion, mutual entanglement and recrystallization of the polypropylene molecular chains in the hot-melt interface.

Description

Technical Field

[0001] This invention relates to the field of modified polypropylene pipes and power cable protection pipes, specifically to a high-strength MPP cable protection pipe and its preparation method that combines high pipe strength, heat fusion joint strength, and long-term connection reliability. Background Technology

[0002] Modified polypropylene cable protection pipes are characterized by their light weight, resistance to chemical corrosion, good insulation performance, high heat resistance, convenient construction, and ability to be heat-fused. They are suitable for cable laying in areas such as roads, railways, buildings, riverbeds, and underground utility tunnels.

[0003] During trenchless construction, MPP cable protection pipes need to withstand combined loads such as axial traction, bending, torsion, sand and gravel scraping, soil compression, and local impact. To improve the axial tensile strength, ring stiffness, and compressive strength of the pipe, existing technologies typically add reinforcing phases such as chopped glass fibers, chopped basalt fibers, acicular wollastonite, and magnesium salt whiskers to the polypropylene matrix, and add nucleating agents to improve the crystallization rate and crystallinity of polypropylene.

[0004] However, while reinforcing phases and nucleating agents improve the strength of the pipe body, they can negatively impact the hot-melt bonding performance of the pipe. Inorganic reinforcing phases cannot diffuse across the hot-melt interface and may accumulate near the fusion surface during hot-melt extrusion, thus reducing the number of polypropylene molecular chains participating in interfacial interdiffusion. Nucleating agents accelerate the cooling and crystallization of the molten interface, shortening the time that polypropylene segments remain in a mobile and diffusible state.

[0005] As the content of reinforcing phase and nucleating agent increases, the degree of interdiffusion, entanglement, and recrystallization of polypropylene molecular chains in the hot-melt interface decreases, easily leading to a phenomenon where the tube body has high strength but the hot-melt joint strength is significantly lower. The hot-melt joint thus becomes a weak area under trenchless traction, long-term earth pressure, and cable thermal cycling conditions.

[0006] Simply reducing the content of reinforcing phase and nucleating agent in the entire pipe can improve the performance of hot-melt connection, but it will reduce the axial tensile strength, ring stiffness, and traction resistance of the pipe. If the bearing area of ​​the pipe end is increased by simply increasing the wall thickness at the pipe end, setting a variable diameter section, or installing an independent connecting sleeve, the degree of molecular chain diffusion at the hot-melt interface cannot be improved at the material level, and new stress concentrations may be generated at the root of the variable diameter section, the mechanical sleeve surface, or the interface between different materials.

[0007] Another possible solution is to set a heat-fused connection region with low reinforcement content at the end of the reinforced tube. However, if the reinforced region and the low-reinforcement connection region are connected by a component abrupt change, there will still be significant differences between the two in terms of elastic modulus, crystallinity, crystallization shrinkage rate and coefficient of thermal expansion. Under long-term temperature changes and continuous load conditions, microcracks are prone to occur from the component abrupt change interface.

[0008] Furthermore, when switching between two different polypropylene melt formulations in a conventional extrusion system, component tailing can easily occur due to differences in melt viscosity, flow channel stagnation, and changes in extrusion back pressure, causing the actual axial transition zone to deviate from the set length. If there is a lack of coordinated control between the extruder output, metering pump speed, and die pressure, it can also lead to a decrease in pipe diameter, wall thickness, transition section length, and component gradient repeatability.

[0009] For axial pipe sections formed under different cooling conditions, the instantaneous cooling rate during the production process is difficult to judge solely based on the appearance of the finished product. Therefore, using only process parameters such as cooling medium temperature, cooling medium flow rate, or average cooling rate to define different pipe sections may be detrimental to finished product quality inspection and product structure identification.

[0010] The cooling history of polypropylene pipes directly affects their crystallinity. By applying relatively strong cooling to the hot-melt joint zone, further crystallization in this area after extrusion can be suppressed, resulting in a lower initial crystallinity in the hot-melt joint section compared to the reinforced load-bearing section. Therefore, the difference in crystallinity between different pipe sections can be considered a direct product result of zoned cooling.

[0011] The difference in crystallinity can be objectively obtained by taking samples from the reinforced load-bearing section and the hot-melt connection section of the final pipe and measuring their enthalpy of fusion using differential scanning calorimetry. It does not require knowing or inferring the flow rate, temperature, or instantaneous cooling rate of the cooling medium during the pipe production process.

[0012] Therefore, there is a need to provide a new high-strength MPP cable protection pipe that allows the reinforcing phase and nucleating agent to be mainly distributed in the reinforcing load-bearing section that bears the mechanical load, so that the hot-melt connection section at the pipe end has a lower content of reinforcing phase and nucleating agent and a greater number of diffusible polypropylene segments, and forms a continuous gradient of composition and crystallinity between the reinforcing load-bearing section and the hot-melt connection section, thereby taking into account the strength of the pipe body, the strength of the joint, and the long-term connection reliability.

[0013] Meanwhile, a preparation method that can stably form the above-mentioned axial continuous gradient structure is also needed, so that the two melts can be stably switched in a predetermined ratio during continuous extrusion, and the total melt flow and pressure fluctuations can be reduced by closed-loop control between the metering pump, extruder and die pressure. Summary of the Invention

[0014] The purpose of this invention is to provide a high-strength MPP cable protection pipe and its preparation method, in order to solve the problem that when adding reinforcing phases and nucleating agents to improve the strength of MPP pipe body, the reinforcing phases and nucleating components hinder the interdiffusion of polypropylene molecular chains at the hot-melt interface, resulting in the strength of the hot-melt joint being lower than that of the reinforced pipe body.

[0015] Another objective of this invention is to reduce abrupt changes in the content of the reinforcing phase, nucleating agent, elastic modulus, crystallinity, and thermal shrinkage rate between the reinforced load-bearing section and the hot-melt connection section by using a gradient transition section that is continuously and integrally formed along the pipe axis, thereby improving the initial equal strength and thermal aging reliability of the hot-melt joint.

[0016] Another objective of this invention is to provide an adjustable melt distributor and its closed-loop control method, which enables stable ratio switching between reinforced MPP melt and weldable MPP melt during continuous extrusion, and controls the fluctuation of total melt mass flow rate and die inlet pressure within a predetermined range.

[0017] To achieve the above objectives, the present invention provides the following technical solution: a high-strength MPP cable protection pipe, comprising an axially continuous integrally formed reinforcing load-bearing section, heat-fusion connecting sections located at both ends of the reinforcing load-bearing section, and gradient transition sections located between the reinforcing load-bearing section and the heat-fusion connecting sections, each section penetrating the pipe wall thickness; the reinforcing load-bearing section is formed of a reinforcing MPP composition, which, based on 100 parts by weight of polypropylene matrix, comprises 8-25 parts by weight of reinforcing phase, 0.05-0.30 parts by weight of nucleating agent, 2-10 parts by weight of polyolefin elastomer, and 0.3-2 parts by weight of compatibilizer; the heat-fusion connecting section is formed of a weldable MPP composition. The weldable MPP composition comprises, based on 100 parts by weight of polypropylene matrix, no more than 2 parts by weight of reinforcing phase, no more than 0.02 parts by weight of nucleating agent, and 10-35 parts by weight of propylene-ethylene random copolymer; the reinforced MPP composition and the weldable MPP composition respectively contain the same grade of propylene homopolymer and propylene-ethylene block copolymer, and at 230°C and 2.16 kg load, the ratio of the melt mass flow rate of the weldable MPP composition to that of the reinforced MPP composition is 0.65-1.35; in the gradient transition section, the content of reinforcing phase and nucleating agent continuously decreases in the direction towards the hot-melt connection section, while the content of propylene-ethylene random copolymer continuously increases.

