Flame-retardant antistatic polyethylene pipe and method for producing the same

Abstract

The application provides a kind of flame-retardant antistatic polyethylene pipe and its preparation method, wherein a small amount of double-shell gradient functionalized core-shell microspheres are added;The double-shell gradient functionalized core-shell microspheres include: an inner core composed of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate, the inner core surface is coated with a silica nano protective layer;An inner shell layer coated on the surface of the inner core, the inner shell layer includes polyether ester amide and four acicular zinc oxide whiskers dispersed therein;An outer shell layer coated on the surface of the inner shell layer, the outer shell layer includes polyether ester amide, carboxylated carbon nanotubes and nano magnesium hydroxide;A gradient multilayer flame-retardant barrier is constructed, which can achieve V-0 flame-retardant level with low filler content;At the same time, at least part of the needle tips of the four acicular zinc oxide whiskers penetrate the inner shell layer and are embedded in the outer shell layer, forming a mechanical interlocking structure and a three-path redundant conductive network, with durable and stable antistatic performance.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a flame-retardant and antistatic polyethylene pipe and its preparation method. Background Technology

[0002] Polyethylene pipes are widely used in mining ventilation, oil and gas transportation, and municipal water supply and drainage due to their lightweight, corrosion resistance, and ease of construction. However, polyethylene is inherently flammable (limiting oxygen index is only about 17%) and has high surface resistivity, posing a fire hazard and static electricity ignition risk in flammable and explosive environments such as coal mines, chemical plants, and oil and gas plants. Therefore, it is necessary to endow pipes with both flame-retardant and antistatic properties.

[0003] To simultaneously impart flame-retardant and antistatic properties to polyethylene pipes, existing technologies mainly employ the following technical approaches: One method involves directly blending flame retardants and conductive fillers into the polyethylene matrix. This method requires a high filler content (usually exceeding 25 wt%), at which point the elongation at break of the pipe is often below 150%, the processing fluidity is significantly reduced, and sag defects occur during the extrusion of large-diameter pipes.

[0004] Another approach involves encapsulating flame retardants within a polymer shell to create core-shell microspheres, which are then blended with polyethylene. However, existing core-shell microspheres have a single-shell structure, resulting in insufficient interfacial bonding strength between the shell and the polyethylene matrix (below 15 MPa). Conductive fillers tend to agglomerate within the shell, leading to discontinuous conductive networks and unstable antistatic properties. Furthermore, the elongation at break under high filler content remains low (150–200%), and the sag problem during large-diameter extrusion remains unresolved.

[0005] Therefore, how to simultaneously achieve high-efficiency flame retardancy, long-lasting antistatic properties, excellent mechanical properties, and stable large-diameter extrusion of polyethylene pipes with low filler content is a technical problem that still needs to be solved in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a flame-retardant and antistatic polyethylene pipe to solve the problems of decreased mechanical properties, insufficient interfacial bonding strength of single-shell core-shell microspheres, and discontinuous conductive network caused by high filling amount in the direct blending filling method in the prior art. At the same time, this invention will also provide a method for preparing the flame-retardant and antistatic polyethylene pipe.

[0007] To achieve the above and other related objectives, the present invention provides the following technical solutions: In a first aspect, the present invention provides a flame-retardant and antistatic polyethylene pipe, comprising the following raw materials in parts by weight: 100 parts of high-density polyethylene; 14-16 portions of double-shell gradient functionalized core-shell microspheres; 2-4 parts of conductive carbon black; 1.0-1.5 parts of maleic anhydride-grafted polyethylene; 0.2-0.3 parts of ethylene-glycidyl methacrylate copolymer; 2.5-3.5 parts of polyolefin elastomer; 1-2 parts of ethylene-octene copolymer; 0.6-0.8 parts of compound lubricant; 0.1-0.3 parts of compound antioxidant; 0.5-1 part of charring agent; The double-shell gradient functionalized core-shell microspheres include: The core is composed of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate, and the surface of the core is covered with a silica nano-protective layer. An inner shell layer covering the surface of the core, the inner shell layer comprising polyether ester amide and tetra-needle-shaped zinc oxide whiskers dispersed therein; An outer shell layer covering the surface of the inner shell layer, the outer shell layer comprising polyether ester amide, carboxylated carbon nanotubes and nano-magnesium hydroxide; In this process, at least some of the tips of the four-needle zinc oxide whiskers penetrate the inner shell layer and are embedded in the outer shell layer, that is, the tips of the four-needle zinc oxide whiskers protrude from the surface of the inner shell layer and are covered and anchored by the outer shell layer.

[0008] Preferably, the mass ratio of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate in the core is (2.5-3.5):1:(0.1-0.13); more preferably, the mass ratio of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate in the core is 3:1:0.12.

[0009] Preferably, the ammonium polyphosphate has a particle size of 3-5 μm and is silanized; the melamine polyphosphate has a degree of polymerization of not less than 15 and a decomposition temperature of not less than 320°C.

[0010] Preferably, the kernel has a particle size of 3-6 μm.

[0011] Preferably, the thickness of the silicon dioxide nano-protective layer is 0.1-0.2 μm.

[0012] Preferably, the tetra-needle zinc oxide whiskers in the inner shell layer account for 18-22% of the mass of the inner shell layer, the aspect ratio of the tetra-needle zinc oxide whiskers is 20-30, the length of a single needle is 1-2 μm, and the surface is treated with a silane coupling agent. The inner shell layer also includes 0.5-1% hindered phenolic antioxidant and 2-3% nano-silica by mass of the polyether ester amide; The thickness of the inner shell (excluding the length of the exposed tetra-needle zinc oxide whiskers) is 0.8-1.0 μm.

[0013] Preferably, at least 60% (referring to the penetration ratio) of the tips of the four needle-shaped zinc oxide whiskers penetrate the inner shell layer and embed themselves in the outer shell layer.

