Bfrp-u sheathed double-layer axial hollow-wall pipe and preparation method thereof

Abstract

The application relates to the technical field of structural wall pipes for buried drainage, and particularly discloses a BFRP-U armored plastic double-layer axial hollow wall pipe and a preparation method thereof. The pipe comprises an inner layer, an intermediate layer and an outer layer which are combined in sequence from inside to outside; the inner layer is a pure PVC-U layer; the intermediate layer is made of BFRP reinforced PVC-U composite material; the outer layer is a pure PVC-U layer or an anti-aging modified PVC-U layer; the BFRP reinforced PVC-U composite material comprises PVC-U resin, surface-modified short-cut basalt fibers, toughening modifier, nano filler, heat stabilizer, processing aid, lubricant, coupling agent and antioxidant. The hollow wall pipe can be used for buried laying in the fields of municipal drainage, industrial wastewater transportation, urban rainwater and sewage pipe network, farmland water conservancy irrigation and the like, and has the advantages of stable and firm layer interface combination, high overall mechanical property, good corrosion resistance, high long-term creep resistance and high low-temperature impact resistance.

Description

Technical Field

[0001] This application relates to the technical field of structural wall pipes for buried drainage, and more specifically, it relates to a BFRP-U armored double-layer axial hollow wall pipe and its preparation method. Background Technology

[0002] Rigid polyvinyl chloride (PVC) structural wall pipe systems for buried drainage are widely used in municipal drainage, buried sewage, and farmland irrigation projects due to their excellent chemical corrosion resistance, good cost performance, and mature processing technology. Meanwhile, double-layer axially hollow wall pipes are usually made by extruding single-layer PVC-U material, and the axially hollow structure achieves a balance between high ring stiffness and material consumption ratio.

[0003] Technicians in related fields have improved the performance of PVC-U pipes by adding reinforcing materials or using multi-layer composite structures; however, the interfacial bonding between the reinforcing materials and the PVC-U matrix is ​​often weak, the uniformity of fiber dispersion is difficult to control, and multi-layer composite structures usually require step-by-step processing, resulting in low production efficiency, increased energy consumption, and easy introduction of stress concentration points. As a result, the improvement in the tensile strength, flexural modulus and ring stiffness of the pipes is limited, the creep ratio is high during long-term use, the low-temperature impact resistance is not significantly improved, and the overall durability is affected. Summary of the Invention

[0004] To address the issue of overall durability being compromised by adding reinforcing materials or employing multi-layer composite structures, this application provides a BFRP-U armored double-layer axial hollow wall tube and its preparation method.

[0005] In the first aspect, this application provides a BFRP-U armored double-layer axial hollow wall tube, which adopts the following technical solution:

[0006] A BFRP-U armored double-layer axial hollow wall tube includes an inner layer, a middle layer, and an outer layer joined sequentially from the inside to the outside; the inner layer is a pure PVC-U layer; the middle layer is made of BFRP-reinforced PVC-U composite material; and the outer layer is a pure PVC-U layer or an anti-aging modified PVC-U layer.

[0007] The BFRP-reinforced PVC-U composite material comprises, by weight: 90-110 parts PVC-U resin, 15-35 parts surface-modified chopped basalt fiber, 5-15 parts toughening modifier, 3-10 parts nanofiller, 3-6 parts heat stabilizer, 1-4 parts processing aid, 0.5-2 parts lubricant, 0.5-2 parts coupling agent, and 0.1-0.5 parts antioxidant.

[0008] By adopting the above technical solutions, the tensile strength and flexural modulus of the composite material are improved by using surface-modified chopped basalt fibers as the reinforcing phase, and the pipe is given higher ring stiffness. At the same time, the addition of toughening modifiers improves the brittleness of the composite material at low temperatures, and the introduction of nanofillers further refines the matrix and enhances the interfacial bonding between fibers and resin. The combination of heat stabilizers, processing aids and lubricants ensures the thermal stability and melt flow of the composite material during processing, the coupling agent ensures the interfacial bonding between basalt fibers and PVC-U matrix, and the antioxidant provides long-term thermo-oxidative stability. Therefore, the final BFRP-U armored pipe is superior to unreinforced ordinary PVC-U pipe in terms of mechanical properties, low-temperature toughness and long-term durability.

[0009] Preferably, the toughening modifier is one or more of chlorinated polyethylene, acrylate copolymers, or methyl methacrylate-butadiene-styrene copolymer; the nanofiller is one or more of nano-calcium carbonate, nano-montmorillonite, or nano-silica.

[0010] By adopting the above technical solution, toughening agents such as chlorinated polyethylene and methyl methacrylate-butadiene-styrene copolymer are used. These agents, as elastomer phases, are dispersed in the PVC-U matrix, which can induce crazing and shear bands, effectively absorbing and dispersing impact energy, thereby reducing the low-temperature brittleness of the material. The addition of nanofillers not only plays a reinforcing role, but also acts as heterogeneous nucleation points in the matrix, refining spherulites. At the same time, their huge specific surface area helps to prevent the agglomeration of short-cut fibers and promotes the uniform dispersion of fibers in the matrix, thereby synergistically improving the overall strength and toughness of the composite material.

[0011] Preferably, the heat stabilizer is one or more of a calcium-zinc composite stabilizer, a rare earth stabilizer, or an organotin stabilizer; and the processing aid is an acrylate processing aid.

[0012] By adopting the above technical solution, environmentally friendly heat stabilizers such as calcium-zinc composite stabilizers can effectively capture hydrogen chloride produced by the decomposition of PVC resin at the processing temperature, prevent catalytic degradation, and ensure the stability of the melt during processing. The addition of acrylate processing aids can promote the melting and plasticization of PVC resin, reduce melt viscosity and processing torque, make composite materials containing fibers and fillers easier to process, and improve the uniformity of the melt and the surface quality of the extruded preform, providing a melt foundation for the subsequent shaping and cooling process.

[0013] Preferably, the lubricant is stearic acid or calcium stearate; the coupling agent is selected from one or more of KH-550, KH-560 or KH-570; and the antioxidant is a composite system of hindered phenolic antioxidant and phosphite auxiliary antioxidant.

