Flame-retardant wear-resistant multilayer polyethylene pipe and method for producing the same

Abstract

The application discloses a kind of fire-retardant wear-resistant multilayer polyethylene pipe and preparation method thereof.The pipe is sequentially from inside to outside for inner layer composite layer, intermediate pressure-bearing layer and outer layer fire-retardant composite layer.The outer layer constructs halogen-free fire-retardant system with magnesium hydroxide and low softening point glass powder, and introduces boron-containing polysiloxane ceramic precursor polymer and multifunctional olefin crosslinking agent, and cooperates with maleic anhydride grafted polyethylene to realize interface regulation.The glass powder is subjected to composite surface treatment with zirconium phosphate primer and polysilazane topcoat, and is added in the third stage of melt blending in the latter section.By three-stage melt blending and negative pressure exhaust, the thermal history and volatile matter are controlled, and a multilayer pipe with fire-retardant and heat release suppression, inner wall wear resistance and pressure-bearing reliability is obtained.

Description

Technical Field

[0001] This invention relates to the technical field of polyethylene pipes, and in particular to a flame-retardant and wear-resistant multilayer polyethylene pipe and its preparation method. Background Technology

[0002] In polyethylene pipes, flame retardancy and wear resistance are typically achieved by adding a large amount of flame-retardant filler or wear-resistant phase to the same matrix. However, flame-retardant systems often require high levels of inorganic filler to achieve combustion inhibition, which can easily lead to deterioration of melt rheology, decreased toughness, and adverse effects on the pressure-bearing reliability of the pipe. Wear-resistant phases (such as UHMWPE) improve wear resistance but introduce risks related to dispersion and molding appearance. If combined with a high-filler flame-retardant system, this can further compress the extrusion process window. Secondly, relying solely on the endothermic and water-release flame retardancy of metal hydroxides often results in a loose residual layer after combustion, making it difficult to maintain a continuous shielding structure on the outer surface in a fire, thus leading to insufficient flame retardant effect and structural retention capacity.

[0003] Therefore, how to ensure that the pipe material has stable flame retardancy (including the maintenance of the outer layer structure in the fire) and wear resistance of the inner surface without sacrificing the continuous phase of the pressure-bearing layer, while maintaining the processing window for industrial co-extrusion molding. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and to provide a flame-retardant and wear-resistant multilayer polyethylene pipe and its preparation method.

[0005] To achieve the above objectives, the first aspect of the present invention provides a flame-retardant and wear-resistant multilayer polyethylene pipe, wherein the pipe comprises, from the inside out, an inner composite layer, a middle pressure-bearing layer and an outer flame-retardant composite layer.

[0006] The outer flame-retardant composite layer comprises, based on 100 parts by weight of polyethylene matrix, 70-110 parts of magnesium hydroxide, 10-40 parts of glass powder, 0.5-6 parts of boron-containing polysiloxane ceramic precursor polymer, 0.2-3 parts of polyfunctional olefin crosslinking agent, 2-8 parts of maleic anhydride-grafted polyethylene, 0.20-0.50 parts of antioxidant, and 0.20-0.80 parts of lubricant.

[0007] As a further improvement of the present invention, the boron-containing polysiloxane ceramic precursor polymer is polyborosiloxane.

[0008] As a further improvement of the present invention, the multifunctional olefin crosslinking agent is divinylbenzene.

[0009] As a further improvement of the present invention, the glass powder is a low softening point glass powder, and the low softening point glass powder is subjected to composite surface treatment to form a coating layer, the mass of the coating layer being 0.3 to 3.0% of the mass of the glass powder.

[0010] As a further improvement of the present invention, the coating layer includes an inorganic base coating and a pre-ceramicized outer coating; the inorganic base coating is a zirconium phosphate layer, and the mass of the inorganic base coating is 0.1 to 1.0% of the mass of the glass powder.

[0011] As a further improvement of the present invention, the pre-ceramicized outer coating is formed by curing polysilazane, and the mass of the pre-ceramicized outer coating is 0.2 to 2.0% of the mass of the glass powder.