[0018] Preferably, the reinforcing phase is selected from one or more of chopped glass fibers, chopped basalt fibers, acicular wollastonite, and magnesium salt whiskers, and the average length of the reinforcing phase is 0.15-2.50 mm and the aspect ratio is 8-60; the reinforcing phase is surface-treated with a silane coupling agent and maleic anhydride-grafted polypropylene, and the mass of the maleic anhydride-grafted polypropylene coating layer is 0.5%-3.0% of the mass of the reinforcing phase.

[0019] Preferably, the polypropylene matrix in both the reinforced MPP composition and the weldable MPP composition is composed of 40-70 parts by weight of propylene homopolymer and 30-60 parts by weight of propylene-ethylene block copolymer, totaling 100 parts by weight; the isotactic index of the propylene homopolymer is not less than 95%, and the ethylene content in the propylene-ethylene block copolymer is 5%-15% by weight; the nucleating agent is a substituted aryl phosphate α-crystalline nucleating agent, and the polyolefin elastomer is selected from one or two of ethylene-octene copolymer and olefin block copolymer.

[0020] Preferably, the ethylene content in the propylene-ethylene random copolymer is 1.5% to 4.5% by mass, the melting peak temperature is 135 to 150°C, and the melt mass flow rate at 230°C and 2.16 kg load is 0.3 to 2.0 g / 10 min; the axial length of each hot-melt connection section is 0.3 to 1.2 times the nominal outer diameter D of the high-strength MPP cable protection pipe and not less than 50 mm, and the axial length of each gradient transition section is 0.1 to 0.5 times the nominal outer diameter D.

[0021] Preferably, when the reinforced bearing section, gradient transition section and hot-melt connection section are tested by differential scanning calorimetry, the crystallinity of the hot-melt connection section is 5 to 15 percentage points lower than that of the reinforced bearing section, and the crystallinity of the gradient transition section decreases continuously in the direction toward the hot-melt connection section; the difference in melting peak temperature between the hot-melt connection section and the reinforced bearing section does not exceed 12°C.

[0022] Preferably, after two high-strength MPP cable protection pipes of the same specification are butt-welded through their respective hot-melt connection sections, the axial tensile strength of the hot-melt joint is not less than 90% of the axial tensile strength of the reinforcing load section, and the tensile failure location is outside the hot-melt fusion surface; after the hot-melt joint is aged in an air environment at 80°C for 1000 hours, its axial tensile strength retention rate is not less than 85%, and the axial tensile strength of the aged hot-melt joint is not less than 85% of the axial tensile strength of the aged reinforcing load section.

[0023] Preferably, a method for preparing a high-strength MPP cable protection pipe includes the following steps: S1. Prepare reinforced MPP material and weldable MPP material separately. The reinforced MPP material, based on 100 parts by weight of polypropylene matrix, includes 8-25 parts by weight of reinforcing phase, 0.05-0.30 parts by weight of nucleating agent, 2-10 parts by weight of polyolefin elastomer, and 0.3-2 parts by weight of compatibilizer. The weldable MPP material, based on 100 parts by weight of the polypropylene matrix, includes no more than 2 parts by weight of reinforcing phase, no more than 0.02 parts by weight of nucleating agent, and 10-35 parts by weight of propylene-ethylene random copolymer. S2. The reinforced MPP material and the weldable MPP material are respectively fed into the first extruder and the second extruder for plasticization to obtain reinforced MPP melt A and weldable MPP melt B. The reinforced MPP melt A and the weldable MPP melt B are then fed into an adjustable melt distributor. The adjustable melt distributor includes a first metering channel and a second metering channel for conveying reinforced MPP melt A and weldable MPP melt B respectively, a first servo metering pump and a second servo metering pump respectively disposed on the first metering channel and the second metering channel, a manifold connected to the two metering channels, and a static mixing channel disposed downstream of the manifold. The static mixing channel is connected to the annular extrusion die. S3. During the continuous extrusion process, the mass flow ratio of reinforced MPP melt A and weldable MPP melt B is controlled by the first servo metering pump and the second servo metering pump, so that the adjustable melt distributor sequentially outputs reinforced MPP melt A, a mixed melt with a continuously changing ratio of the two melts, weldable MPP melt B, and a mixed melt with a continuously changing ratio of the two melts in the opposite direction, so that the continuous tube blank periodically forms a reinforced bearing zone, a gradient transition zone, and a hot melt connection zone along the axial direction. S4. The axial length of each complete hot-melt connection zone in the continuous tube blank is twice the length of the single-end hot-melt connection section of the finished tube. After cutting at the axial midpoint of the complete hot-melt connection zone, a hot-melt connection end is formed for each of the two adjacent tubes. The continuous tube blank is sized, cooled in sections, and pulled. The section between the axial midpoints of the two adjacent hot-melt connection zones is used as a cutting unit to obtain a high-strength MPP cable protection pipe with hot-melt connection sections at both ends.

[0024] Preferably, the adjustable melt distributor further includes a distributor housing, a first melt pressure sensor disposed at the inlet of the first metering channel, a second melt pressure sensor disposed at the inlet of the second metering channel, and a die pressure sensor disposed at the inlet of the annular extrusion die; the outlets of the first metering channel and the second metering channel are alternately connected to the annular confluence cavity in the circumferential direction, and the static mixing channel is provided with 2 to 6 stages of split-flow superimposed mixing units; with the total melt volume flow rate at the outlet of the static mixing channel as Q, the internal volume between the outlet of the first metering channel and the outlet of the static mixing channel as V, and V / Q as 0.5 to 3.0 s, so as to reduce axial tailing during the melt ratio switching process.

[0025] Preferably, the first servo metering pump, the second servo metering pump, the first extruder, and the second extruder are connected to a controller. The controller performs proportional control, total flow control, and feed pressure control based on a preset mass flow rate ratio curve r(t) of the weldable MPP melt B, a set value for the total melt mass flow rate, and a set value for the annular extrusion die inlet pressure. Specifically, the proportional control adjusts the speed ratio of the first servo metering pump and the second servo metering pump based on the ratio of the actual mass flow rate of the weldable MPP melt B to the actual total mass flow rate of the two melts. The total flow control maintains the total mass flow rate of the two melts by synchronously adjusting the speeds of the two servo metering pumps. The feed pressure control adjusts the screw speeds of the first extruder and the second extruder based on the detection values ​​of the first melt pressure sensor, the second melt pressure sensor, and the die pressure sensor, so that the fluctuation of the total melt mass flow rate relative to the set value does not exceed 3%, and the fluctuation of the annular extrusion die inlet pressure relative to the set value does not exceed 5%.

[0026] Preferably, the controller determines the arrival times of the reinforced bearing zone, gradient transition zone, and hot-melt connection zone to the zoned cooling device based on the melt ratio switching signal of the adjustable melt distributor, the distance from the annular extrusion die outlet to the zoned cooling device, and the traction speed of the continuous billet, and controls the medium flow rate and medium temperature of the corresponding cooling zones respectively; the average cooling rate of the hot-melt connection zone during the process of decreasing from 140°C to 100°C is 1.5 to 3.0 times that of the average cooling rate of the reinforced bearing zone, so that the crystallinity of the hot-melt connection section in the high-strength MPP cable protection pipe is 5 to 15 percentage points lower than that of the reinforced bearing section, and the difference in melting peak temperature between the hot-melt connection section and the reinforced bearing section does not exceed 12°C.