[0014] By adopting the above technical solutions, the use of stearic acid as a lubricant reduces friction between the melt and the metal surface of the processing equipment, improves material conveying and extrusion flowability, and prevents overheating and decomposition of the material. The use of silane coupling agents to treat basalt fibers allows the hydrolyzed silanol groups to combine with the hydroxyl groups on the fiber surface, while the organic functional groups interact physically or chemically with the PVC-U resin matrix, thereby constructing an interfacial layer between the fiber and the resin and improving the interfacial shear strength. Furthermore, the use of a composite antioxidant system, where the antioxidant not only captures free radicals but also decomposes hydrogen peroxide, effectively inhibits the thermo-oxidative aging of the composite material during processing and long-term use, maintaining the long-term stability of the material's properties.

[0015] Secondly, this application provides a method for preparing a BFRP-U armored double-layer axial hollow wall tube, which adopts the following technical solution:

[0016] A method for preparing a BFRP-U armored double-layer axially hollow wall tube includes the following steps:

[0017] S1. Surface modification of basalt fiber: After cleaning and drying the chopped basalt fiber, surface treatment is performed with coupling agent solution, followed by drying to obtain surface-modified basalt fiber.

[0018] S2. Preparation of intermediate layer composite material: PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are mixed and heated, toughening modifier and nanofiller are added, mixing is continued and then cooled to obtain matrix mixture; surface-modified basalt fiber is mixed evenly with matrix mixture; the mixture is added to a twin-screw extruder for melt blending and granulation to obtain BFRP reinforced PVC-U composite material granules;

[0019] S3, Three-layer co-extrusion molding: Using a three-layer co-extrusion production line, the inner layer material, the middle layer composite material and the outer layer material are added to the corresponding extruders, melted and plasticized, and then extruded through a three-layer co-extrusion die to form a tube blank with a three-layer structure.

[0020] S4. Shaping and Cooling: The extruded melt is shaped by a vacuum shaping sleeve to form an axially hollow structure on the outer wall, the shape of reinforcing ribs and the spiral texture on the outer surface, and then cooled to obtain a shaped tube.

[0021] S5. Traction Cutting: Traction and cutting of the shaped pipe obtained in S4;

[0022] S6. Flaring and forming: One end of the pipe is heated and flared to form a socket end. During the flaring process, the first sealing groove, the second sealing groove, the test pressure hole threaded interface, the axial reinforcing rib, and the end face reinforcing flange are formed simultaneously. The other end of the pipe is the spigot end. During the forming process, the first sealing step, the second sealing step, and the lubrication guide groove are formed simultaneously.

[0023] By adopting the above technical solution, the interface between fibers and resin is first improved through coupling agent treatment, providing a foundation for the preparation of high-performance composite materials. Then, the fibers are uniformly dispersed and impregnated in the matrix through twin-screw extrusion melt blending to prepare intermediate layer granules. Subsequently, a three-layer co-extrusion technology is used to achieve one-time composite molding of the inner layer, the reinforcing intermediate layer, and the functional outer layer, ensuring the integrity of the pipe wall structure and the interlayer bonding force. Finally, through an integrated flaring molding process, the sealing and reinforcement structure of the socket are completed simultaneously in one processing step. Therefore, this method can efficiently and continuously produce integrated composite pipes with excellent mechanical properties, reliable sealing interfaces, and complex outer wall structures.

[0024] Preferably, in step S1, the chopped basalt fibers are first ultrasonically cleaned in acetone for 30–60 min and vacuum dried at 60–80 °C for 2–4 h; simultaneously, the coupling agent is dissolved in an ethanol aqueous solution, the pH is adjusted to 4–5, and hydrolyzed for 30 min to obtain a coupling agent solution; then the dried fibers are added to the coupling agent solution, stirred and reacted for 1–2 h, and then dried at 80–100 °C to constant weight.

[0025] By adopting the above technical solution, ultrasonic cleaning with acetone can effectively remove textile-type wetting agents and contaminants from the fiber surface, exposing more active hydroxyl sites; hydrolysis of the coupling agent under weakly acidic conditions is beneficial to the formation and stabilization of silanol bonds; the subsequent stirring reaction in the solution allows the hydrolyzed coupling agent molecules to fully chemically bond and physically adsorb with the clean fiber surface, thereby forming a uniform coupling agent molecular layer on the fiber surface; the final drying process removes the solvent and cures the coupling agent coating. Therefore, the compatibility and interfacial adhesion between the obtained surface-modified basalt fiber and PVC-U resin are improved, providing conditions for obtaining high interfacial shear strength in subsequent composite materials.

[0026] Preferably, in step S2, the PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are first stirred and heated to 80-90°C, then toughening modifier and nanofiller are added, and stirring is continued to 100-110°C, and the discharged material is cooled to below 40°C; while during melt blending, the extrusion temperature is controlled at 160-180°C, and the screw speed is 150-300 rpm.

[0027] By adopting the above technical solution, the resin is first mixed with the stabilizing system and lubricant at medium and low temperatures, so that it is initially coated on the surface of the resin particles, playing a role in pre-lubrication and pre-stabilization. Then, the temperature is raised and toughening agents and nanofillers are added. The higher temperature is used to soften and initially disperse the toughening agent, while the nanofillers are initially embedded in the resin. This cooling step prevents the material from agglomerating and facilitates subsequent dry mixing with fibers. In the twin-screw melt blending stage, the relatively low extrusion temperature is controlled to avoid thermal degradation of PVC resin, while ensuring that the melt has sufficient fluidity to fully wet and coat the fibers. The appropriate screw speed provides sufficient shear force to disperse the fibers and nanofillers while avoiding excessive mechanical damage to the fibers. Therefore, this combination of process parameters ensures that the composite material maintains stable processing performance while obtaining good fiber dispersion and interfacial bonding.

[0028] Preferably, in step S3, the co-extrusion die temperature is 175-185°C.