[0012] A second aspect of the present invention provides a method for preparing the flame-retardant and wear-resistant multilayer polyethylene pipe as described above, comprising the following steps:

[0013] S1: Magnesium hydroxide, boron-containing polysiloxane ceramic precursor polymer and multifunctional olefin crosslinking agent are mixed and then melt-blended in the first stage. During the first stage melt-blending process, negative pressure is vented to obtain pretreated filler masterbatch.

[0014] S2. Polyethylene matrix and maleic anhydride-grafted polyethylene are added to the pretreated filler masterbatch obtained in step S1 for a second stage of melt blending.

[0015] S3. After step S2, glass powder is added for the third stage of melt blending and granulation to obtain the outer flame-retardant composite layer granules.

[0016] S4. The inner composite layer granules, the middle pressure-bearing layer granules and the outer flame-retardant composite layer granules are co-extruded, sized and cooled to obtain the pipe.

[0017] As a further improvement of the present invention, the first stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperatures of each temperature zone are set to 170°C, 180°C, 185°C, 185°C and 180°C respectively, the screw speed is 300 rpm, the material residence time in the machine is 7 minutes, and negative pressure exhaust is set in the second to fourth temperature zones, with an exhaust pressure of -0.085 MPa.

[0018] As a further improvement of the present invention, the second stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperatures of each temperature zone are set sequentially as 165°C, 175°C, 180°C, 185°C, 185°C, and 180°C, the screw speed is 240 rpm, the material residence time in the machine is 5 minutes, and a negative pressure exhaust is set in the third temperature zone with an exhaust pressure of -0.070 MPa.

[0019] As a further improvement of the present invention, the third stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperatures of each temperature zone are set sequentially as 165°C, 175°C, 180°C, 185°C, 185°C, and 180°C. Glass powder is added to the molten material in step S2 through a side feed port. The glass powder is added between the fourth and fifth temperature zones. After the glass powder is added, melt blending continues for 2 minutes, and the screw speed is 180 rpm.

[0020] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0021] (1) By configuring the pipe material from the inside out as an inner composite layer, an intermediate pressure-bearing layer, and an outer flame-retardant composite layer, the filler system that bears the requirements of wear resistance and flame retardancy is concentrated in the outer layer, while the pressure bearing and dimensional stability are mainly provided by the intermediate pressure-bearing layer. This avoids the interference of the high-filling flame-retardant system on the melt flow and crystallization morphology of the pressure-bearing layer, thus ensuring both pressure bearing reliability and wear resistance stability of the outer surface in the structure. The inner composite layer, through the synergy of high-density polyethylene and ultra-high molecular weight polyethylene, enables the inner surface to maintain lower wear and a more stable surface state under friction and scouring conditions, and reduces the risk of overall embrittlement caused by the high filling of the outer layer.

[0022] (2) By using magnesium hydroxide as the main flame-retardant filler in the outer layer, while ensuring the endothermic decomposition and residual basis of the halogen-free system, boron-containing polysiloxane ceramic precursor polymer and multifunctional olefin crosslinking agent are introduced together, so that the precursor has the conditions for controlled crosslinking and preferential enrichment near the inorganic filler during the processing thermal history. This enrichment process not only brings about the enhancement of condensed phase residue in the conventional sense, but more importantly, it changes the interfacial state of the magnesium hydroxide surface during the processing, reducing its interference with the formation of subsequent compatible systems and glass phases, thereby providing a more controllable environment for interfacial interaction in subsequent steps. The boron-containing polysiloxane is limited to polyborosiloxane, which is conducive to the formation of a more stable boron-containing silicon inorganic skeleton in the heating stage, making the adhesion and continuity of the outer residual phase less prone to batch fluctuations. The multifunctional olefin crosslinking agent is limited to divinylbenzene, which makes crosslinking triggering easier to occur in the processing temperature range and reach an appropriate degree of crosslinking in a short time, making the precursor more inclined to form a local network around the filler rather than homogeneous dilution, which makes the precursor's regulation of the interfacial environment more stable.