[0014] More preferably, at least 70% (referring to the penetration ratio) of the tips of the four needle-shaped zinc oxide whiskers penetrate the inner shell layer and embed themselves in the outer shell layer.

[0015] Preferably, the average depth to which the tips of the four needle-shaped zinc oxide whiskers are embedded in the outer shell layer is 70-85% of the thickness of the outer shell layer.

[0016] Preferably, the carboxylated carbon nanotubes in the outer shell layer account for 6-8% of the mass of the outer shell layer, and the degree of carboxylation of the carboxylated carbon nanotubes is 2.5-3.5%. The nano-magnesium hydroxide accounts for 10-14% of the mass of the outer shell layer, and it is hexagonal with a particle size of 40-80 nm, and is surface-treated with stearic acid or silane coupling agent. The thickness of the outer shell layer is 1.0-1.2 μm.

[0017] Preferably, the outer shell layer further includes a reversible crosslinking agent comprising 0.3-0.5% of the mass of the polyether ester amide, wherein the reversible crosslinking agent is a furan-maleimide dynamic covalent crosslinking agent.

[0018] Preferably, the particle size of the double-shell gradient functionalized core-shell microspheres is 7-12 μm.

[0019] Preferably, the raw material also includes zinc borate, which accounts for 1-2% of the total mass of the raw material.

[0020] Preferably, the grafting rate of the maleic anhydride-grafted polyethylene is not less than 0.8%; and the glass transition temperature of the polyolefin elastomer is not higher than -55°C.

[0021] In a second aspect, the present invention provides a method for preparing the above-mentioned flame-retardant and antistatic polyethylene pipe, in order to solve the problems in the prior art that the core-shell microsphere preparation process is difficult to achieve a double-shell gradient functionalized structure and that sag defects are difficult to eliminate in the extrusion molding of large-diameter pipes.

[0022] The preparation method of the flame-retardant and antistatic polyethylene pipe includes the following steps: Step 1: Preparation of double-shell gradient functionalized core-shell microspheres: Ammonium polyphosphate, melamine polyphosphate and melamine cyanurate are mixed and then granulated by spray drying to form core particles. After drying, the core particles are spray-coated with silica sol in a fluidized bed to form a silica nano-protective layer, thus obtaining a protected core. Polyether ester amide was dissolved in a solvent to prepare a solution, and tetra-needle-shaped zinc oxide whiskers, antioxidants and nano-silica were added. After ultrasonic dispersion, the protected core was coated with thin layers multiple times in a fluidized bed to form an inner shell layer. Another polyether ester amide solution was taken, and carboxylated carbon nanotubes, nano magnesium hydroxide and dispersing agent were added. After being dispersed by ultrasonication, shearing and grinding, the solution was further coated in a fluidized bed to form an outer shell layer, thereby obtaining the double-shell gradient functionalized core-shell microspheres. Step 2: Mixing and granulation: Weigh each raw material according to the formula, first mix high-density polyethylene, polyolefin elastomer and ethylene-octene copolymer, then add composite lubricant, maleic anhydride grafted polyethylene, ethylene-glycidyl methacrylate copolymer, composite antioxidant, charring agent, conductive carbon black and the double-shell gradient functionalized core-shell microspheres, mix at high speed and then add the mixture to a twin-screw extruder, and granulate to obtain granules; Step 3: Pipe extrusion molding: The granules are added to a single-screw pipe extrusion production line for extrusion. After segmented gradient cooling and vacuum shaping, the material is then annealed to obtain the flame-retardant and antistatic polyethylene pipe.

[0023] Preferably, in step one, the spray drying granulation process includes: adding deionized water and 1.5-2.5% polyvinyl alcohol (PVA) as a binder to ammonium polyphosphate, melamine polyphosphate, and melamine cyanurate, mixing them evenly to prepare a suspension with a solid content of 35-45%, pumping the suspension into a fluidized bed granulation device, producing granules, which are then screened by airflow classification, and collecting particles with a particle size of 3-6 μm as core particles.

[0024] Preferably, in step one, the core particles are vacuum dried at 100°C until the moisture content is no higher than 0.1%; the silica sol spray coating is carried out in a fluidized bed to form a silica nano-protective layer with a thickness of 0.1-0.2 μm.

[0025] Preferably, in step one, the polyether ester amide is dissolved in toluene to prepare a solution; after ultrasonic dispersion, the protected core is coated with a thin layer 3-4 times to form an inner shell layer with a thickness of 0.8-1.0 μm.

[0026] Preferably, in step one, the carboxylated carbon nanotubes and nano-magnesium hydroxide are dispersed by ultrasonication, shearing, and grinding, and then coated in a fluidized bed to form an outer shell layer with a thickness of 1.0-1.2 μm.

[0027] Preferably, in step two, the high-speed mixing temperature is 40-65℃ and the mixing time is 6-9 min.

[0028] Preferably, in step two, the temperature zones of the twin-screw extruder are 160℃ / 170℃ / 175℃ / 175~180℃ / 175~180℃ / 180℃, the screw speed is 180-200 r / min, and the dual-stage exhaust vacuum is -0.09~-0.10 MPa.

[0029] Preferably, in step three, in the extrusion production line, the melt after being melted and plasticized passes through a fish-scale-shaped guide plate orientation device.

[0030] More preferably, in step three, the angle of the fish scales of the fish scale-shaped guide plate is 30-45°, the spacing is 2-3mm, and the surface is coated with polytetrafluoroethylene; the temperature of the shaping section is controlled at 182-188℃, and the traction speed ratio is 1.06-1.08:1.

[0031] Preferably, in step three, the segmented gradient cooling is divided into 5-6 segments, the cooling water temperature is 15±2℃, and the cooling rate decreases in a gradient of 20℃ / min, 18℃ / min, and 15℃ / min; the negative pressure of the vacuum shaping is -0.03 to -0.05 MPa, and the pressure fluctuation does not exceed ±0.005 MPa; the annealing treatment is performed at 80℃ for 2 hours.