[0029] By adopting the above technical solution, the mold temperature of 175 to 185°C is higher than the melting temperature of PVC-U resin, which ensures that the three-layer melt has high fluidity and adhesion when they come together, so that the inner, middle and outer layers form a composite pipe blank in the mold. At the same time, this temperature range is also within the safe window for PVC-U processing, preventing material decomposition due to excessive temperature. Therefore, this temperature setting can ensure that the three-layer structure is tightly composited and ensure the quality of interlayer bonding of the pipe.

[0030] Preferably, in step S4, the cooling and shaping of the tube is controlled to below 40°C; the cross-section of the reinforcing rib is controlled to be trapezoidal, with the angle between the two sides and the radial direction being 5° to 15°, the spiral texture pitch being 0.5 to 1.0 times the nominal outer diameter of the tube, and the texture depth being 0.5 to 2.0 mm.

[0031] By adopting the above technical solutions, the vacuum shaping and cooling process to below 40°C ensures the stability of the pipe's shape and dimensions and releases internal stress. The reinforcing ribs are designed with trapezoidal cross-sections with included angles, which provide lateral support and circumferential stiffness when subjected to external loads. The angle range balances structural strength and demolding feasibility. Controlling the pitch and depth of the spiral texture to form a continuous concave-convex structure on the outer surface of the pipe not only further improves the pipe's circumferential stiffness but also increases the friction coefficient with the backfill material during construction and laying, preventing pipe displacement. Therefore, this shaping and cooling process, combined with the structural parameter design, enhances the pipe's structural mechanical properties and engineering applicability while giving it the specified appearance.

[0032] Preferably, during process S6, the taper of the sealing section at the socket end is controlled to be 1:100 to 1:200, the spacing between the sealing grooves is 0.2 to 0.3 times the nominal outer diameter of the pipe and not less than 30 mm; the depth of the sealing groove is controlled to match the diameter of the sealing ring, and the compression rate is 15% to 25%.

[0033] By adopting the above technical solutions, the appropriate taper of the sealing section facilitates spigot installation and ensures that the sealing ring can be smoothly inserted and positioned due to the precise control of taper, spacing and compression ratio parameters. The reasonable sealing groove spacing provides a sealing chamber for the double sealing ring, which is the structural basis for forming a reliable double sealing barrier and realizing the independent pressure test function of the interface. Controlling the compression ratio of the sealing ring within a specific range can ensure sufficient contact stress to achieve sealing under normal conditions, and also provide elastic compensation space for the sealing ring to maintain effective sealing when the pipe is subjected to interface deflection or displacement caused by foundation deformation or temperature changes.

[0034] In summary, this application has the following beneficial effects:

[0035] 1. This application adopts a three-layer co-extrusion one-time molding structure consisting of an inner pure PVC-U layer, a middle BFRP reinforced PVC-U composite material layer, and an outer pure PVC-U or anti-aging layer. The middle layer uses surface-modified short-cut basalt fiber as the reinforcing phase, which enables the fiber to play a reinforcing role in load-bearing, improves the tensile strength, flexural modulus and ring stiffness of the pipe body, improves the impact brittleness of the material in low temperature environment, and reduces the long-term creep rate. Therefore, it achieves the effect of improving the comprehensive mechanical properties and long-term durability of the pipe wall.

[0036] 2. This application preferably adopts a toughening modifier system and a three-layer color-separated visual design. Since the toughening agent forms an elastic dispersed phase in the PVC-U matrix, it can induce silver streaks and absorb impact energy, and work synergistically with nanofillers to further improve the low-temperature toughness of the composite material. At the same time, the inner, middle and outer layers are given different colors and the interlayer color difference is controlled. Since this design forms a distinct visual hierarchy and brand recognition characteristics, the product is easily identifiable by the naked eye at the construction site. Therefore, while improving the low-temperature toughness of the material, it also achieves the effect of strengthening brand anti-counterfeiting and quality control.

[0037] 3. The method of this application involves setting two axially spaced sealing grooves on the inner wall of the pipe socket end and setting a corresponding sealing step on the spigot end to install double sealing rings. At the same time, a pressure test hole is set on the outer wall of the socket end, which is connected to the circumferential space between the two sealing rings. This forms a physically isolated and independently pressurized sealing chamber, which ensures zero leakage through the double barrier even when the joint is subjected to radial deformation, angular deflection, or even negative pressure conditions. It also allows the construction party to quickly and directly verify the sealing performance of each joint before backfilling, thereby improving the reliability of pipeline system connections, the controllability of construction quality, and the safety of long-term operation. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating a method for preparing a BFRP-U armored double-layer axial hollow wall tube as proposed in this application. Detailed Implementation

[0039] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0040] Example 1: This example provides a BFRP-U armored double-layer axial hollow wall tube, comprising an inner layer, a middle layer, and an outer layer joined sequentially from the inside out; the inner layer is a pure PVC-U layer; the middle layer is made of BFRP reinforced PVC-U composite material; and the outer layer is a pure PVC-U layer.

[0041] The BFRP-reinforced PVC-U composite material comprises, by weight: 90 parts PVC-U resin, 15 parts surface-modified chopped basalt fiber, 5 parts toughening modifier, 3 parts nanofiller, 3 parts heat stabilizer, 1 part processing aid, 0.5 parts lubricant, 0.5 parts coupling agent, and 0.1 parts antioxidant.

[0042] Among them, the toughening modifier is chlorinated polyethylene; the nanofiller is nano-calcium carbonate; the heat stabilizer is calcium-zinc composite stabilizer; the processing aid is acrylate processing aid; the lubricant is stearic acid; the coupling agent is KH-550; and the antioxidant is a composite system of hindered phenolic antioxidant and phosphite auxiliary antioxidant.

[0043] The preparation method of the above-mentioned BFRP-U armored double-layer axial hollow wall tube includes the following steps:

[0044] S1. Surface modification of basalt fiber: After cleaning and drying the chopped basalt fiber, surface treatment is performed with coupling agent solution, followed by drying to obtain surface-modified basalt fiber.