[0023] (3) By limiting the glass powder to low softening point glass powder and performing composite surface treatment to form a coating layer, the tendency of the glass powder to absorb moisture and release gas during the melt blending stage is reduced, and the direct hard agglomeration between particles is suppressed, so that it can still maintain effective dispersion under the short residence time of the later stage of mixing. The mass ratio of the coating layer is controlled within a small range, so that the glass powder retains the viscous flow and gap-filling ability brought by the low softening point, while avoiding the significant delay of the softening behavior of the glass phase due to excessive coating thickness. Furthermore, by sequentially forming a zirconium phosphate inorganic base coating and a pre-ceramicized outer coating formed by polysilazane curing on the surface of the glass powder, the coating layer has more stable adhesion and hydrolysis resistance characteristics, weakens the influence of magnesium hydroxide and processing volatiles on the surface state of the glass powder, and makes the surface of the glass powder have stronger precursor compatibility. During the heating stage, the tendency of the polysilazane outer coating to transform into an inorganic network is conducive to the formation of a stronger bond and bridge between the glass phase and the outer residual phase, thereby reducing the tendency of residual layer powdering and cracking.

[0024] (4) By dividing the preparation of the outer flame-retardant composite layer granules into three stages of melt blending and fixing the feeding sequence as follows: first, treat magnesium hydroxide and precursor and crosslinking agent, then introduce polyethylene matrix and maleic anhydride-grafted polyethylene, and finally add glass powder, so that the action path of the outer layer system changes from parallel superposition to controlled sequential coupling. The first stage of melt blending, combined with negative pressure exhaust, reduces the moisture and volatile content of the system in the key stage of precursor crosslinking enrichment and interfacial network establishment, reduces bubble nucleus formation and interfacial instability factors, and at the same time allows the precursor to establish a local network more fully near the inorganic filler. The second stage of melt blending introduces maleic anhydride-grafted polyethylene under milder shear and controlled exhaust conditions, so that it can play a greater role in the interfacial environment that has been stabilized in the previous stage, and avoids its effectiveness from decreasing under high moisture and strong alkaline exposure conditions, thereby improving interfacial adhesion and intralayer uniformity. The third stage of melt blending employs a rear-stage side-feeding method for adding glass powder, with the side-feeding position positioned between the rear temperature zones. Simultaneously, the mixing time after side-feeding is shortened and the screw speed reduced. This prevents the glass powder from undergoing the prolonged high-shear process in the preceding stage, which would lead to breakage and agglomeration, and also avoids its participation in mixing during stages with relatively higher moisture content. Since the precursor interface network and compatible system are already established when the glass powder is added, the glass powder is more easily captured by the interface network and forms a more continuous interstitial closed phase in the melt, thus forming a dense shielding path earlier during the heating stage. The effect of this process is not limited to a conventional improvement in the pore-sealing ability of the glass powder; rather, the degree of glass powder continuity amplifies the compactness and anti-pulverization ability of the residual layer, making it easier to obtain a more stable outer layer structure with the same amount of glass powder.

[0025] (5) By limiting the temperature range, screw speed, residence time, and negative pressure exhaust position and pressure of the first, second, and third stages of melt blending, the relationship between precursor crosslinking and enrichment, interfacial network stability, effective integration of the compatible system, and continuous glass powder production in the later stages can be made more controllable. The higher speed and longer residence time in the first stage, combined with stronger negative pressure exhaust, are conducive to the pre-construction of the precursor network under conditions of lower volatile matter. The second stage reduces the damage to the already formed interfacial network by lowering the speed and shortening the residence time, while maintaining sufficient plasticizing and dispersing capacity to achieve effective distribution of the compatible system. The third stage is achieved by side feeding in the same extruder, and by further reducing the speed and shortening the mixing time in the later stages, the glass powder can be more fully integrated with the interfacial network without being excessively sheared. Therefore, the outer flame-retardant system forms a more stable interfacial structure during the processing stage, making it easier for the synergy between glass phase sealing, inorganic bonding of precursors and magnesium hydroxide residue to be transformed into a continuous and dense outer residual structure during the heating stage. This simultaneously inhibits pore channel expansion and residual layer powdering and cracking, and makes the wear resistance of the outer surface more stable under thermal shock and friction conditions. Detailed Implementation