[0027] The continuous decrease or continuous increase referred to in this invention means that when at least 5 equidistant sampling positions are selected along the axial direction in the gradient transition section, the content of the reinforcing phase and nucleating agent at adjacent sampling positions does not increase sequentially, the content of propylene-ethylene random copolymer does not decrease sequentially, and there is no independent component abrupt interface.

[0028] Compared with the prior art, the present invention provides a high-strength MPP cable protection pipe and its preparation method, which has the following beneficial effects: the reinforcing phase and nucleating agent are mainly concentrated in the reinforcing load section, which can maintain a high axial tensile strength and ring stiffness of the pipe body; the content of reinforcing phase and nucleating agent in the hot melt connection section is low, which is conducive to the interdiffusion, entanglement and recrystallization of polypropylene molecular chains at the hot melt interface.

[0029] The reinforced MPP composition and the weldable MPP composition use the same grade of polypropylene matrix, and the ratio of the melt mass flow rate of the two compositions is controlled at 0.65 to 1.35, which can reduce the flow mismatch and local enrichment of the two melts.

[0030] The gradient transition section allows the content of the reinforcing phase, nucleating agent, and propylene-ethylene random copolymer to change continuously along the axial direction, reducing abrupt changes in modulus, crystallinity, and thermal shrinkage between the reinforcing load-bearing section and the hot-melt connection section.

[0031] The reinforced load-bearing section, gradient transition section, and hot-melt connection section are formed by continuous integral extrusion, eliminating the need for adhesive surfaces, mechanical fitting surfaces, or secondary molding interfaces, thus reducing the risk of interface peeling caused by independent connection structures.

[0032] The adjustable melt distributor has a small melt retention volume, and with the help of closed-loop flow and pressure control, it can reduce the tailing of the ratio switching and improve the stability of axial gradient length, pipe diameter and wall thickness.

[0033] Partitioned cooling results in a 5-15 percentage point lower crystallinity in the hot-melt joint section compared to the reinforced load-bearing section. This difference in crystallinity is a direct product consequence of partitioned cooling and can be objectively observed through DSC testing of the final product, without needing to deduce the cooling medium flow rate, cooling temperature, or instantaneous cooling rate during production from the final product.

[0034] The lower initial crystallinity of the hot-melt joint section improves the effective segment diffusion capacity of this region during the hot-melt process; the higher crystallinity of the reinforced load-bearing section helps to maintain the strength, ring stiffness and dimensional stability of the pipe body.

[0035] The initial axial tensile strength of the resulting hot-melt joint can reach more than 90% of the axial tensile strength of the reinforced load-bearing section, and it can still maintain high joint strength and non-fusion surface failure characteristics after aging at 80℃ for 1000 hours. Attached Figure Description

[0036] Figure 1 This is a DSC curve of the first temperature rise of the reinforced bearing section and the hot-melt connection section in Embodiment 2 of the present invention; Figure 2 This is a crystallinity distribution curve along the pipe axis in Embodiment 2 of the present invention; Figure 3 This is a cross-sectional SEM image of the fusion interface of the thermofusion joint in Embodiment 2 of the present invention. Figure 4 This is a SEM image of the reinforced phase dispersion and interface bonding of the reinforced bearing section in Embodiment 2 of the present invention. Figure 5 This is a SEM image of the tensile fracture surface of the thermofusion joint in Embodiment 2 of the present invention. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] Please see Figures 1-5 This invention provides a technical solution for a high-strength MPP cable protection pipe and its preparation method: The present invention will be further described below with reference to embodiments and comparative examples. Unless otherwise stated, all parts are parts by weight and all percentages are mass percentages.

[0039] All embodiments and comparative examples prepared MPP cable protection pipes with a nominal outer diameter D of 160 mm, a wall thickness of 12 mm, and a single length of 6000 mm.

[0040] raw material HPP-1 is a propylene homopolymer with an isotactic index of 95% and a melt mass flow rate of 0.8 g / 10 min at 230°C and 2.16 kg.

[0041] HPP-2 is a propylene homopolymer with an isotactic index of 97% and a melt flow rate of 0.7 g / 10 min.

[0042] HPP-3 is a propylene homopolymer with an isotactic index of 98% and a melt flow rate of 0.6 g / 10 min.

[0043] BPP-1 is a propylene-ethylene block copolymer with an ethylene content of 5% and a melt flow rate of 0.7 g / 10 min.

[0044] BPP-2 is a propylene-ethylene block copolymer with an ethylene content of 10% and a melt flow rate of 0.6 g / 10 min.

[0045] BPP-3 is a propylene-ethylene block copolymer with an ethylene content of 15% and a melt flow rate of 0.5 g / 10 min.

[0046] RPP-L is a random copolymer of propylene and ethylene with an ethylene content of 1.5% by mass, a melting peak temperature of 150℃, and a melt flow rate of 0.3 g / 10 min.

[0047] RPP-M is a random copolymer of propylene and ethylene with an ethylene content of 3.0% by mass, a melting peak temperature of 143℃, and a melt flow rate of 1.0 g / 10 min.

[0048] RPP-H is a random copolymer of propylene and ethylene with an ethylene content of 4.5% by mass, a melting peak temperature of 135℃, and a melt flow rate of 2.0 g / 10 min.

[0049] The polyolefin elastomer is made of ethylene-octene copolymer (POE) or olefin block copolymer (OBC).

[0050] The compatibilizer used is maleic anhydride-grafted polypropylene with a grafting rate of 0.8%.

[0051] The nucleating agent used is a substituted aryl phosphate α-crystal nucleating agent.

[0052] The compound antioxidant is composed of hindered phenolic antioxidant and phosphite antioxidant in a mass ratio of 1:1.

[0053] Example 1 The polypropylene matrix in the reinforced MPP composition consists of 40 parts HPP-1 and 60 parts BPP-1.

[0054] Based on 100 parts of polypropylene matrix, add 8 parts of chopped glass fiber, 0.05 parts of nucleating agent, 2 parts of POE, 0.30 parts of maleic anhydride-grafted polypropylene, 0.5 parts of composite antioxidant and 0.3 parts of processing aid.

[0055] The average length of the chopped glass fibers is 0.15 mm, and the aspect ratio is 8. The mass of the maleic anhydride-grafted polypropylene coating on the surface of the chopped glass fibers is 0.5% of the mass of the glass fibers.

[0056] The weldable MPP composition uses the same HPP-1 and BPP-1 as polypropylene matrix in a ratio of 40:60.

[0057] Based on 100 parts of polypropylene matrix, add 10 parts RPP-L, 0.5 parts composite antioxidant and 0.2 parts processing aid, without adding reinforcing phase and nucleating agent.

[0058] The MFR of the reinforced MPP composition is 1.20 g / 10 min, and the MFR of the weldable MPP composition is 0.78 g / 10 min, with a ratio of 0.65.

[0059] The temperature of the first extruder is 175–195℃, and the temperature of the second extruder is 165–185℃. The reinforcing phase is added through the side feed port at 60% of the effective length of the screw in the first extruder.