[0045] First, the short-cut basalt fibers were ultrasonically cleaned in acetone for 30 minutes and then vacuum dried at 60°C for 4 hours. Simultaneously, the coupling agent was dissolved in an ethanol aqueous solution, the pH was adjusted to 4, and hydrolysis was carried out for 30 minutes to obtain a coupling agent solution. Subsequently, the dried fibers were added to the coupling agent solution, stirred and reacted for 1 hour, and then dried at 80°C to constant weight.

[0046] S2. Preparation of intermediate layer composite material: PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are mixed and heated, toughening modifier and nanofiller are added, and after further mixing and cooling, a matrix mixture is obtained; surface-modified basalt fiber is mixed evenly with the matrix mixture; the mixture is added to a twin-screw extruder for melt blending and granulation to obtain BFRP reinforced PVC-U composite material granules.

[0047] First, PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are stirred and heated to 80°C. Then, toughening modifier and nanofiller are added, and stirring is continued to 100°C. The material is discharged and cooled to below 40°C. During melt blending, the extrusion temperature is controlled at 160°C and the screw speed is 150 rpm.

[0048] S3, Three-layer co-extrusion molding: Using a three-layer co-extrusion production line, the inner layer material, the middle layer composite material and the outer layer material are added to the corresponding extruders respectively. After being melted and plasticized, they are compounded and extruded through a three-layer co-extrusion die to form a tube blank with a three-layer structure.

[0049] The temperature of the co-extrusion die is 175℃.

[0050] S4. Shaping and Cooling: The extruded melt is shaped by a vacuum shaping sleeve to form an axially hollow structure on the outer wall, the shape of reinforcing ribs, and the spiral texture on the outer surface, and then cooled to obtain a shaped pipe.

[0051] Among them, the cooling control shaped the pipe to below 40℃; the control reinforcing rib cross section is trapezoidal, the two sides are at an angle of 5° with the radial direction, the spiral texture pitch is 0.5 times the nominal outer diameter of the pipe, and the texture depth is 0.5mm.

[0052] S5. Traction Cutting: Traction and cutting of the shaped pipe obtained in S4.

[0053] S6. Flaring and forming: One end of the pipe is heated and flared to form a socket end. During the flaring process, the first sealing groove, the second sealing groove, the test pressure hole threaded interface, the axial reinforcing rib, and the end face reinforcing flange are formed simultaneously. The other end of the pipe is the spigot end. During the forming process, the first sealing step, the second sealing step, and the lubrication guide groove are formed simultaneously.

[0054] Among them, the taper of the sealing section at the socket end is controlled to be 1:100, the spacing of the sealing groove is 0.2 times the nominal outer diameter of the pipe and not less than 30mm; the depth of the sealing groove is controlled to match the diameter of the sealing ring, and the compression rate is 15%.

[0055] Example 2: This example provides a BFRP-U armored double-layer axial hollow wall tube, comprising an inner layer, a middle layer, and an outer layer joined sequentially from the inside out; the inner layer is a pure PVC-U layer; the middle layer is made of BFRP-reinforced PVC-U composite material; and the outer layer is an anti-aging modified PVC-U layer.

[0056] The BFRP-reinforced PVC-U composite material comprises, by weight: 100 parts PVC-U resin, 25 parts surface-modified chopped basalt fiber, 10 parts toughening modifier, 6.5 parts nanofiller, 4.5 parts heat stabilizer, 2.5 parts processing aid, 1.25 parts lubricant, 1.25 parts coupling agent, and 0.3 parts antioxidant.

[0057] Among them, the toughening modifier is an acrylate copolymer; the nanofiller is nano-montmorillonite; the heat stabilizer is a rare earth stabilizer; the processing aid is an acrylate processing aid; the lubricant is calcium stearate; the coupling agent is KH-560; and the antioxidant is a composite system of hindered phenolic antioxidant and phosphite auxiliary antioxidant.

[0058] The preparation method of the above-mentioned BFRP-U armored double-layer axial hollow wall tube includes the following steps:

[0059] S1. Surface modification of basalt fiber: After cleaning and drying the chopped basalt fiber, surface treatment is performed with coupling agent solution, followed by drying to obtain surface-modified basalt fiber.

[0060] First, the short-cut basalt fibers were ultrasonically cleaned in acetone for 45 minutes and then vacuum dried at 70°C for 3 hours. Simultaneously, the coupling agent was dissolved in an ethanol aqueous solution, the pH was adjusted to 4.5, and hydrolyzed for 30 minutes to obtain a coupling agent solution. Then, the dried fibers were added to the coupling agent solution, stirred and reacted for 1.5 hours, and then dried at 90°C to constant weight.

[0061] S2. Preparation of intermediate layer composite material: PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are mixed and heated, toughening modifier and nanofiller are added, and after further mixing and cooling, a matrix mixture is obtained; surface-modified basalt fiber is mixed evenly with the matrix mixture; the mixture is added to a twin-screw extruder for melt blending and granulation to obtain BFRP reinforced PVC-U composite material granules.

[0062] First, PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are stirred and heated to 85°C. Then, toughening modifier and nanofiller are added, and stirring is continued to 105°C. The material is discharged and cooled to below 40°C. During melt blending, the extrusion temperature is controlled at 170°C and the screw speed is 225 rpm.

[0063] S3, Three-layer co-extrusion molding: Using a three-layer co-extrusion production line, the inner layer material, the middle layer composite material and the outer layer material are added to the corresponding extruders respectively. After being melted and plasticized, they are compounded and extruded through a three-layer co-extrusion die to form a tube blank with a three-layer structure.

[0064] The temperature of the co-extrusion die is 180℃.

[0065] S4. Shaping and Cooling: The extruded melt is shaped by a vacuum shaping sleeve to form an axially hollow structure on the outer wall, the shape of reinforcing ribs, and the spiral texture on the outer surface, and then cooled to obtain a shaped pipe.

[0066] Among them, the cooling control shaped the pipe to below 40℃; the control reinforcing rib cross section is trapezoidal, the two sides are at an angle of 10° with the radial direction, the spiral texture pitch is 0.75 times the nominal outer diameter of the pipe, and the texture depth is 1.25mm.