[0026] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0027] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0030] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0031] Example 1

[0032] The raw materials used in this embodiment are as follows:

[0033] The polyethylene matrix is ​​selected from BorSafeHE3490-LS, a polyethylene pipe-grade resin supplied by Borealis.

[0034] The magnesium hydroxide was selected from Magnifin H-5 supplied by Huber Advanced Materials.

[0035] The glass powder was selected from low melting point glass powder D235 supplied by AnyPowder.

[0036] The boron-containing polysiloxane ceramic precursor polymer was selected from polyborosiloxane PA43109 supplied by Guangdong Wengjiang Chemical Reagent Co., Ltd.

[0037] The multifunctional olefin crosslinking agent is selected from divinylbenzene (80%, isomer mixture, catalog number D103376) supplied by Aladdin Company.

[0038] Maleic anhydride-grafted polyethylene was selected from FUSABOND E MB100D supplied by Dow Chemical Company.

[0039] The antioxidants were selected from Irganox 1010 and Irgafos 168 supplied by BASF.

[0040] The lubricant used was calcium stearate L-155 supplied by Blachford.

[0041] The polysilazane was selected from Durazane 1800 supplied by Merck.

[0042] The zirconium oxychloride octahydrate and phosphoric acid used to prepare the zirconium phosphate layer were selected from zirconium oxychloride octahydrate and phosphoric acid (85%) supplied by Sinopharm Chemical Reagent Co., Ltd.

[0043] The ultra-high molecular weight polyethylene powder was selected from GUR 4120 supplied by Celanese.

[0044] The formula for the outer flame-retardant composite layer, based on 100 parts by weight of the polyethylene matrix, is as follows:

[0045] 90 parts magnesium hydroxide, 25 parts glass powder, 3.0 parts polyborosiloxane, 1.0 part divinylbenzene, 5.0 parts maleic anhydride-grafted polyethylene, 0.30 parts antioxidant, and 0.50 parts lubricant.

[0046] The antioxidant 0.30 parts consist of 0.20 parts of Irganox 1010 and 0.10 parts of Irgafos 168.

[0047] The composite surface treatment of the glass powder is completed before extrusion begins, as follows:

[0048] 10 kg of low softening point glass powder was dried at 110℃ for 2 h and then cooled to room temperature. A 2 wt% zirconium oxychloride aqueous solution and a 2 wt% phosphoric acid aqueous solution were prepared. The glass powder was added to the zirconium oxychloride aqueous solution and dispersed for 15 min. Then, the phosphoric acid aqueous solution was added dropwise, and the pH of the system was adjusted to 2.2 with ammonia. The mixture was stirred at 60℃ for 60 min to allow zirconium phosphate to deposit in situ on the surface of the glass powder. After filtration and washing with deionized water until the conductivity of the filtrate stabilized, the powder was dried at 120℃ for 3 h to obtain glass powder with an inorganic undercoat. This glass powder was then added to a mixed solvent of ethylene glycol dimethyl ether and xylene, and Durazane 1800 solution was added and stirred for 30 min to achieve uniform wetting and coating. After desolventizing under reduced pressure, the powder was heat-treated at 160℃ for 1 h to cure the polysilazane into a film, resulting in modified glass powder with a pre-ceramicized outer coating. The mass of the pre-ceramicized outer coating was controlled to be approximately 1% of the glass powder mass, and the total mass of the two layers was 1.5% of the glass powder mass.

[0049] The inner composite layer granules and the intermediate bearing layer granules are prepared separately before step S4:

[0050] The inner composite layer granules are obtained by adding 20 parts by weight of ultra-high molecular weight polyethylene, 0.30 parts by weight of antioxidant and 0.30 parts by weight of lubricant to 100 parts by weight of polyethylene, and then melt-blending and granulating.