[0060] The melt distributor is equipped with a 2-stage static mixing unit with a V / Q ratio of 0.5s.

[0061] The length of the single-end hot-melt connection section is 50mm, and the length of the gradient transition section is 16mm. The gradient adopts a linear ratio change curve.

[0062] The average cooling rate of the hot-melt joint zone is 1.5 times that of the reinforced support zone. This cooling process results in the crystallinity of the hot-melt joint section being 5.0 percentage points lower than that of the reinforced support section.

[0063] Example 2 The polypropylene matrix in the reinforced MPP composition consists of 55 parts HPP-2 and 45 parts BPP-2.

[0064] Based on 100 parts of polypropylene matrix, 16.5 parts of chopped glass fiber, 0.175 parts of nucleating agent, 6 parts of POE, 1.15 parts of maleic anhydride-grafted polypropylene, 0.5 parts of composite antioxidant and 0.3 parts of processing aid are added.

[0065] The average length of the chopped glass fibers is 1.0 mm, and the aspect ratio is 30. The mass of the maleic anhydride-grafted polypropylene coating is 1.75% of the mass of the glass fibers.

[0066] The weldable MPP composition uses the same HPP-2 and BPP-2 as polypropylene matrices in a ratio of 55:45.

[0067] Based on 100 parts of polypropylene matrix, add 1.0 part chopped glass fiber, 0.010 part nucleating agent, 22.5 parts RPP-M, 0.5 parts composite antioxidant and 0.2 parts processing aid.

[0068] The MFR of both the reinforced MPP composition and the weldable MPP composition was 1.00 g / 10 min, and the ratio between the two was 1.00.

[0069] The temperature of the first extruder is 185–205℃, and the temperature of the second extruder is 175–195℃. The reinforcing phase is added through the side feed port at 70% of the effective length of the screw in the first extruder.

[0070] The melt distributor is equipped with a 4-stage static mixing unit with a V / Q ratio of 1.5s.

[0071] The length of the single-end hot-melt connection section is 120mm, and the length of the gradient transition section is 48mm. The gradient adopts an S-shaped ratio change curve.

[0072] The average cooling rate of the hot-melt joint zone is 2.2 times that of the reinforced support zone. This cooling process results in the crystallinity of the hot-melt joint section being 10.1 percentage points lower than that of the reinforced support section.

[0073] Example 3 The polypropylene matrix in the reinforced MPP composition consists of 70 parts HPP-3 and 30 parts BPP-3.

[0074] Based on 100 parts of polypropylene matrix, add 25 parts magnesium salt whiskers, 0.30 parts nucleating agent, 10 parts OBC, 2.00 parts maleic anhydride grafted polypropylene, 0.5 parts composite antioxidant and 0.3 parts processing aid.

[0075] The magnesium salt whiskers have an average length of 2.50 mm and an aspect ratio of 60. The maleic anhydride-grafted polypropylene coating accounts for 3.0% of the whisker mass.

[0076] The weldable MPP composition uses the same HPP-3 and BPP-3 as polypropylene matrices in a ratio of 70:30.

[0077] Based on 100 parts of polypropylene matrix, add 2.0 parts magnesium salt whiskers, 0.020 parts nucleating agent, 35 parts RPP-H, 0.5 parts composite antioxidant and 0.2 parts processing aid.

[0078] The MFR of the reinforced MPP composition is 0.80 g / 10 min, and the MFR of the weldable MPP composition is 1.08 g / 10 min, with a ratio of 1.35.

[0079] The temperature of the first extruder is 195–215℃, and the temperature of the second extruder is 185–205℃. The reinforcing phase is added through the side feed port at 80% of the effective length of the screw in the first extruder.

[0080] The melt distributor is equipped with a 6-stage static mixing unit with a V / Q ratio of 3.0 s.

[0081] The length of the single-end heat-fusion connection section is 192mm, which is 1.2 times the nominal outer diameter D; the length of the gradient transition section is 80mm, which is 0.5 times the nominal outer diameter D.

[0082] The gradient is represented by an S-shaped ratio change curve.

[0083] The average cooling rate of the hot-melt joint zone is 3.0 times that of the reinforced support zone. Through this cooling process, the crystallinity of the hot-melt joint section is 15.0 percentage points lower than that of the reinforced support section.

[0084] Example 4 The polypropylene matrix in the reinforced MPP composition consists of 55 parts HPP-2 and 45 parts BPP-2.

[0085] Based on 100 parts of polypropylene matrix, 16.5 parts of chopped basalt fiber, 0.175 parts of nucleating agent, 6 parts of POE, 1.15 parts of maleic anhydride-grafted polypropylene, 0.5 parts of composite antioxidant and 0.3 parts of processing aid are added.

[0086] The average length of the chopped basalt fibers is 0.80 mm, the aspect ratio is 28, and the coating mass is 1.5% of the basalt fiber mass.

[0087] The weldable MPP composition uses the same polypropylene matrix and adds 0.8 parts chopped basalt fiber, 0.008 parts nucleating agent, 20 parts RPP-M, 0.5 parts composite antioxidant and 0.2 parts processing aid.

[0088] The MFR ratio of the two compositions is 0.93.

[0089] The single-end hot-melt connection section is 80 mm long, and the gradient transition section is 32 mm long. The melt distributor is equipped with a 3-stage mixing unit with a V / Q ratio of 1.0 s. The gradient uses a linear curve, and the cooling rate ratio is 1.8.

[0090] The partitioned cooling process reduces the crystallinity of the hot-melt joint section by 9.9 percentage points compared to the reinforced load-bearing section.

[0091] Example 5 The polypropylene matrix in the reinforced MPP composition consists of 55 parts HPP-2 and 45 parts BPP-2.

[0092] Based on 100 parts of polypropylene matrix, add 16.5 parts of needle-shaped wollastonite, 0.175 parts of nucleating agent, 6 parts of OBC, 1.15 parts of maleic anhydride-grafted polypropylene, 0.5 parts of composite antioxidant and 0.3 parts of processing aid.

[0093] The needle-shaped wollastonite has an average length of 0.35 mm, an aspect ratio of 20, and a coating mass of 2.0% of the wollastonite mass.

[0094] The weldable MPP composition uses the same polypropylene matrix and adds 1.2 parts needle-shaped wollastonite, 0.012 parts nucleating agent, 25 parts RPP-M, 0.5 parts composite antioxidant and 0.2 parts processing aid.

[0095] The MFR ratio of the two compositions is 1.07.

[0096] The single-end hot-melt connection section is 160mm long, and the gradient transition section is 64mm long. The melt distributor is equipped with 5 mixing units, with a V / Q ratio of 2.0s. The gradient adopts an S-shaped curve, and the cooling rate ratio is 2.5.

[0097] The partitioned cooling process reduces the crystallinity of the hot-melt joint section by 8.7 percentage points compared to the reinforced load-bearing section.

[0098] Example 6 The polypropylene matrix in the reinforced MPP composition consists of 55 parts HPP-2 and 45 parts BPP-2.

[0099] Based on 100 parts of polypropylene matrix, the following are added: 8.25 parts chopped glass fiber and 8.25 parts needle-shaped wollastonite, 0.175 parts nucleating agent, 3 parts POE, 3 parts OBC, 1.15 parts maleic anhydride-grafted polypropylene, 0.5 parts composite antioxidant and 0.3 parts processing aid.