[0067] S5. Traction Cutting: Traction and cutting of the shaped pipe obtained in S4.

[0068] S6. Flaring and forming: One end of the pipe is heated and flared to form a socket end. During the flaring process, the first sealing groove, the second sealing groove, the test pressure hole threaded interface, the axial reinforcing rib, and the end face reinforcing flange are formed simultaneously. The other end of the pipe is the spigot end. During the forming process, the first sealing step, the second sealing step, and the lubrication guide groove are formed simultaneously.

[0069] Among them, the taper of the sealing section at the socket end is controlled to be 1:150, the spacing of the sealing groove is 0.25 times the nominal outer diameter of the pipe and not less than 30mm; the depth of the sealing groove is controlled to match the diameter of the sealing ring, and the compression rate is 20%.

[0070] Example 3: This example provides a BFRP-U armored double-layer axial hollow wall tube, comprising an inner layer, a middle layer, and an outer layer joined sequentially from the inside out; the inner layer is a pure PVC-U layer; the middle layer is made of BFRP-reinforced PVC-U composite material; and the outer layer is an anti-aging modified PVC-U layer.

[0071] The BFRP-reinforced PVC-U composite material comprises, by weight: 110 parts PVC-U resin, 35 parts surface-modified chopped basalt fiber, 15 parts toughening modifier, 10 parts nanofiller, 6 parts heat stabilizer, 4 parts processing aid, 2 parts lubricant, 2 parts coupling agent, and 0.5 parts antioxidant.

[0072] Among them, the toughening modifier is methyl methacrylate-butadiene-styrene copolymer; the nanofiller is nano silica; the heat stabilizer is organotin stabilizer; the processing aid is acrylate processing aid; the lubricant is calcium stearate; the coupling agent is KH-570; and the antioxidant is a composite system of hindered phenolic antioxidant and phosphite auxiliary antioxidant.

[0073] The preparation method of the above-mentioned BFRP-U armored double-layer axial hollow wall tube includes the following steps:

[0074] S1. Surface modification of basalt fiber: After cleaning and drying the chopped basalt fiber, surface treatment is performed with coupling agent solution, followed by drying to obtain surface-modified basalt fiber.

[0075] First, the short-cut basalt fibers were ultrasonically cleaned in acetone for 60 minutes and then vacuum dried at 80°C for 2 hours. Simultaneously, the coupling agent was dissolved in an ethanol aqueous solution, the pH was adjusted to 5, and hydrolysis was carried out for 30 minutes to obtain a coupling agent solution. Subsequently, the dried fibers were added to the coupling agent solution, stirred and reacted for 2 hours, and then dried at 100°C to constant weight.

[0076] S2. Preparation of intermediate layer composite material: PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are mixed and heated, toughening modifier and nanofiller are added, and after further mixing and cooling, a matrix mixture is obtained; surface-modified basalt fiber is mixed evenly with the matrix mixture; the mixture is added to a twin-screw extruder for melt blending and granulation to obtain BFRP reinforced PVC-U composite material granules.

[0077] First, PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are stirred and heated to 90°C. Then, toughening modifier and nanofiller are added, and stirring is continued to 110°C. The material is discharged and cooled to below 40°C. During melt blending, the extrusion temperature is controlled at 180°C and the screw speed is 300 rpm.

[0078] S3, Three-layer co-extrusion molding: Using a three-layer co-extrusion production line, the inner layer material, the middle layer composite material and the outer layer material are added to the corresponding extruders respectively. After being melted and plasticized, they are compounded and extruded through a three-layer co-extrusion die to form a tube blank with a three-layer structure.

[0079] The temperature of the co-extrusion die is 185℃.

[0080] S4. Shaping and Cooling: The extruded melt is shaped by a vacuum shaping sleeve to form an axially hollow structure on the outer wall, the shape of reinforcing ribs, and the spiral texture on the outer surface, and then cooled to obtain a shaped pipe.

[0081] Among them, the cooling control shaped the pipe to below 40℃; the control reinforcing rib cross section is trapezoidal, the two sides are at an angle of 15° with the radial direction, the spiral texture pitch is 1.0 times the nominal outer diameter of the pipe, and the texture depth is 2.0mm.

[0082] S5. Traction Cutting: Traction and cutting of the shaped pipe obtained in S4.

[0083] S6. Flaring and forming: One end of the pipe is heated and flared to form a socket end. During the flaring process, the first sealing groove, the second sealing groove, the test pressure hole threaded interface, the axial reinforcing rib, and the end face reinforcing flange are formed simultaneously. The other end of the pipe is the spigot end. During the forming process, the first sealing step, the second sealing step, and the lubrication guide groove are formed simultaneously.

[0084] Among them, the taper of the sealing section at the socket end is controlled to be 1:200, the spacing of the sealing groove is 0.3 times the nominal outer diameter of the pipe and not less than 30mm; the depth of the sealing groove is controlled to match the diameter of the sealing ring, and the compression rate is 25%.

[0085] Example 4: This example provides DN400SN8 grade pipe, which includes an inner layer, a middle layer and an outer layer bonded together from the inside out; the inner layer is a pure PVC-U layer; the middle layer is made of BFRP reinforced PVC-U composite material; the outer layer is an anti-aging modified PVC-U layer;

[0086] I. Material Formula

[0087] The intermediate layer BFRP-reinforced PVC-U composite material is made from the following raw materials in parts by weight: SG-5 type PVC-U resin with a K value of 65: 100 parts; surface-modified basalt fiber with a length of 4 mm and treated with KH-560: 25 parts; CPE toughening agent: 8 parts; ACR processing aid: 2 parts; nano calcium carbonate: 5 parts; calcium-zinc composite stabilizer: 4 parts; stearic acid: 0.5 parts; polyethylene wax: 0.5 parts; antioxidant 1010: 0.2 parts; antioxidant 168: 0.1 parts;

[0088] Inner layer: SG-5 type pure PVC-U: 100 parts, calcium zinc stabilizer: 4 parts, ACR: 2 parts, stearic acid: 0.5 parts, polyethylene wax: 0.5 parts, titanium dioxide: 1 part (to make the inner layer light gray RAL7035).