[0051] The intermediate pressure-bearing layer granules are obtained by melt blending and granulation of polyethylene pipe-grade resin with 0.3 parts by weight of antioxidant.

[0052] The preparation method of flame-retardant and wear-resistant multilayer polyethylene pipe is as follows:

[0053] S1: 90 parts of magnesium hydroxide, 3.0 parts of polyborosiloxane, and 1.0 part of divinylbenzene are premixed in a high-speed mixer for 6 minutes and then fed into a co-rotating twin-screw extruder for the first stage of melt blending. The temperature settings of each zone of the co-rotating twin-screw extruder are 170℃, 180℃, 185℃, 185℃, and 180℃ respectively, the screw speed is 300 rpm, and the material residence time in the machine is 7 minutes. Negative pressure exhaust is set in the second to fourth temperature zones, and the exhaust pressure is -0.085MPa. The discharged material is water-cooled, drawn into strips, and granulated to obtain pretreated filler masterbatch.

[0054] S2: The pretreated filler masterbatch obtained in step S1 is added to the main feed port of the co-rotating twin-screw extruder according to the outer layer formula. At the same time, 100 parts of polyethylene matrix, 5.0 parts of maleic anhydride grafted polyethylene, 0.30 parts of antioxidant and 0.50 parts of lubricant are added from the main feed port for the second stage of melt blending. The temperature settings of each temperature zone of the co-rotating twin-screw extruder are 165℃, 175℃, 180℃, 185℃, 185℃ and 180℃ respectively, the screw speed is 240 rpm and the material residence time in the machine is 5 minutes. The third temperature zone is set with negative pressure exhaust, and the exhaust pressure is -0.070MPa to obtain the melt material section without glass powder.

[0055] S3: Based on the molten material in step S2, add 25 parts of the above modified glass powder through the side feed port, with the side feed port located between the fourth and fifth temperature zones; after adding the glass powder, adjust the screw speed to 180 rpm and continue melting and blending for 2 minutes, then discharge the material, water-cool it into strips, and cut it into pellets to obtain the outer flame-retardant composite layer pellets.

[0056] S4: The inner composite layer granules, the middle pressure-bearing layer granules, and the outer flame-retardant composite layer granules are respectively added to three extruders. Through the three-layer co-extrusion die, the inner composite layer, the middle pressure-bearing layer, and the outer flame-retardant composite layer are formed sequentially from the inside to the outside. After extrusion, the pipes are vacuum sizing and spray cooling for shaping, and then pulled and cut to obtain the finished flame-retardant and wear-resistant multilayer polyethylene pipe.

[0057] Example 2

[0058] The only difference between this embodiment and Embodiment 1 is that the boron-containing polysiloxane ceramic precursor polymer in the outer flame-retardant composite layer is organic polyborosilazane IOTA-9120, selected from Anhui Aiyota Silicon Oil Co., Ltd.

[0059] Example 3

[0060] The only difference between this embodiment and Embodiment 1 is that the multifunctional olefin crosslinking agent in the outer flame-retardant composite layer is triallyl isocyanurate, selected from the product model T123406 of Shanghai Aladdin Biochemical Technology Co., Ltd.

[0061] Example 4

[0062] The only difference between this embodiment and Embodiment 1 is that the total amount of the composite surface treatment coating layer of the low softening point glass powder is adjusted from 1.50% of the glass powder mass to 0.30% of the glass powder mass, wherein the mass of the inorganic base coating zirconium phosphate layer is 0.1% of the glass powder mass, and the mass of the pre-ceramicized outer coating polysilazane curing layer is 0.2% of the glass powder mass.

[0063] Example 5

[0064] The only difference between this embodiment and Embodiment 1 is that the glass powder is added at a different location and under different mixing conditions in the third stage of melt blending. The glass powder is still added after the second stage of melt blending, but instead of being added through the side feed port between the fourth and fifth temperature zones, it is added through the main feed port at the beginning of the third stage of melt blending. After the glass powder is added, melt blending continues for 4 minutes, and the screw speed is set to 220 rpm.