[0032] As described above, the flame-retardant and antistatic polyethylene pipe and its preparation method of the present invention have the following beneficial effects: 1. This invention incorporates self-developed double-shell gradient functionalized core-shell microspheres into polyethylene pipes. The core layer utilizes a ternary intumescent flame-retardant system of ammonium polyphosphate, melamine polyphosphate, and melamine cyanurate, combined with an inner shell layer of polyether ester amide / tetraneously shaped zinc oxide whiskers for synergistic flame retardancy, and an outer shell layer of polyether ester amide / carboxylated carbon nanotubes / nano-magnesium hydroxide composite flame-retardant layer, constructing a gradient multilayer flame-retardant barrier that achieves a V-0 flame retardant rating even with low filler content. Simultaneously, the tetraneously shaped zinc oxide whisker framework, the carboxylated carbon nanotube nanopathways, and the conductive carbon black in the polyethylene matrix collectively construct a three-path redundant conductive network. Even when a single conductive path fails, antistatic properties are maintained, and the pipe's volume resistivity does not exceed 5 × 10⁻⁶. 7 Ω·cm, with long-lasting and stable antistatic properties.

[0033] 2. In the double-shell gradient functionalized core-shell microspheres, at least some of the tips of the four needle-like zinc oxide whiskers penetrate the inner shell and embed into the outer shell. The penetration depth of the outer shell is 70-85% of the thickness of the outer shell, forming a three-in-one interface interlocking structure of "mechanical locking + conductive path + flame retardant triggering". This ensures that the interfacial bonding force between the inner and outer shell layers is not less than 20 MPa, which is more than 33% higher than that of the existing single-shell core-shell microspheres (interfacial bonding force is less than 15 MPa). At the same time, it ensures that the elongation at break of the pipe is not less than 400% and the tensile strength is not less than 23 MPa, achieving a balance between rigidity and toughness.

[0034] 3. The synergistic compatibility system of maleic anhydride-grafted polyethylene and ethylene-glycidyl methacrylate copolymer improves the interfacial bonding between the core-shell microspheres and the high-density polyethylene matrix. The dual toughening system of polyolefin elastomer and ethylene-octene copolymer synergistically enhances the toughness of the pipe. Combined with segmented gradient cooling and annealing processes to optimize the crystal morphology, the tensile strength of the pipe is not less than 23 MPa and the elongation at break is not less than 400%. The comprehensive mechanical properties are significantly better than the existing technology (tensile strength of about 11 MPa and elongation at break of about 385%).

[0035] 4. During the pipe extrusion molding process, a fish-scale-shaped guide plate orientation device is adopted. Combined with the temperature of the shaping section and the segmented gradient cooling process, the melt strength is effectively improved and the melt orientation is controlled. This solves the melt sag defect during the extrusion of large-diameter pipes with a nominal diameter of 400 mm and above, and realizes stable production of large-diameter pipes without melt sag. The ellipticity of the pipe does not exceed 0.5%, and the wall thickness uniformity is not less than 98%. Detailed Implementation

[0036] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0037] Unless otherwise specified, all raw materials used in the following examples are commercially available products.

[0038] High-density polyethylene (HDPE, grade 5000S) was purchased from Sinopec Yanshan Petrochemical Company, with a melt flow rate of 0.9 g / 10 min (190℃, 2.16 kg).

[0039] Maleic anhydride-grafted polyethylene (MAH-g-PE, grafting rate 0.9%), ethylene-glycidyl methacrylate copolymer (E-GMA), polyolefin elastomer (POE, grade 8842, glass transition temperature -58℃), and ethylene-octene copolymer (EOC, grade DC7447) were all purchased from Dow Chemical (China) Investment Co., Ltd.

[0040] Ammonium polyphosphate (APP, particle size 3-5 μm, silanized, degree of polymerization ≥1000) was purchased from Zhejiang Wansheng Co., Ltd.; melamine polyphosphate (MPP, degree of polymerization ≥15, decomposition temperature ≥320℃) and melamine cyanurate (MCA) were purchased from Shandong Brothers Technology Co., Ltd.

[0041] The four needle-shaped zinc oxide whiskers (T-ZnOw, aspect ratio 20-30, single needle length 1-2μm, surface treated with KH-550 silane coupling agent) were purchased from Panasonic Electric Works (China) Co., Ltd.

[0042] Carboxylated carbon nanotubes (COOH-CNTs, degree of carboxylation 2.5–3.5%, diameter 10–20 nm, length 5–15 μm) were purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences.

[0043] Nano magnesium hydroxide (hexagonal crystal system, particle size 40-80nm, surface treated with stearic acid) was purchased from Anhui Longhua Nanomaterials Co., Ltd.

[0044] The conductive carbon black (brand name Ketjenblack EC-600JD) was purchased from AkzoNobel.

[0045] The furan-maleimide dynamic covalent reversible crosslinking agent (Diels-Alder type, furan group grafting rate of about 15%) was prepared in the laboratory, and the preparation method was based on the literature synthesis.

[0046] The composite lubricant is a mixture of methyl silicone oil (viscosity 50 mPa·s) and calcium stearate in a mass ratio of 2:1; the composite antioxidant is a mixture of phenolic antioxidant 1010 and phosphite antioxidant 168 in a mass ratio of 1:1; and the charring agent is pentaerythritol.

[0047] Polyether ester amide (PEEA, brand name PEBAX2533) was purchased from Arkema (China) Chemical Co., Ltd. Silica sol (30% by mass, particle size 10–20 nm) was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0048] Nano-silica (vapor phase method, particle size 10-20 nm, hydrophobically treated with hexamethyldisilazane) was purchased from Degussa.