[0100] The average length of the composite reinforcing phase is 0.70 mm, the average aspect ratio is 25, and the coating layer mass is 1.75% of the reinforcing phase mass.

[0101] The weldable MPP composition uses the same polypropylene matrix and adds 1.0 part of composite reinforcing phase, 0.010 part of nucleating agent, 28 parts of RPP-M, 0.5 parts of composite antioxidant and 0.2 parts of processing aid.

[0102] The MFR ratio of the two compositions is 1.15.

[0103] The single-end hot-melt connection section is 100mm long, and the gradient transition section is 40mm long. The melt distributor is equipped with 4 mixing units, with a V / Q ratio of 1.2s. The gradient adopts an S-shaped curve, and the cooling rate ratio is 2.0.

[0104] The partitioned cooling process reduces the crystallinity of the hot-melt joint section by 11.8 percentage points compared to the reinforced load-bearing section.

[0105] In Comparative Example 1, the amount of chopped glass fiber in the weldable MPP composition was increased from 1.0 part in Example 2 to 16.5 parts, while other components and process parameters remained the same as in Example 2.

[0106] In Comparative Example 2, the nucleating agent in the weldable MPP composition was increased from 0.010 parts in Example 2 to 0.050 parts, while other components and process parameters remained the same as in Example 2.

[0107] Comparative Example 3: The enhanced MPP composition and the weldable MPP composition remained the same as in Example 2, but the gradient transition length was adjusted from 48 mm to 0, allowing the two compositions to switch directly along the axial direction.

[0108] Comparative Example 4: By replacing the random copolymer in the weldable MPP composition, the MFR ratio of the weldable MPP composition to the reinforced MPP composition was increased from 1.00 in Example 2 to 1.72. Other components and process parameters were the same as in Example 2.

[0109] In Comparative Example 5, the first servo metering pump, the second servo metering pump, the first extruder, and the second extruder all operate in open loop at a preset fixed speed, without feedback correction based on the total flow and pressure sensor readings. Other components and process parameters are consistent with those in Example 2.

[0110] Compared to Example 6, only the zoned cooling is removed. The same cooling medium temperature and flow rate are used in the enhanced load-bearing zone, gradient transition zone, and hot-melt connection zone.

[0111] In Comparative Example 6, only a crystallinity difference of 2.2 percentage points was formed between the hot-melt connection section and the reinforced load-bearing section, which did not reach the 5-15 percentage points limit of the present invention, indicating that the crystallinity difference is a direct product result of the partitioned cooling.

[0112] Hot melt connection conditions Two pipes of the same group and specifications are joined together by heat fusion.

[0113] Before hot melting, the oxide layer at the pipe end is removed by mechanical planing to ensure that the two end faces are flat and parallel to each other.

[0114] The temperature of the hot melt plate is 210±5℃, the heating pressure is 0.15±0.02MPa, the heat absorption time is 70s, the hot plate removal and end face switching time does not exceed 5s, the butt joint pressure is 0.20±0.02MPa, and the pressure holding and cooling time is 15min.

[0115] Each group used the same hot-melt equipment, hot-melt plate, fixtures, and environmental conditions.

[0116] After the heat fusion is completed, the joint is placed in an environment of 23±2℃ for no less than 24 hours before test samples are prepared.

[0117] Performance testing methods 1. Melt mass flow rate The melt mass flow rate of the reinforced MPP composition and the weldable MPP composition was tested according to GB / T 3682.1 at a test temperature of 230℃ and a nominal load of 2.16kg.

[0118] Each composition was tested in parallel 5 times, and the arithmetic mean was taken.

[0119] 2. Crystallinity and melting peak temperature Crystallinity and melting peak temperature were determined by differential scanning calorimetry according to GB / T 19466.3.

[0120] Samples were taken from the middle of the reinforced load-bearing section, different axial positions of the gradient transition section, and the middle of the hot-melt connection section.

[0121] The sampling location of the reinforced load-bearing section specimen should be at least 1 time the nominal outer diameter D from the boundary of the gradient transition section; the sampling location of the hot-melt connection section specimen should be located in the axial middle of the hot-melt connection section, avoiding the cutting surface and the subsequent hot-melt influence area.

[0122] The sample should be taken from the middle of the tube wall in the thickness direction to reduce the influence of the difference in cooling conditions between the inner and outer walls on the test results.

[0123] The sample mass was 5–10 mg, and the temperature was increased from 25 °C to 200 °C at a rate of 10 °C / min under nitrogen protection.

[0124] To preserve the initial crystallization state formed during pipe extrusion and zoned cooling, the melting enthalpy and melting peak temperature were calculated using the first heating curve.

[0125] The crystallinity Xc of the polypropylene phase is calculated according to the following formula: .

[0126] in, The measured enthalpy of fusion for the sample is given in J / g; 207 J / g is the reference enthalpy of fusion for fully crystalline polypropylene. This represents the mass fraction of polypropylene polymers in the sample.

[0127] The difference in crystallinity between the reinforced load-bearing section and the hot-melt connection section is calculated according to the following formula: ΔXc=Xc1-Xc2.

[0128] Wherein, Xc1 represents the crystallinity of the reinforced bearing section, and Xc2 represents the crystallinity of the hot-melt connection section.

[0129] No fewer than 5 DSC samples were prepared for each pipe section. The arithmetic mean of the crystallinity of the reinforced load-bearing section and the hot-melt connection section was calculated, and then the difference in crystallinity between the two was calculated.

[0130] When the average crystallinity of the hot-melt joint is 5 to 15 percentage points lower than the average crystallinity of the reinforced load-bearing section, the final product is deemed to have the differentiated crystal structure described in this invention.

[0131] The crystallinity difference can be directly measured from the finished pipe and is a result of zoned cooling. During final product testing, there is no need to measure or prove the average cooling rate during pipe production.

[0132] 3. Axial tensile properties of the tube body The axial tensile properties of the reinforced load-bearing section were tested in accordance with GB / T 8804.3.

[0133] Five samples were tested in each group, and the arithmetic mean was taken.

[0134] 4. Axial tensile properties of heat fusion joints The sampling method and failure mode determination principle of the hot melt joint shall be specified in GB / T 19810, and the tensile test conditions of polyolefin pipes specified in GB / T8804.3 shall be used for comparison test.

[0135] Five tensile specimens spanning the fusion surface were uniformly prepared in the circumferential direction of each hot-melt joint.

[0136] The axial tensile strength of a hot-melt joint is calculated by dividing the maximum tensile load by the minimum effective cross-sectional area of ​​the specimen.

[0137] The initial joint strength ratio is calculated according to the following formula: Initial joint strength ratio = initial axial tensile strength of the heat fusion joint / initial axial tensile strength of the reinforced load-bearing section × 100%.

[0138] Record the main failure locations of the specimen, including the hot-melt fusion surface, the gradient transition section, and the reinforced load-bearing section.

[0139] The coefficient of variation of the strength results of five hot-melt joint samples was calculated to evaluate the consistency of the pipe's axial gradient and welding performance.

[0140] 5. Hot air aging Hot air aging shall be carried out in accordance with GB / T 7141.

[0141] The pipe sections that have completed the heat fusion connection are placed in a forced ventilation aging chamber at 80±2℃ for aging for 1000 hours.

[0142] After aging, the samples were placed in an environment of 23±2℃ for 24 hours and then the axial tensile strength of the reinforced load-bearing section and the hot melt joint were tested respectively.