[0065] Comparative Example 1

[0066] The only difference between this comparative example and Example 1 is that the outer flame-retardant composite layer does not contain boron-containing polysiloxane ceramic precursor polymer or polyfunctional olefin crosslinking agent; the other outer layer components and their amounts remain unchanged. The outer flame-retardant composite layer, based on 100 parts by weight of the polyethylene matrix, consists of 90 parts magnesium hydroxide, 25 parts modified glass powder, 5 parts maleic anhydride-grafted polyethylene, 0.3 parts antioxidant, and 0.5 parts lubricant.

[0067] The preparation method is as follows:

[0068] S1: 90 parts of magnesium hydroxide were premixed in a high-speed mixer for 3 minutes and then fed into a co-rotating twin-screw extruder for the first stage of melt blending. The temperature settings of each zone of the co-rotating twin-screw extruder were 170℃, 180℃, 185℃, 185℃, and 180℃, respectively. The screw speed was 300 rpm, and the material residence time in the machine was 7 minutes. Negative pressure exhaust was set in the second to fourth temperature zones, with an exhaust pressure of -0.085MPa. The discharged material was water-cooled, drawn into strips, and granulated to obtain pretreated filler masterbatch.

[0069] S2: The pretreated filler masterbatch obtained in step S1 is added to the main feed port of a co-rotating twin-screw extruder according to the outer layer formula. At the same time, 100 parts of polyethylene matrix, 5.0 parts of maleic anhydride grafted polyethylene, 0.30 parts of antioxidant and 0.50 parts of lubricant are added from the main feed port for the second stage of melt blending. The temperatures of each temperature zone are set to 165℃, 175℃, 180℃, 185℃, 185℃ and 180℃ respectively. The screw speed is 240 rpm and the material residence time in the machine is 5 minutes. Negative pressure exhaust is set in the third temperature zone with an exhaust pressure of -0.070MPa to obtain the melt material section without glass powder.

[0070] S3: Based on the molten material in step S2, add 25 parts of modified glass powder through the side feed port, which is located between the fourth and fifth temperature zones. After adding the glass powder, adjust the screw speed to 180 rpm and continue to melt and blend for 2 minutes. Discharge the material, water-cool it into strips, and cut it into pellets to obtain the outer flame-retardant composite layer pellets.

[0071] S4: The inner composite layer granules, the middle pressure-bearing layer granules, and the outer flame-retardant composite layer granules are respectively added to three extruders. Through the three-layer co-extrusion die, the inner composite layer, the middle pressure-bearing layer, and the outer flame-retardant composite layer are formed sequentially from the inside to the outside. After extrusion, the material is sized by vacuum and cooled by spraying. Then it is pulled and cut to obtain the finished multilayer polyethylene pipe.

[0072] Comparative Example 2

[0073] The only difference between this comparative example and Example 1 is that a boron-containing polysiloxane ceramic precursor polymer is added to the outer flame-retardant composite layer, but no polyfunctional olefin crosslinking agent is added; the other outer layer components and their amounts remain unchanged. The formulation of the outer flame-retardant composite layer, based on 100 parts by weight of the polyethylene matrix, is: 90 parts magnesium hydroxide, 25 parts modified glass powder, 3.0 parts boron-containing polysiloxane ceramic precursor polymer, 5.0 parts maleic anhydride-grafted polyethylene, 0.30 parts antioxidant, and 0.50 parts lubricant.

[0074] The preparation method is as follows:

[0075] S1: 90 parts of magnesium hydroxide and 3.0 parts of boron-containing polysiloxane ceramic precursor polymer are premixed in a high-speed mixer for 6 minutes and then fed into a co-rotating twin-screw extruder for the first stage of melt blending. The temperature settings of each zone of the co-rotating twin-screw extruder are 170℃, 180℃, 185℃, 185℃, and 180℃ respectively, the screw speed is 300 rpm, and the material residence time in the machine is 7 minutes. Negative pressure exhaust is set in the second to fourth temperature zones, and the exhaust pressure is -0.085MPa. The discharged material is water-cooled, drawn into strips, and granulated to obtain pretreated filler masterbatch.