[0049] The hindered phenolic antioxidant is antioxidant 1076, purchased from BASF.

[0050] Zinc borate (2ZnO·3B2O3·3.5H2O, particle size 3-5μm) was purchased from Borax Company, USA.

[0051] The solvent used, toluene, was of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd.

[0052] In the following embodiments, the limiting oxygen index (LOI) was determined according to GB / T 2406.2; the vertical flammability rating (UL-94) was determined according to GB / T 2408; the volume resistivity was determined according to GB / T 1410; the tensile strength and elongation at break were determined according to GB / T 1040.2, with a tensile rate of 50 mm / min; the interfacial bonding strength was determined using the tensile shear method (refer to GB / T 7124); and the peak heat release rate (HRRpeak) was determined using a cone calorimeter according to ISO 5660-1, with a heat flux of 50 kW / m³. 2 Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10 °C / min; the ovality and wall thickness uniformity of the pipe were determined according to GB / T 8806.

[0053] The characterization method for the penetration depth of tetra-needle-shaped zinc oxide whiskers is as follows: A suitable amount of double-shell gradient functionalized core-shell microsphere samples were taken, embedded in epoxy resin, cryogenically sectioned, and observed under a field emission scanning electron microscope (FE-SEM). At least 100 microsphere cross-sections clearly showing the interface between the four needle-like zinc oxide whisker tips and the outer shell were randomly selected, and measured using image analysis software. (1) The number of four needle-shaped zinc oxide whisker tips that penetrate the inner shell and embed in the outer shell is denoted as N, and the penetration ratio of the tips is N / 100×100%.

[0054] (2) The depth of a single whisker tip embedded in the outer shell layer is denoted as L_embed, which is the vertical distance from the outer surface of the inner shell layer to the tip of the whisker; the total thickness of the outer shell layer at the corresponding position of the whisker tip is denoted as L_shell; calculate the penetration depth ratio of each whisker in (1) = (L_embed / L_shell) × 100%; take the arithmetic mean of all measurements as the “average penetration depth” of the sample.

[0055] Example 1 This embodiment provides a method for preparing flame-retardant and antistatic polyethylene pipes, including the following steps: Step 1: Preparation of double-shell gradient functionalized core-shell microspheres: Weigh 300 g of APP, 100 g of MPP, and 12 g of MCA (mass ratio 3:1:0.12), add deionized water and 2% PVA as a binder, mix thoroughly to prepare a suspension with a solid content of 40%. Pump the suspension into a fluidized bed granulation device to produce granules. After airflow classification and screening, collect particles with a diameter of approximately 3-4 μm. Place the granules in a vacuum drying oven and vacuum dry at 100℃ for 4 h until the moisture content is no higher than 0.1%. Then transfer them to a fluidized bed coating device. Dilute 30% silica sol with deionized water to a mass fraction of 10%, and coat the core mixture in the fluidized bed using a peristaltic pump spray method. The inlet air temperature is 80℃, the spray rate is 5 mL / min, and the coating time is 60 min. A protected core with a silica nano-protective layer with a surface thickness of approximately 0.15 μm is obtained, denoted as core A.

[0056] 78 g of PEEA was dissolved in 500 mL of toluene and stirred at 60 °C. 20 g of T-ZnOw (20% of the inner shell mass), 0.62 g of hindered phenolic antioxidant 1076 (0.8% of PEEA mass), and 1.95 g of nano-silica (2.5% of PEEA mass) were added to the PEEA solution and ultrasonically dispersed for 30 min (600 W power, 20 kHz frequency) to obtain the inner shell coating solution. Core A was placed in a fluidized bed and coated with the above coating solution in a thin layer. The inlet air temperature was 60 °C, the spray rate was 3 mL / min, and each coating was dried for 15 min. This process was repeated three times to form an inner shell layer with a thickness of approximately 0.9 μm (the thickness of the polymer layer, excluding the exposed length of T-ZnOw), resulting in a core / inner shell composite microsphere, denoted as microsphere B.

[0057] Separately, 82 g of PEEA was dissolved in 500 mL of toluene. 6.5 g of COOH-CNTs (7% of the outer shell mass) and 11.5 g of nano-magnesium hydroxide (approximately 12% of the outer shell mass) were added to the PEEA solution. The mixture was first ultrasonically dispersed for 20 min, then sheared and dispersed using a high-speed shear emulsifier (10000 r / min, 10 min), and finally milled for 30 min to obtain the outer shell coating solution. Microspheres B were placed in a fluidized bed and coated with the above outer shell coating solution. The inlet air temperature was 65℃, the spray rate was 4 mL / min, and the coating was repeated three times. After each coating, the microspheres were dried for 20 min to form an outer shell layer with a thickness of approximately 1.1 μm, resulting in a double-shell gradient functionalized core-shell microsphere, denoted as core-shell microsphere C.

[0058] Characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the core-shell microsphere C had a particle size of approximately 8 μm and a total coating thickness of approximately 2.15 μm. Using the aforementioned characterization method for the penetration depth of four-needle zinc oxide whiskers, it was observed that 82% of the T-ZnOw whiskers penetrated the inner shell and embedded themselves in the outer shell, with the average penetration depth of these whiskers being approximately 77% of the outer shell thickness.

[0059] Step 2: Mixing and granulation: Weigh the raw materials according to the following proportions: 100 parts by weight of HDPE, 15 parts by weight of core-shell microspheres C, 3 parts by weight of conductive carbon black, 1.2 parts by weight of MAH-g-PE, 0.25 parts by weight of E-GMA, 3 parts by weight of POE, 1.5 parts by weight of EOC, 0.7 parts by weight of composite lubricant, 0.2 parts by weight of composite antioxidant, and 0.75 parts by weight of charring agent (pentaerythritol).