[0143] The aging retention rate of the joint is calculated according to the following formula: Joint aging retention rate = tensile strength of heat fusion joint after aging / tensile strength of heat fusion joint before aging × 100%.

[0144] The aging strength ratio is calculated according to the following formula: Aging strength ratio = Tensile strength of heat fusion joint after aging / Tensile strength of reinforced load-bearing section after aging × 100%.

[0145] 6. Ring stiffness The ring stiffness of the pipe was tested in accordance with GB / T 9647.

[0146] Five loops were tested in each group, and the arithmetic mean was taken.

[0147] The ring stiffness of the pipes was tested separately for the unaged pipes and the pipes aged at 80℃ for 1000 hours.

[0148] The ring stiffness retention rate is calculated according to the following formula: Ring stiffness retention rate = (ring stiffness after aging / initial ring stiffness) × 100%.

[0149] 7. Impact resistance The impact resistance performance is tested in accordance with the industry standard for trenchless plastic cable conduits and the clockwise rotation method specified in GB / T 14152. The results are expressed as the true impact rate (TIR).

[0150] 8. Longitudinal Retraction Rate The longitudinal shrinkage rate is tested in accordance with the MPP conduit test conditions specified in the industry standard for trenchless plastic cable conduits, and in accordance with GB / T 6671 and its current amendments.

[0151] When this application document was prepared, GB / T 6671—2001 "Determination of longitudinal shrinkage rate of thermoplastic pipes" and its No. 1 amendment were adopted.

[0152] 9. Fluctuations in total melt flow rate and die pressure Total melt flow rate and die pressure are process control indicators and are not the sole indicators for judging the zonal cooling effect in the final pipe.

[0153] After continuous extrusion and stable operation for 30 minutes, the total mass flow rate of the two melts and the die inlet pressure were recorded at 1-second sampling intervals.

[0154] Total mass flow rate fluctuation is calculated using the following formula: Total mass flow rate volatility = (Qmax - Qmin) / (2Q average) × 100%.

[0155] The pressure fluctuation rate at the die inlet is calculated using the following formula: Die inlet pressure fluctuation rate = (Pmax - Pmin) / (2P average) × 100%.

[0156] The actual gradient length is determined by sampling every 5 mm along the axial direction in the gradient transition section and detecting the ash content or the mass fraction of the reinforcing phase.

[0157] The actual gradient length deviation rate is calculated using the following formula: Gradient length deviation rate = |actual gradient length - set gradient length| / set gradient length × 100%.

[0158] Performance test results Table 1. Results of extrusion stability and thermal performance tests

[0159] As shown in Table 1, the total melt mass flow rate fluctuation rate of Examples 1 to 6 does not exceed 3%, the die inlet pressure fluctuation rate does not exceed 5%, and the gradient length deviation rate does not exceed 5%.

[0160] Examples 1, 2, and 3 yielded crystallinity differences of 5.0, 10.1, and 15.0 percentage points, respectively. These crystallinity differences were obtained through DSC testing at different axial positions of the final pipe, and were not calculated based on the cooling medium temperature or cooling rate during the production process. Therefore, the crystallinity differences in Table 1 are objective test results that directly characterize the final product structure.

[0161] Comparative Example 4 only increased the MFR ratio to 1.72, and its gradient length deviation rate increased to 18.4%, indicating that melt flow performance mismatch will cause low viscosity melt to flow preferentially and actual gradient deformation.

[0162] After the closed-loop control was removed in Comparative Example 5, the total flow fluctuation rate and the die head pressure fluctuation rate increased to 7.2% and 10.5%, respectively, and the gradient length deviation rate increased to 19.1%, indicating that a fixed pump speed cannot effectively compensate for the viscosity and back pressure changes under different mixing ratios.

[0163] In Comparative Example 6, after eliminating the zonal cooling, the crystallinity difference between the connecting section and the supporting section decreased from 10.1 percentage points in Example 2 to 2.2 percentage points. With the material formulation, gradient structure, melt distributor, and closed-loop control remaining the same, eliminating zonal cooling resulted in a significant decrease in the crystallinity difference, demonstrating that the 5–15 percentage point crystallinity difference is a direct product result of zonal cooling.

[0164] Table 2 Initial strength and thermal aging properties of hot melt joints

[0165] As shown in Table 2, the initial strength equalization rate of the hot-melt joints in Examples 1 to 6 was 91.3% to 95.6%, all higher than 90%. After aging for 1000 hours, the joint strength retention rate was 86.7% to 91.3%, and the aging strength equalization rate was 87.3% to 92.9%, indicating that the main failure location was not on the hot-melt fusion surface.

[0166] Comparative Example 1 only increased the reinforcing phase content of the hot-melt joint section, and its initial equal strength rate of the joint decreased from 95.6% in Example 2 to 77.1%, and the aging equal strength rate decreased to 66.1%.

[0167] Comparative Example 2 only increased the nucleating agent content in the hot-melt joint section, which increased the crystallinity of the joint section, reduced the initial equal strength rate of the joint to 82.4%, and reduced the aging equal strength rate to 72.8%.

[0168] In Comparative Example 3, after the gradient transition section was removed, the initial strength ratio and aging strength ratio of the joint decreased to 86.5% and 75.2%, respectively, with the failure mainly caused by the component abrupt change interface. This result indicates that direct component abrupt changes can create preferential failure regions, and a continuous gradient transition structure is beneficial for reducing performance abrupt changes between different functional sections and improving long-term connection reliability.

[0169] Comparative Example 4 showed that even when the MFR ratio was exceeded, the joint strength ratio and long-term retention rate both decreased significantly.

[0170] After the closed-loop control was removed in Comparative Example 5, the coefficient of variation of joint strength increased to 9.4%, which was significantly higher than 2.1% in Example 2.

[0171] In Comparative Example 6, after the zonal cooling was removed, although the material formulation and composition gradient remained unchanged, the crystallinity difference between the hot-melt joint section and the reinforced load-bearing section was only 2.2 percentage points, the initial equal strength rate of the joint was 89.0%, and the aging equal strength rate was 78.5%.

[0172] The results between Example 2 and Comparative Example 6 show that the 5-15 percentage point crystallinity difference formed by partitioned cooling has a direct correlation with the strength and long-term reliability of the heat fusion joint.

[0173] Table 3 Basic Mechanical Properties of Pipes

[0174] Table 3 shows that Examples 1-6 improve the strength of the hot melt joint while maintaining high ring stiffness, resistance to external impact, and longitudinal dimensional stability.

[0175] The hot-melt connection section and the gradient transition section only account for a small part of the total axial length of the pipe. The area that bears the main mechanical load is still the reinforced load-bearing section. Therefore, setting a low-reinforcement content connection section does not cause a significant decrease in the ring stiffness of the entire pipe.

[0176] Figure 1 In the graph, the horizontal axis represents temperature, and the vertical axis represents heat flow, with the heat absorption direction upwards. The solid line represents the reinforced load-bearing section, and the dashed line represents the hot-melt connection section. The difference in the melting peak area between the two curves indicates that the crystallinity of the reinforced load-bearing section is higher than that of the hot-melt connection section.

[0177] Figure 2 In the figure, the horizontal axis represents the axial position from the pipe end face, and the vertical axis represents the apparent crystallinity. The curve shows that the crystallinity gradually increases from the hot-melt connection section through the gradient transition section to the reinforced load-bearing section, and remains relatively stable within each stable functional section.