[0089] Outer layer: Pure PVC-U: 100 parts, calcium and zinc stabilizer: 4 parts, ACR: 2 parts, stearic acid: 0.5 parts, polyethylene wax: 0.5 parts, blue masterbatch: 2 parts (to make the outer layer dark blue RAL5002), ultraviolet absorber: 0.5 parts.

[0090] II. Pipe structural parameters:

[0091] Basic dimensions: nominal outer diameter 500mm, total wall thickness 15mm.

[0092] Layer thickness and color: Inner layer thickness 3.5mm (23.3%), light gray (RAL7035, L*=75); Middle layer thickness 8.0mm (53.4%), basalt natural color (RAL7013, L*=35); Outer layer thickness 3.5mm (23.3%), dark blue (RAL5002, L*=45), surface sprayed with 30μm transparent acrylic protective layer;

[0093] External structure: The cross-section of the reinforcing rib is trapezoidal, with a top base width of 4mm, a bottom base width of 6mm, a height of 8mm, and an angle of 8° between the two sides and the radial direction; the spiral texture has a pitch of 400mm, a depth of 1.2mm, and a width of 3mm; there are 6 axial reinforcing ribs at the socket end, evenly distributed, with a height of 6mm and a length of 80mm; and the end face reinforcing flange has a height of 10mm and a width of 15mm.

[0094] Joint structure: First, the inner diameter of the socket end is controlled at 508.0mm, the distance between the first and second sealing grooves is 90mm, the groove depth is 5mm, and the groove width is 10mm; while the outer diameter of the spigot end is 506.0mm, and the height of the first and second sealing steps is 4.5mm; the sealing ring is an O-ring rubber with a cross-sectional diameter of 8.5mm, a Shore hardness of 62A, and a compression ratio of 20%; at the same time, the diameter of the test pressure hole is controlled at 6mm, and it is equipped with an M10 threaded interface; the taper of the sealing section is 1:150.

[0095] III. Preparation method:

[0096] The process involves modifying the surface of basalt fiber, preparing the intermediate layer composite material, three-layer co-extrusion molding, shaping and cooling, traction cutting and flaring molding, as described above, to obtain the finished product.

[0097] IV. Performance Testing:

[0098] The following performance tests were conducted on the BFRP-U armored double-layer axial hollow wall tubes prepared according to the above formula and process:

[0099] Ring stiffness test: According to GB / T9647-2015, three pipe section specimens were tested. Under constant speed loading, the load value when the radial deformation of the specimen was 3% of the inner diameter of the pipe was measured, and the average ring stiffness was calculated.

[0100] Drop hammer impact test:

[0101] 23℃ impact: According to GB / T14152-2001, 10 specimens were impacted at 23±2℃ using a drop hammer of specified mass.

[0102] -10℃ Low Temperature Impact: After pretreating the sample in an environment of -10±1℃ for at least 4 hours, the impact test is carried out immediately.

[0103] Long-term hydrostatic test and creep ratio: According to GB / T18042-2000, the pipe was subjected to a long-term hydrostatic test. The creep ratio after 1000 hours was calculated by measuring the change of circumferential strain of the pipe over time under different stress levels.

[0104] Tensile property test: According to GB / T8804.3-2003, dumbbell-shaped specimens are cut from the pipe and tested on a universal testing machine; the tensile strength and flexural modulus of the material are measured.

[0105] Interfacial shear strength test: The interfacial shear strength between the surface-modified basalt fiber and the PVC-U matrix was determined by short beam shear or microdroplet debonding method.

[0106] Interlayer color difference measurement: Use a colorimeter to measure the color difference between the inner layer and the middle layer, and between the middle layer and the outer layer on the cross-section of the pipe, and calculate the color difference value ΔE in the CIELab color space.

[0107] Sealing performance test:

[0108] Hydrostatic sealing: At 23±2℃, apply a hydrostatic pressure of 0.05MPa to the assembled socket joint, maintain the pressure for 15 minutes, and observe the joint.

[0109] Sealing under deformation conditions: Compress the spigot end radially to a deformation of 10%, or apply a radial load to the socket end to deform it by 5%, and then maintain pressure at 0.05 MPa hydrostatic pressure for 15 minutes and observe the interface.

[0110] Sealing under angular deflection conditions: For this DN500 pipe, deflect the axis of the socket and spigot by 1.5°, and then maintain pressure at 0.05MPa hydrostatic pressure for 15 minutes, and observe the joint.

[0111] Independent pressure test of the interface: Inject water at a pressure of 0.1 MPa into the circumferential space between the first and second sealing rings through the test hole on the outer wall of the socket, maintain the pressure for 5 minutes, and measure the pressure drop value.

[0112] Table 1: Summary of Performance Test Data for Example 4

[0113] Testing items Measured value Feature parameters of this application Ring stiffness (SN12.5 grade) 14.8kN / m2 ≥14.0kN / m2 23℃ shock TIR 2% ≤5% -10℃ shock TIR 5% ≤10% creep ratio 1.7 ≤2.0 Tensile strength 54MPa ≥50MPa Flexural modulus 2720MPa ≥2500MPa Interfacial shear strength 9.2MPa ≥8MPa Interlayer color difference - E Internal-Chinese: 38. Chinese-External: 24 ≥10 0.05MPa sealing No leakage No leakage The socket is deformed by 10% + 0.05 MPa No leakage No leakage Angle deflection 1.5° + 0.05MPa No leakage No leakage Independent pressure test of the interface (0.1MPa, 5min) Pressure drop 0.007 MPa ≤0.01MPa

[0114] Example 5: This example provides BFRP-U armored double-layer axial hollow wall tubes with different fiber contents; the basalt fiber content of the intermediate layer is adjusted to 15%, 20%, and 30%, while other conditions are the same as in Example 4, and performance testing is performed, as follows:

[0115] Ring stiffness test: conducted according to GB / T9647-2015; when the fiber content is 15%, the ring stiffness is 13.8 kN / m²; when the content is 20%, the ring stiffness increases to 14.5 kN / m²; when the content increases to 30%, the ring stiffness reaches 15.2 kN / m²; the results show that the fiber content is positively correlated with the ring stiffness.