[0060] HDPE, POE, and EOC were added to a high-speed mixer and premixed at 50°C for 3 min. Then, composite lubricant, MAH-g-PE, E-GMA, composite antioxidant, charring agent, conductive carbon black, and core-shell microspheres C were added, and the mixture was mixed at 55°C for 8 min to obtain a compound. The compound was then fed into a co-rotating twin-screw extruder with temperature zones of 160°C / 170°C / 175°C / 178°C / 178°C / 180°C, a screw speed of 195 r / min, dual-stage venting, a vacuum of -0.095 MPa, and underwater pelletizing to obtain pellet D.

[0061] Step 3: Pipe extrusion molding: Granules D are added to a single-screw pipe extrusion production line. After melting and plasticizing, the melt passes through a fish-scale guide plate orientation device (fish scale angle 35°, fish scale spacing 2.5 mm, surface coated with polytetrafluoroethylene). The temperature of the shaping section is controlled at 185℃, the traction speed ratio is 1.07:1, and after 6 stages of gradient cooling (cooling water temperature 15℃, cooling rate decreasing sequentially at 20℃ / min, 20℃ / min, 18℃ / min, 18℃ / min, 15℃ / min, 15℃ / min) and vacuum shaping (negative pressure -0.04 MPa, pressure fluctuation ≤ ±0.005 MPa), DN110 pipe is produced. After annealing at 80℃ for 2 h, flame-retardant and antistatic polyethylene pipe is produced, denoted as pipe E1.

[0062] Performance tests were conducted on the E1 tubing, and the results are as follows: LOI was 32.8%, UL-94 vertical burning rating was V-0, initial decomposition temperature was 313℃, char residue at 700℃ was 29.5%, HRRpeak was 162 kW / m², and volume resistivity was 3.2 × 10⁻⁶. 7Ω·cm; tensile strength is 24.5 MPa, elongation at break is 435%; the interfacial bonding force between the core-shell microspheres and the HDPE matrix is ​​21 MPa; continuous extrusion production of DN400 pipes is free of sag, the pipe ellipticity is 0.4%, and the wall thickness uniformity is 98.5%.

[0063] Example 2 This embodiment provides a method for preparing flame-retardant and antistatic polyethylene pipes. The difference from Example 1 lies only in the introduction of a furan-maleimide dynamic covalent reversible crosslinking agent into the outer shell polyether ester amide and the adjustment of the fish-scale-shaped guide plate parameters. Specifically, it includes the following steps: Step 1: Preparation of bi-shell gradient functionalized core-shell microspheres containing reversible crosslinking agents: The core preparation and inner shell coating process were the same as in Example 1, resulting in microsphere B.

[0064] The preparation of the outer shell coating solution differs from that in Example 1 in that: 0.33 g of furan-maleimide dynamic covalent reversible crosslinking agent (0.4% of PEEA mass) is added to a toluene solution of 82 g of PEEA, along with 6.5 g of COOH-CNTs and 11.5 g of nano magnesium hydroxide. The remaining dispersion process is the same as in Example 1.

[0065] The outer shell coating process of Example 1 was followed to obtain a double-shell gradient functionalized core-shell microsphere containing a reversible crosslinking agent, denoted as core-shell microsphere C2.

[0066] Step 2: Mixing and granulation: The raw material ratio is the same as in Example 1, except that core-shell microspheres C2 are used instead of core-shell microspheres C. The remaining operations are the same as in Example 1 to obtain granules D2.

[0067] Step 3: Pipe extrusion molding: The difference from Example 1 is that the angle of the fish scales in the fish scale-shaped guide plate is adjusted to 40° and the spacing is 2.5 mm. The other process parameters are the same as in Example 1, and flame-retardant and antistatic polyethylene pipe is obtained, which is denoted as pipe E2.

[0068] The furan-maleimide dynamic covalent reversible crosslinking agent undergoes a reverse Diels-Alder reaction at extrusion processing temperatures (above 175℃), resulting in the breakage of crosslinking bonds and a decrease in the melt viscosity of the outer shell PEEA, which is beneficial for the dispersion of microspheres in the matrix and the extrusion molding of large-diameter pipes. At pipe operating temperatures (below 130℃), the Diels-Alder reaction recurs, the crosslinking bonds are restored, and the toughness of the outer shell and the interfacial bonding force with the matrix are enhanced.

[0069] Performance tests were conducted on the E2 pipe, and the results are as follows: LOI was 33.1%, UL-94 vertical combustion rating reached V-0, and HRRpeak was 159 kW / m. 2 The volume resistivity is 3.0 × 10⁻⁶. 7 Ω·cm; tensile strength is 24.8 MPa, elongation at break is 455%; interfacial bonding strength is 22 MPa; melt flow index (190℃, 2.16 kg) is about 15% higher than that of pipe E1; continuous extrusion production of DN400 pipe is free of melt sag, pipe ellipticity is 0.3%, and wall thickness uniformity is 99.0%.

[0070] Example 3 This embodiment provides a method for preparing flame-retardant and antistatic polyethylene pipes. The only difference from Embodiment 1 is that zinc borate is added to the pipe raw materials as a synergistic flame retardant. Specifically, it includes the following steps: Step 1: The preparation of core-shell microspheres is exactly the same as in Example 1, resulting in core-shell microsphere C.

[0071] Step 2: Mixing and granulation: The raw material ratio was the same as in Example 1, except that zinc borate was added at a rate of 1.5% of the total raw material mass. HDPE, POE, and EOC were premixed in a high-speed mixer, followed by the addition of composite lubricant, MAH-g-PE, E-GMA, composite antioxidant, charring agent, conductive carbon black, zinc borate, and core-shell microspheres C. The mixture was then mixed at high speed at 55°C for 8 minutes. The remaining granulation process was the same as in Example 1, yielding granules D3.

[0072] Step 3: The pipe extrusion molding process is the same as in Example 1, and flame-retardant and antistatic polyethylene pipe is obtained, which is denoted as pipe E3.