[0178] Figure 3 In the diagram, the dashed line marks the boundary of the fusion zone, with the heat-affected zones of the two pipes above and below the interface, respectively. The fusion zone exhibits a continuous and dense microstructure, with no obvious delamination, through cracks, or large pores, indicating that the joint interface is well-fused.

[0179] Figure 4 In the experiment, samples were taken from the middle of the tube wall, subjected to liquid nitrogen embrittlement, drying, and gold sputtering, and then observed under an accelerating voltage of 5–10 kV. In the figure, 1 represents the chopped glass fiber reinforcement phase, 2 represents the polypropylene matrix, 3 represents the residual polypropylene matrix layer attached to the fiber surface, 4 represents the interfacial bonding area between the fiber and the matrix, and 5 represents the matrix fracture texture. The reinforcement phase was relatively uniformly dispersed, with no obvious agglomeration or continuous debonding gaps, indicating good interfacial bonding between the fiber and the polypropylene matrix.

[0180] Figure 5In the experiment, after the thermofusion joint fractured under axial tensile testing, samples from the fracture area were taken, cleaned, dried, and sputter-coated with gold, and then observed using a scanning electron microscope. In the figure, 1 represents the fracture area of ​​the reinforcing load-bearing section, 2 represents the thermofusion joint area, 3 represents the thermofusion interface, 4 represents the ductile tear texture of the polypropylene matrix, and 5 represents the fracture or pull-out marks of the reinforcing phase. The fracture morphology shows that the cracks mainly occurred inside the reinforcing load-bearing section, and the fusion interface remained continuous with no obvious interface debonding, indicating that the thermofusion joint has good interfacial bonding performance.

[0181] The core technical problem solved by this invention is: how to improve the strength of MPP pipe body by using reinforcing phase and nucleating agent, while avoiding the reinforcing phase and nucleating component from hindering the interdiffusion of polypropylene molecular chains at the hot melt interface, so that the hot melt joint can achieve a strength that is basically equal to that of the reinforced pipe body, and maintain high reliability after long-term thermal aging.

[0182] The only difference between Example 2 and Comparative Example 1 was the content of the reinforcing phase in the hot-melt joint. The initial strength equalization rate of the joint in Example 2 was 95.6%, while that in Comparative Example 1 was 77.1%; the aging strength equalization rate of Example 2 was 92.0%, while that in Comparative Example 1 was 66.1%. These results demonstrate that reducing the content of the reinforcing phase in the joint has a significant effect on restoring the molecular chain diffusion ability of the hot-melt interface.

[0183] The only difference between Example 2 and Comparative Example 2 was the nucleating agent content in the connecting segment. In Comparative Example 2, the crystallinity of the connecting segment increased significantly, and the initial and aged strength ratios of the joint decreased to 82.4% and 72.8%, respectively, demonstrating that a low nucleating agent content in the connecting segment can prolong the effective diffusion time of the hot-melt interface.

[0184] The only difference between Example 2 and Comparative Example 3 was the presence or absence of a gradient transition section. After removing the gradient, the failure of the sample shifted from the reinforced load-bearing section to the failure at the abrupt component interface, and the aging strength rate decreased by 16.8 percentage points, indicating that the continuous gradient structure significantly contributes to long-term reliability.

[0185] Examples 2 and 4 show that even if the two compositions use the same polypropylene matrix, if the MFR ratio exceeds 0.65 to 1.35, component enrichment and gradient length deviation will still occur, indicating that the same polypropylene matrix system and MFR matching need to work together.

[0186] Example 2 and Comparative Example 5 show that, under the condition that the material formulation, structure and cooling process are exactly the same, closed-loop control can reduce the total flow fluctuation rate from 7.2% to 1.2%, the die head pressure fluctuation rate from 10.5% to 2.5%, and the joint strength variation coefficient from 9.4% to 2.1%.

[0187] The only difference between Example 2 and Comparative Example 6 is whether or not partitioned cooling is used. In Example 2, partitioned cooling is used, and the crystallinity of its hot-melt joint section is 10.1 percentage points lower than that of the reinforced load-bearing section; in Comparative Example 6, partitioned cooling is not used, and the difference in crystallinity is only 2.2 percentage points.

[0188] Example 2 and Comparative Example 6 have the same material formulation, continuous gradient structure, melt distributor and closed-loop control conditions, so the significant difference in crystallinity between the two can be attributed to partitioned cooling.

[0189] This indicates that the 5-15 percentage point lower crystallinity of the hot-melt joint section compared to the reinforced load-bearing section is a direct result of the partitioned cooling process, rather than a random phenomenon caused by differences in the composition.

[0190] The crystallinity difference can be directly obtained by performing DSC testing on the final pipe material, without needing to prove what kind of cooling device, cooling medium flow rate, or cooling medium temperature was used during product manufacturing. This gives the differentiated crystal structure of the present invention an objective, clear, and repeatable basis for product characterization.

[0191] The crystallinity difference of Example 2 was 10.1 percentage points, and its initial joint strength was 95.6% and its aging strength was 92.0%; the crystallinity difference of Comparative Example 6 was 2.2 percentage points, and its initial joint strength was 89.0% and its aging strength was 78.5%.

[0192] This result further demonstrates that the low-reinforcement joint segment, continuous component gradient, and crystallinity difference of 5–15 percentage points have a synergistic effect. Simply setting a low-reinforcement joint segment and component gradient without forming a corresponding crystallinity difference cannot achieve the same joint strength and long-term reliability.

[0193] In summary, while maintaining the strength of the reinforced bearing section and the ring stiffness of the pipe, the present invention enables the initial axial tensile strength of the hot-melt joint to reach more than 90% of the axial tensile strength of the reinforced bearing section, and enables the hot-melt joint to maintain a joint strength retention rate of not less than 85% and an aging equal strength rate of not less than 85% after aging at 80℃ for 1000h.

Claims

1. A high-strength MPP cable protection pipe, characterized in that, The device includes a reinforced load-bearing section integrally formed along the axial direction, hot-melt connection sections located at both ends of the reinforced load-bearing section, and gradient transition sections located between the reinforced load-bearing section and the hot-melt connection section, each section penetrating the pipe wall thickness; the reinforced load-bearing section is formed of a reinforced MPP composition, which, based on 100 parts by weight of polypropylene matrix, includes 8-25 parts by weight of reinforcing phase, 0.05-0.30 parts by weight of nucleating agent, 2-10 parts by weight of polyolefin elastomer, and 0.3-2 parts by weight of compatibilizer; the hot-melt connection section is formed of a weldable MPP composition, which, based on 100 parts by weight of polypropylene matrix, includes... The composition comprises no more than 2 parts by weight of reinforcing phase, no more than 0.02 parts by weight of nucleating agent, and 10-35 parts by weight of propylene-ethylene random copolymer; the reinforced MPP composition and the weldable MPP composition use propylene homopolymers of the same composition and performance grade, and propylene-ethylene block copolymers of the same composition and performance grade. Under a load of 2.16 kg at 230 °C, the ratio of the melt mass flow rate of the weldable MPP composition to that of the reinforced MPP composition is 0.65-1.35; in the gradient transition section, the content of reinforcing phase and nucleating agent continuously decreases along the direction towards the hot-melt connection section, while the content of propylene-ethylene random copolymer continuously increases.