[0116] Tensile strength test: conducted according to GB / T8804.3-2003. As the fiber content increased from 15% to 30%, the tensile strength of the composite material gradually increased from 48 MPa to 56 MPa, showing a reinforcing effect.

[0117] -10℃ Low-Temperature Drop Impact Test: Conducted under low-temperature conditions according to GB / T14152-2001. When the fiber content is 15%, the TIR is 8%; when the content is 20%, the TIR decreases to 6%; and when the content is 30%, the TIR further decreases to 4%.

[0118] Table 2: Summary of Performance Test Data from Example 5

[0119] Fiber content Ring stiffness (kN / m2) Tensile strength (MPa) -10℃ shock TIR (%) 15% 13.8 48 8 20% 14.5 52 6 30% 15.2 56 4

[0120] Example 6: With a fixed fiber content of 25%, the type of toughening modifier was changed, and other conditions remained the same as in Example 4. Performance testing was then conducted, as detailed below:

[0121] -10℃ low-temperature drop hammer impact test: conducted according to GB / T14152-2001; when CPE is used as toughening agent, TIR is 5%; when MBS is used, TIR is optimal at 4%; when ACR is used, TIR is 7%; when CPE and MBS are compounded (5:3), TIR reaches 3%; indicating that MBS and its compound system with CPE can better improve the low-temperature brittleness of PVC-U / BFRP composite materials.

[0122] Tensile strength test: conducted according to GB / T8804.3-2003; the tensile strength of the CPE system is 54MPa, the MBS system is 52MPa, the ACR system is 53MPa, and the CPE / MBS compound system reaches 55MPa.

[0123] Table 3: Summary of Performance Test Data from Example 6

[0124] toughening agent -10℃ shock TIR (%) Tensile strength (MPa) CPE 5 54 MBS 4 52 ACR 7 53 CPE+MBS 3 55

[0125] Comparative Example 1: This comparative example provides a non-BFRP reinforced ordinary PVC-U double-layer axial hollow wall pipe; other conditions are the same as in Example 4, and performance tests are performed as follows:

[0126] Ring stiffness test: According to GB / T9647-2015, the measured ring stiffness is 11.8kN / m², which meets the basic requirements of GB / T18477.3-2019 for SN12.5 grade, but does not meet the characteristic performance index of ≥14.0kN / m² proposed in this invention.

[0127] -10℃ Low Temperature Drop Impact Test: According to GB / T14152-2001, the True Impact Rate (TIR) ​​reached 22%, exceeding the requirement of ≤10%, indicating that ordinary PVC-U material is too brittle at low temperatures.

[0128] Tensile strength and flexural modulus tests: According to GB / T8804.3-2003, the tensile strength was 41MPa and the flexural modulus was 2050MPa, both of which are lower than the required ≥50MPa and ≥2500MPa.

[0129] Creep ratio test: According to GB / T18042-2000, the creep ratio is 2.6, which exceeds the required limit of ≤2.0, indicating that its long-term resistance to deformation is poor.

[0130] Table 4: Summary of Performance Test Results for Comparative Example 1

[0131] Testing items Measured value Does it satisfy the present invention? Ring stiffness (SN12.5 grade) 11.8 kN / m² No (<14.0) -10℃ shock TIR 22% No (>10%) Tensile strength 41 MPa No (<50) Flexural modulus 2050 MPa No (<2500) creep ratio 2.6 No (>2.0)

[0132] Comparative Example 2: This comparative example provides a single-seal ring connector, which adopts a single-seal ring structure. Other aspects are the same as in Example 4, and performance testing is performed as follows:

[0133] Sealing test under deformation conditions: According to the method for sealing test of elastic sealing ring connection in GB / T19472.2-2017, the end of the socket was radially deformed by 10% and then pressure was maintained under a hydrostatic pressure of 0.05MPa; the result showed a slight leakage, which could not meet the sealing requirements under the working conditions.

[0134] Negative pressure sealing test: At 23±2℃, apply a negative pressure of -0.03MPa to the interface and maintain the pressure for 15 minutes; Since there is only one sealing ring, the sealing ring may be sucked in or fail to seal properly under negative pressure, resulting in a pressure drop exceeding the allowable value.

[0135] Interface independent pressure test function verification: Since the single sealing ring structure cannot form a closed circumferential space for independent pressurization within the socket, it does not have the function of independently testing the sealing performance of the interface through the pressure test hole.

[0136] Table 5: Summary of Performance Test Results for Comparative Example 2

[0137] Test Project result The socket is deformed by 10% + 0.05 MPa Minor leakage occurred -0.03MPa negative pressure The sealing ring is drawn in, and the pressure drop is -0.035MPa. Independent pressure testing of interfaces Unable to achieve

[0138] Based on the above test data, the following conclusions can be drawn:

[0139] Combined with Examples 4 and 5 and Comparative Example 1, and with Tables 1, 2 and 4, it can be seen that BFRP reinforcement can improve the ring stiffness, tensile strength and low-temperature toughness of the pipe, indicating that the introduction of basalt fiber improves the mechanical properties and brittleness resistance of the PVC-U matrix, and is a factor in achieving high-performance pipes.

[0140] As can be seen from Example 5 and Table 2, with the increase of basalt fiber content, the ring stiffness and tensile strength of the pipe are correspondingly enhanced, and the low-temperature impact performance is improved. This indicates that the fiber content is positively correlated with the overall mechanical properties of the composite material, and the reinforcing effect of the fiber is strengthened with the increase of the content.

[0141] As can be seen from Example 6 and Table 3, the use of MBS toughening agent or its compounding with CPE can improve the low-temperature brittleness of PVC-U / BFRP composite material and synergistically enhance the tensile strength. This indicates that the type of toughening agent and its compounding system have an impact on the low-temperature toughness and strength of the material, and the selection of toughening agent can enhance the overall performance of the material.