[0073] Zinc borate releases water of crystallization during combustion, absorbs heat, and works synergistically with the APP / MPP / MCA expansion system to promote char densification, inhibit droplet formation, and further enhance flame retardant performance.

[0074] Performance tests were conducted on the E3 pipe material, and the results are as follows: LOI of 35.4%, UL-94 vertical combustion rating of V-0, char residue at 700℃ of 32.1%, and HRRpeak of 148 kW / m². 2 The volume resistivity is 3.5 × 10⁻⁶. 7 Ω·cm; tensile strength is 24.1 MPa, elongation at break is 421%; interfacial bond strength is 20 MPa. Pipe E3 is suitable for mining applications requiring higher flame retardancy ratings.

[0075] Example 4 This embodiment sets up a comparative example to compare the comprehensive performance of the polyethylene pipe prepared by the present invention with that of the prior art, so as to verify the technical effect brought by the present invention.

[0076] Comparative Example 1, following the technical solution disclosed in CN111925582A, prepared flame-retardant and antistatic polyethylene pipes using a direct blending filling method. 70 parts by weight of HDPE, 15 parts by weight of pentaerythritol phosphate melamine salt, 10 parts by weight of multi-scale conductive powder (a composite of conductive carbon black and carbon nanotubes in a 9:1 mass ratio), and 0.2 parts by weight of composite antioxidant were milled and dispersed in a solid-state mechanical chemical reactor. The mixture was then melt-extruded at 160–200°C and shaped using the pipe extrusion process described in step three of Example 1 (without using fish-scale guide plates). The resulting pipe was designated as Comparative Pipe F1. The filler content was approximately 29 wt% of the total mass.

[0077] Comparative Example 2 uses single-shell core-shell microspheres (without tetrapter zinc oxide whisker penetrating structure) to prepare tubing. The preparation method of single-shell core-shell microspheres is as follows: based on the same core A as in Example 1, only one shell layer is prepared. The shell layer contains PEEA (80% of the shell layer mass), COOH-CNTs (7% of the shell layer mass), and nano-magnesium hydroxide (12% of the shell layer mass), and does not contain T-ZnOw. The shell layer thickness is about 2.0 μm. The remaining formulation and preparation process are the same as in Example 1. The resulting tubing is designated as control tubing F2.

[0078] Comparative Example 3 is based on Example 1, but the POE and EOC dual toughening system is removed, that is, the raw materials do not contain POE and EOC, the amount of HDPE is adjusted to 105 parts by weight (keeping the total amount basically unchanged), and the rest of the formula and preparation process are the same as in Example 1. The pipe is obtained and is referred to as comparative pipe F3.

[0079] The comprehensive performance of pipe E1, comparative pipe F1, comparative pipe F2, and comparative pipe F3 were tested according to the aforementioned test method. The results are shown in Table 1.

[0080] Table 1. Comparison of the comprehensive performance of the pipes prepared in Example 1 and Comparative Examples 1-3

[0081] As shown in Table 1, compared with Comparative Example 1 (direct blending route), pipe E1, with a reduction of about 50% in the amount of functional filler, exhibits a 118% increase in tensile strength, a 13% increase in elongation at break, a nearly one-order-of-magnitude reduction in volume resistivity, a 25% reduction in HRRpeak, and a 19% increase in carbon residue at 700℃. Its overall performance is significantly better than that of the prior art, and this effect is unexpected by those skilled in the art.

[0082] Compared with Comparative Example 2 (a single-shell core-shell microsphere without the four-needle zinc oxide whisker penetration structure), the interfacial bonding strength of tube E1 is improved by 91% (from 11 MPa to 21 MPa), the elongation at break is improved by 120% (from 198% to 435%), the flame retardant rating is improved from V-1 to V-0, and the volume resistivity is reduced by nearly one order of magnitude. This fully demonstrates that the mechanical interlocking junction of the four-needle zinc oxide whiskers penetrating the inner shell and embedding into the outer shell, as well as the three-channel redundant conductive network, bring about unexpected synergistic effects of interface enhancement, toughening, and conductivity improvement.

[0083] Compared with Comparative Example 3 (without POE and EOC dual toughening), the elongation at break of pipe E1 increased by 60% (from 272% to 435%) and the tensile strength increased by 12% (from 21.8 MPa to 24.5 MPa), proving that the dual toughening system of polyolefin elastomer and ethylene-octene copolymer plays an irreplaceable role in achieving a balance between rigidity and toughness.

[0084] Example 5 This embodiment is to verify the stability of the preparation method of the present invention for extrusion molding of large-diameter DN500 pipes.

[0085] A method for preparing flame-retardant and antistatic polyethylene pipes specifically includes the following steps: Step 1: The preparation of core-shell microspheres is exactly the same as in Example 1, resulting in core-shell microsphere C.

[0086] Step 2: The mixing and granulation process is exactly the same as in Example 1, and granules D are obtained.

[0087] Step 3: Pipe Extrusion Molding. Granules D are added to a single-screw pipe extrusion production line (screw diameter 150 mm, length-to-diameter ratio 35:1). After melting and plasticizing, the melt passes through a fish-scale guide plate orientation device (fish-scale angle 35°, fish-scale spacing 3 mm, surface coated with polytetrafluoroethylene). The temperature of the shaping section is controlled at 186℃, the traction speed ratio is 1.06:1, and the pipe undergoes five stages of gradient cooling (cooling water temperature 15℃, cooling rate decreasing sequentially at 20℃ / min, 18℃ / min, 18℃ / min, 15℃ / min, 15℃ / min) and vacuum shaping (negative pressure -0.04 MPa, pressure fluctuation ≤ ±0.005 MPa) to obtain DN500 pipe. After annealing at 80℃ for 2 hours, flame-retardant and antistatic polyethylene pipe is obtained, designated as pipe E5.