2. The high-strength MPP cable protection pipe according to claim 1, characterized in that, The reinforcing phase is selected from one or more of chopped glass fibers, chopped basalt fibers, acicular wollastonite, and magnesium salt whiskers. The average length of the reinforcing phase is 0.15–2.50 mm and the aspect ratio is 8–60. The reinforcing phase is surface-treated with a silane coupling agent and maleic anhydride-grafted polypropylene. The mass of the maleic anhydride-grafted polypropylene coating layer is 0.5%–3.0% of the mass of the reinforcing phase.

3. The high-strength MPP cable protection pipe according to claim 1, characterized in that, Both the reinforced MPP composition and the weldable MPP composition contain a polypropylene matrix composed of 40-70 parts by weight of propylene homopolymer and 30-60 parts by weight of propylene-ethylene block copolymer, totaling 100 parts by weight. The isotactic index of the propylene homopolymer is not less than 95%, and the ethylene content in the propylene-ethylene block copolymer is 5%-15% by mass. The nucleating agent is a substituted aryl phosphate α-crystalline nucleating agent, and the polyolefin elastomer is selected from one or two of ethylene-octene copolymer and olefin block copolymer.

4. The high-strength MPP cable protection pipe according to claim 1, characterized in that, The ethylene mass content in the propylene-ethylene random copolymer is 1.5% to 4.5%, the melting peak temperature is 135 to 150°C, and the melt mass flow rate at 230°C and 2.16 kg load is 0.3 to 2.0 g / 10 min. The axial length of each hot-melt connection section is 0.3 to 1.2 times the nominal outer diameter D of the high-strength MPP cable protection pipe and not less than 50 mm. The axial length of each gradient transition section is 0.1 to 0.5 times the nominal outer diameter D.

5. A high-strength MPP cable protection pipe according to claim 1, characterized in that, When the reinforced bearing section, gradient transition section and hot-melt connection section are tested by differential scanning calorimetry respectively, the crystallinity of the hot-melt connection section is 5 to 15 percentage points lower than that of the reinforced bearing section, and the crystallinity of the gradient transition section decreases continuously in the direction toward the hot-melt connection section; the difference in melting peak temperature between the hot-melt connection section and the reinforced bearing section does not exceed 12°C.

6. A high-strength MPP cable protection pipe according to claim 5, characterized in that, After two high-strength MPP cable protection pipes of the same specification are butt-welded through their respective hot-melt connection sections, the axial tensile strength of the hot-melt joint is not less than 90% of the axial tensile strength of the reinforcing load section, and the tensile failure location is outside the hot-melt fusion surface; after the hot-melt joint is aged in an air environment at 80°C for 1000 hours, its axial tensile strength retention rate is not less than 85%, and the axial tensile strength of the aged hot-melt joint is not less than 85% of the axial tensile strength of the aged reinforcing load section.

7. A method for preparing a high-strength MPP cable protection pipe, characterized in that, Includes the following steps: S1. Prepare reinforced MPP material and weldable MPP material separately. The reinforced MPP material, based on 100 parts by weight of polypropylene matrix, includes 8-25 parts by weight of reinforcing phase, 0.05-0.30 parts by weight of nucleating agent, 2-10 parts by weight of polyolefin elastomer, and 0.3-2 parts by weight of compatibilizer. The weldable MPP material, based on 100 parts by weight of the polypropylene matrix, includes no more than 2 parts by weight of reinforcing phase, no more than 0.02 parts by weight of nucleating agent, and 10-35 parts by weight of propylene-ethylene random copolymer. S2. The reinforced MPP material and the weldable MPP material are respectively fed into the first extruder and the second extruder for plasticization to obtain reinforced MPP melt A and weldable MPP melt B, and the reinforced MPP melt A and weldable MPP melt B are fed into an adjustable melt distributor. The adjustable melt distributor includes a first metering channel and a second metering channel for conveying reinforced MPP melt A and weldable MPP melt B respectively, a first servo metering pump and a second servo metering pump respectively disposed on the first metering channel and the second metering channel, a manifold connected to the two metering channels, and a static mixing channel disposed downstream of the manifold. The static mixing channel is connected to the annular extrusion die. S3. During the continuous extrusion process, the mass flow ratio of reinforced MPP melt A and weldable MPP melt B is controlled by the first servo metering pump and the second servo metering pump, so that the adjustable melt distributor sequentially outputs reinforced MPP melt A, a mixed melt with a continuously changing ratio of the two melts, weldable MPP melt B, and a mixed melt with a continuously changing ratio of the two melts in the opposite direction, so that the continuous tube blank periodically forms a reinforced bearing zone, a gradient transition zone, and a hot melt connection zone along the axial direction. S4. The axial length of each complete hot-melt connection zone in the continuous tube blank is twice the length of the single-end hot-melt connection section of the finished tube. After cutting at the axial midpoint of the complete hot-melt connection zone, a hot-melt connection end is formed for each of the two adjacent tubes. The continuous tube blank is sized, cooled in sections, and pulled. The section between the axial midpoints of the two adjacent hot-melt connection zones is used as a cutting unit to obtain a high-strength MPP cable protection pipe with hot-melt connection sections at both ends.

8. The method for preparing a high-strength MPP cable protection pipe according to claim 7, characterized in that, The adjustable melt distributor further includes a distributor housing, a first melt pressure sensor located at the inlet of the first metering channel, a second melt pressure sensor located at the inlet of the second metering channel, and a die pressure sensor located at the inlet of the annular extrusion die. The outlets of the first and second metering channels are alternately connected to the annular confluence cavity in the circumferential direction. The static mixing channel is provided with 2 to 6 stages of split-flow superimposed mixing units. The total melt volume flow rate at the outlet of the static mixing channel is Q, the internal volume between the outlet of the first metering channel and the outlet of the static mixing channel is V, and V / Q is 0.5 to 3.0 s to reduce axial tailing during the melt ratio switching process.

9. The method for preparing a high-strength MPP cable protection pipe according to claim 8, characterized in that, The first servo metering pump, the second servo metering pump, the first extruder, and the second extruder are connected to a controller. The controller performs proportional control, total flow control, and feed pressure control based on a preset mass flow rate ratio curve r(t) of the weldable MPP melt B, a set value for the total melt mass flow rate, and a set value for the annular extrusion die inlet pressure. Specifically, the proportional control adjusts the speed ratio of the first servo metering pump and the second servo metering pump based on the ratio of the actual mass flow rate of the weldable MPP melt B to the actual total mass flow rate of the two melts. The total flow control maintains the total mass flow rate of the two melts by synchronously adjusting the speeds of the two servo metering pumps. The feed pressure control adjusts the screw speeds of the first extruder and the second extruder based on the detection values ​​of the first melt pressure sensor, the second melt pressure sensor, and the die pressure sensor, so that the fluctuation of the total melt mass flow rate relative to the set value does not exceed 3%, and the fluctuation of the annular extrusion die inlet pressure relative to the set value does not exceed 5%.

10. The method for preparing a high-strength MPP cable protection pipe according to claim 9, characterized in that, The controller determines the arrival times of the reinforced bearing zone, gradient transition zone, and hot-melt connection zone at the zoned cooling device based on the melt ratio switching signal of the adjustable melt distributor, the distance from the annular extrusion die outlet to the zoned cooling device, and the traction speed of the continuous billet, and controls the medium flow rate and medium temperature of the corresponding cooling zones respectively; the average cooling rate of the hot-melt connection zone during the process of decreasing from 140°C to 100°C is 1.5 to 3.0 times the average cooling rate of the reinforced bearing zone.

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