[0142] As can be seen from Example 4 and Comparative Example 2, and in conjunction with Tables 1 and 5, the dual-seal ring structure can ensure more reliable sealing performance under deformation, deflection, or negative pressure conditions, and supports independent pressure testing of the interface, highlighting the superiority and synergistic protection of the multi-seal design compared to the single-seal ring under complex working conditions.

[0143] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A BFRP-U armored double-layer axial hollow wall tube, characterized in that, The hollow wall tube is formed by three-layer co-extrusion in one step, including an inner layer, a middle layer and an outer layer joined together from the inside to the outside; the inner layer is a pure PVC-U layer; The intermediate layer is made of BFRP-reinforced PVC-U composite material; the outer layer is a pure PVC-U layer or an anti-aging modified PVC-U layer. The BFRP-reinforced PVC-U composite material comprises, by weight: 90-110 parts PVC-U resin, 15-35 parts surface-modified chopped basalt fiber, 5-15 parts toughening modifier, 3-10 parts nanofiller, 3-6 parts heat stabilizer, 1-4 parts processing aid, 0.5-2 parts lubricant, 0.5-2 parts coupling agent, and 0.1-0.5 parts antioxidant.

2. The BFRP-U armored double-layer axial hollow wall tube according to claim 1, characterized in that, The toughening modifier is one or more of chlorinated polyethylene, acrylate copolymers, or methyl methacrylate-butadiene-styrene copolymers; the nanofiller is one or more of nano-calcium carbonate, nano-montmorillonite, or nano-silica.

3. The BFRP-U armored double-layer axial hollow wall tube according to claim 1, characterized in that, The heat stabilizer is one or more of calcium-zinc composite stabilizers, rare earth stabilizers, or organotin stabilizers; the processing aid is an acrylate processing aid.

4. The BFRP-U armored double-layer axial hollow wall tube according to claim 1, characterized in that, The lubricant is stearic acid or calcium stearate; the coupling agent is selected from one or more of KH-550, KH-560 or KH-570; the antioxidant is a composite system of hindered phenolic antioxidant and phosphite auxiliary antioxidant.

5. A method for preparing a BFRP-U armored double-layer axial hollow wall tube, characterized in that, The BFRP-U armored double-layer axial hollow wall tube according to any one of claims 1-4 comprises the following steps: S1. Surface modification of basalt fiber: After cleaning and drying the chopped basalt fiber, surface treatment is performed with coupling agent solution, followed by drying to obtain surface-modified basalt fiber. S2. Preparation of intermediate layer composite material: PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are mixed and heated, toughening modifier and nanofiller are added, mixing is continued and then cooled to obtain matrix mixture; surface-modified basalt fiber is mixed evenly with matrix mixture; the mixture is added to a twin-screw extruder for melt blending and granulation to obtain BFRP reinforced PVC-U composite material granules; S3, Three-layer co-extrusion molding: Using a three-layer co-extrusion production line, the inner layer material, the middle layer composite material and the outer layer material are added to the corresponding extruders, melted and plasticized, and then extruded through a three-layer co-extrusion die to form a tube blank with a three-layer structure. S4. Shaping and Cooling: The extruded melt is shaped by a vacuum shaping sleeve to form an axially hollow structure on the outer wall, the shape of reinforcing ribs and the spiral texture on the outer surface, and then cooled to obtain a shaped tube. S5. Traction Cutting: Traction and cutting of the shaped pipe obtained in S4; S6. Flaring and forming: One end of the pipe is heated and flared to form a socket end. During the flaring process, the first sealing groove, the second sealing groove, the test pressure hole threaded interface, the axial reinforcing rib, and the end face reinforcing flange are formed simultaneously. The other end of the pipe is the spigot end. During the forming process, the first sealing step, the second sealing step, and the lubrication guide groove are formed simultaneously.

6. The method for preparing a BFRP-U armored double-layer axial hollow wall tube according to claim 5, characterized in that, In step S1, the short-cut basalt fibers are first ultrasonically cleaned in acetone for 30-60 minutes and vacuum dried at 60-80°C for 2-4 hours. At the same time, the coupling agent is dissolved in an ethanol aqueous solution, the pH is adjusted to 4-5, and hydrolyzed for 30 minutes to obtain the coupling agent solution. The dried fibers were then added to the coupling agent solution and stirred for 1–2 hours. After being removed, they were dried at 80–100°C to constant weight.

7. The method for preparing a BFRP-U armored double-layer axial hollow wall tube according to claim 5, characterized in that, In step S2, PVC-U resin, heat stabilizer, processing aid, lubricant and antioxidant are first stirred and heated to 80-90°C. Then toughening modifier and nanofiller are added and stirred to 100-110°C. The material is discharged and cooled to below 40°C. During melt blending, the extrusion temperature is controlled at 160-180°C and the screw speed is 150-300 rpm.

8. The method for preparing a BFRP-U armored double-layer axial hollow wall tube according to claim 5, characterized in that, In step S3, the co-extrusion die temperature is 175-185℃.

9. The method for preparing a BFRP-U armored double-layer axial hollow wall tube according to claim 5, characterized in that, In step S4, the cooling control shaped pipe is set to below 40°C; the cross section of the reinforcing rib is controlled to be trapezoidal, with the two sides forming an angle of 5° to 15° with the radial direction, the spiral texture pitch is 0.5 to 1.0 times the nominal outer diameter of the pipe, and the texture depth is 0.5 to 2.0 mm.

10. The method for preparing a BFRP-U armored double-layer axial hollow wall tube according to claim 5, characterized in that, During the S6 process, the taper of the sealing section at the socket end is controlled to be 1:100 to 1:200, and the spacing between the sealing grooves is 0.2 to 0.3 times the nominal outer diameter of the pipe and not less than 30 mm; the depth of the sealing groove is controlled to match the diameter of the sealing ring, and the compression rate is 15% to 25%.

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