[0088] Performance tests were conducted on the E5 grade pipe, and the results are as follows: LOI is 32.5%, UL-94 vertical burning rating reaches V-0; volume resistivity is 3.4 × 10⁻⁶. 7The tensile strength was 24.1 MPa, and the elongation at break was 428%. No sag was observed during the continuous extrusion production of the DN500 pipe. The pipe ellipticity was 0.5%, and the wall thickness uniformity was 98.2%. Its overall performance was comparable to that of Example 1 (DN110).

[0089] Comparative Example 4 uses the exact same raw material formulation and preparation method as Example 5, except for the following change in the pipe extrusion molding in step three: the fish-scale-shaped guide vane orientation device is removed, meaning the melt directly enters the shaping section after melting and plasticizing, without passing through any guide vane device. All other process parameters are the same as in Example 5, and this is designated as Comparative Pipe F4.

[0090] The performance of pipe E5 and control pipe F4 was tested according to the aforementioned test method, and the results are shown in Table 2.

[0091]

[0092] The above results show that, under the same formulation and most process conditions, simply eliminating the fish-scale guide vane orientation device resulted in significant sag defects in DN500 large-diameter pipes. The ellipticity deteriorated from 0.5% to 2.8% (exceeding the ≤1.5% requirement of the national standard GB / T 8806), wall thickness uniformity decreased from 98.2% to 89.2%, and obvious ripples appeared on the surface. This indicates that the fish-scale guide vane orientation device, combined with a shaping section temperature of 182–188℃, a traction speed ratio of 1.06–1.08:1, and a segmented gradient cooling process, can effectively improve melt strength and control melt orientation, successfully solving the sag problem during the extrusion of 500 mm nominal diameter large-diameter pipes and achieving stable, sag-free production of large-diameter pipes.

[0093] Example 6 This embodiment prepares core-shell microspheres with different penetration depths of tetra-needle-shaped zinc oxide whiskers by adjusting the fluidized bed coating process parameters, and verifies the technical rationality of the penetration depth range of 70-85%.

[0094] Four sets of core-shell microspheres were prepared, with the core and inner shell layers prepared using the same process as in Example 1. During the outer shell coating process, the outer shell thickness was controlled by adjusting the fluidized bed inlet air temperature (55–75°C), spray rate (2–6 mL / min), and coating times (2–4 times), thereby obtaining different whisker penetration depth ratios. Five sets of core-shell microspheres were obtained with penetration depths of approximately 60% (core-shell microsphere C6a), 70% (core-shell microsphere C6b), 77% (i.e., core-shell microsphere C from Example 1), 85% (core-shell microsphere C6d), and over 85% (core-shell microsphere C6e, with a penetration depth of approximately 95%, where the outer shell layer was partially punctured).

[0095] Tubes were prepared using the core-shell microspheres of each group according to the formulation and process of Example 1, with all other conditions being exactly the same. Tubes E6a, E6b, E1 (i.e. Example 1), E6d and E6e were obtained. The interfacial bonding force, elongation at break and volume resistivity of each tube were tested, and the results are shown in Table 3.

[0096] Table 3. Effect of different penetration depths of tetraneedle-shaped zinc oxide whiskers on pipe performance

[0097] As shown in Table 3, when the penetration depth is 60% of the outer shell thickness, the interfacial bonding strength is only 14 MPa, the elongation at break is only 320%, and the volume resistivity is 9.8 × 10⁻⁶. 7 The Ω·cm value indicates poor performance; however, when the penetration depth is within the range of 70–85%, the interfacial bonding strength is not less than 20 MPa, the elongation at break is not less than 408%, and the volume resistivity is not higher than 3.6 × 10⁻⁶. 7 Ω·cm, exhibiting excellent overall performance; however, when the penetration depth exceeds 85% (approximately 95%, with localized penetration of the outer shell), the integrity of the outer shell decreases, the conductive network is damaged, and the volume resistivity increases to 1.2 × 10⁻⁶. 8 The interfacial bonding strength and elongation at break also decreased, as measured by Ω·cm. These results demonstrate that the optimal penetration depth for tetraneedle zinc oxide whiskers is 70–85%, and this range represents a reasonable generalization; beyond this range, the overall performance significantly declines.

[0098] It is understood that the polyether ester amide is not limited to PEBAX 2533, but can also be other grades of polyether ester amide, such as PEBAX 3533, PEBAX 4033, etc., as long as it has a soft segment structure compatible with polyethylene matrix and sufficient film-forming properties.

[0099] It is understood that the composite lubricant is not limited to a mixture of methyl silicone oil and calcium stearate, but other lubricant systems, such as a mixture of ethylene bis-stearamide (EBS) and zinc stearate, can also be used. However, the mixture of methyl silicone oil and calcium stearate in a mass ratio of 2:1 exhibits the best lubrication effect in the system of the present invention.

[0100] It is understood that the charring agent is not limited to pentaerythritol, but can also be melamine, starch and other polyhydroxy compounds, but pentaerythritol and the APP / MPP system have the best synergistic charring effect.

[0101] It is understood that the silane coupling agent is not limited to KH-550 (3-aminopropyltriethoxysilane), but can also be KH-570 (3-methacryloyloxypropyltrimethoxysilane), KH-560 (3-glycidyl etheroxypropyltrimethoxysilane), etc. The specific selection should be determined based on the compatibility between the functional groups on the filler surface and the matrix.

[0102] It is understood that the segmented gradient cooling is not limited to 5 to 6 segments. Under the premise of ensuring that the cooling rate decreases in a gradient of 20℃ / min, 18℃ / min, and 15℃ / min, the number of cooling segments can be appropriately adjusted according to the pipe specifications and production line length.

[0103] In summary, this invention constructs a gradient multilayer flame-retardant barrier, achieving a V-0 flame-retardant rating even with low filler content. Simultaneously, at least some of the tips of the four needle-like zinc oxide whiskers penetrate the inner shell layer and embed themselves in the outer shell layer, forming a mechanically interlocked structure and a three-channel redundant conductive network, resulting in durable and stable antistatic performance. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and possesses high industrial applicability.

[0104] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A flame-retardant and antistatic polyethylene pipe, characterized in that, Including the following parts by weight of raw materials: 100 parts of high-density polyethylene; 14-16 portions of double-shell gradient functionalized core-shell microspheres; 2-4 parts of conductive carbon black; 1.0-1.5 parts of maleic anhydride-grafted polyethylene; 0.2-0.3 parts of ethylene-glycidyl methacrylate copolymer; 2.5-3.5 parts of polyolefin elastomer; 1-2 parts of ethylene-octene copolymer; 0.6-0.8 parts of compound lubricant; 0.1-0.3 parts of compound antioxidant; 0.5-1 part of charring agent; The double-shell gradient functionalized core-shell microspheres include: The core is composed of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate, and the surface of the core is covered with a silica nano-protective layer. An inner shell layer covering the surface of the core, the inner shell layer comprising polyether ester amide and tetra-needle-shaped zinc oxide whiskers dispersed therein; An outer shell layer covering the surface of the inner shell layer, the outer shell layer comprising polyether ester amide, carboxylated carbon nanotubes and nano-magnesium hydroxide; In this process, at least some of the tips of the four needle-shaped zinc oxide whiskers penetrate the inner shell layer and embed themselves in the outer shell layer.

2. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, The mass ratio of ammonium polyphosphate, melamine polyphosphate and melamine cyanurate in the core is 3:1:0.12; the particle size of the core is 3-6 μm.

3. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, The thickness of the silica nano-protective layer is 0.1-0.2 μm.

4. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, The inner shell layer contains 18-22% of the mass of tetra-needle zinc oxide whiskers, which have an aspect ratio of 20-30, a single needle length of 1-2 μm, and are surface-treated with a silane coupling agent. The inner shell layer also includes 0.5-1% of the mass of the polyether ester amide, a hindered phenolic antioxidant, and 2-3% of nano-silica. The thickness of the inner shell layer is 0.8-1.0 μm.

5. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, The carboxylated carbon nanotubes in the outer shell layer account for 6-8% of the mass of the outer shell layer, and the degree of carboxylation of the carboxylated carbon nanotubes is 2.5-3.5%; the nano-magnesium hydroxide accounts for 10-14% of the mass of the outer shell layer, has a particle size of 40-80 nm, and is surface-treated with stearic acid or silane coupling agent; the thickness of the outer shell layer is 1.0-1.2 μm.

6. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, At least 60% of the tips of the tetra-needle zinc oxide whiskers penetrate the inner shell layer and embed themselves in the outer shell layer; and the average depth to which the tips of the tetra-needle zinc oxide whiskers are embedded in the outer shell layer is 70-85% of the thickness of the outer shell layer.

7. The flame-retardant and antistatic polyethylene pipe according to claim 1, characterized in that, The outer shell layer also includes a reversible crosslinking agent comprising 0.3-0.5% of the mass of the polyether ester amide, wherein the reversible crosslinking agent is a furan-maleimide dynamic covalent crosslinking agent.

8. A method for preparing a flame-retardant and antistatic polyethylene pipe as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Preparation of double-shell gradient functionalized core-shell microspheres: Ammonium polyphosphate, melamine polyphosphate and melamine cyanurate are mixed and then granulated by spray drying to form core particles. After drying, the core particles are spray-coated with silica sol in a fluidized bed to form a silica nano-protective layer, thus obtaining a protected core. Polyether ester amide was dissolved in a solvent to prepare a solution, and tetra-needle-shaped zinc oxide whiskers, antioxidants and nano-silica were added. After ultrasonic dispersion, the protected core was coated with thin layers multiple times in a fluidized bed to form an inner shell layer. Another polyether ester amide solution was taken, and carboxylated carbon nanotubes, nano magnesium hydroxide and dispersing agent were added. After being dispersed by ultrasonication, shearing and grinding, the solution was further coated in a fluidized bed to form an outer shell layer, thereby obtaining the double-shell gradient functionalized core-shell microspheres. Step 2: Mixing and granulation: Weigh each raw material according to the formula, first mix high-density polyethylene, polyolefin elastomer and ethylene-octene copolymer, then add composite lubricant, maleic anhydride grafted polyethylene, ethylene-glycidyl methacrylate copolymer, composite antioxidant, charring agent, conductive carbon black and the double-shell gradient functionalized core-shell microspheres, mix at high speed and then add the mixture to a twin-screw extruder, and granulate to obtain granules; Step 3: Pipe extrusion molding: The granules are added to a single-screw pipe extrusion production line for extrusion. After segmented gradient cooling and vacuum shaping, the material is then annealed to obtain the flame-retardant and antistatic polyethylene pipe.

9. The method for preparing flame-retardant and antistatic polyethylene pipe according to claim 8, characterized in that, In step one, the spray drying granulation process includes: adding deionized water and 1.5-2.5% polyvinyl alcohol as a binder to ammonium polyphosphate, melamine polyphosphate, and melamine cyanurate, mixing them evenly to prepare a suspension with a solid content of 35-45%, pumping the suspension into a fluidized bed granulation device, producing granules, which are then screened by airflow classification, and collecting particles with a particle size of 3-6 μm as core particles.

10. The method for preparing flame-retardant and antistatic polyethylene pipe according to claim 8, characterized in that, In step three, in the extrusion production line, the melt after melting and plasticizing passes through a fish-scale-shaped guide plate orientation device. The fish-scale-shaped guide plate has a scale angle of 30-45°, a spacing of 2-3 mm, and a surface coated with polytetrafluoroethylene. The temperature of the shaping section is controlled at 182-188℃, and the traction speed ratio is 1.06-1.08:1.

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