[0076] S2: The pretreated filler masterbatch obtained in step S1 is added to the main feed port of a co-rotating twin-screw extruder according to the outer layer formula. At the same time, 100 parts of polyethylene matrix, 5.0 parts of maleic anhydride grafted polyethylene, 0.30 parts of antioxidant and 0.50 parts of lubricant are added from the main feed port for the second stage of melt blending. The temperatures of each temperature zone are set to 165℃, 175℃, 180℃, 185℃, 185℃ and 180℃ respectively. The screw speed is 240 rpm and the material residence time in the machine is 5 minutes. Negative pressure exhaust is set in the third temperature zone with an exhaust pressure of -0.070MPa to obtain the melt material section without glass powder.

[0077] S3: Based on the molten material in step S2, add 25 parts of modified glass powder through the side feed port, which is located between the fourth and fifth temperature zones. After adding the glass powder, adjust the screw speed to 180 rpm and continue to melt and blend for 2 minutes. Discharge the material, water-cool it into strips, and cut it into pellets to obtain the outer flame-retardant composite layer pellets.

[0078] S4: Perform three-layer co-extrusion molding, sizing and cooling, traction and cutting as described in Comparative Example 1 to obtain multi-layer polyethylene pipe finished product.

[0079] Comparative Example 3

[0080] The only difference between this comparative example and Example 1 is that the outer flame-retardant composite layer does not contain boron-containing polysiloxane ceramic precursor polymer, but instead contains boron-free silicone resin. A multifunctional olefin crosslinking agent is still added, while the other outer layer components and their amounts remain unchanged. The boron-free silicone resin used is Wacker Chemie's methylphenyl silicone resin product, model SILRES® REN 50. The formulation of the outer flame-retardant composite layer, based on 100 parts by weight of the polyethylene matrix, is: 90 parts magnesium hydroxide, 25 parts modified glass powder, 3.0 parts boron-free silicone resin, 1.0 part multifunctional olefin crosslinking agent, 5.0 parts maleic anhydride-grafted polyethylene, 0.30 parts antioxidant, and 0.50 parts lubricant.

[0081] The preparation method is as follows:

[0082] S1: 90 parts of magnesium hydroxide, 3.0 parts of boron-free silicone resin, and 1.0 part of polyfunctional olefin crosslinking agent are premixed in a high-speed mixer for 6 minutes and then fed into a co-rotating twin-screw extruder for the first stage of melt blending. The temperature settings of each zone of the co-rotating twin-screw extruder are 170℃, 180℃, 185℃, 185℃, and 180℃ respectively, the screw speed is 300 rpm, and the material residence time in the machine is 7 minutes. Negative pressure exhaust is set in the second to fourth temperature zones, and the exhaust pressure is -0.085MPa. The discharged material is water-cooled, drawn into strips, and granulated to obtain pretreated filler masterbatch.

[0083] S2: The pretreated filler masterbatch obtained in step S1 is added to the main feed port of a co-rotating twin-screw extruder according to the outer layer formula. At the same time, 100 parts of polyethylene matrix, 5.0 parts of maleic anhydride grafted polyethylene, 0.30 parts of antioxidant and 0.50 parts of lubricant are added from the main feed port for the second stage of melt blending. The temperatures of each temperature zone are set to 165℃, 175℃, 180℃, 185℃, 185℃ and 180℃ respectively. The screw speed is 240 rpm and the material residence time in the machine is 5 minutes. Negative pressure exhaust is set in the third temperature zone with an exhaust pressure of -0.070MPa to obtain the melt material section without glass powder.

[0084] S3: Based on the molten material in step S2, add 25 parts of modified glass powder through the side feed port, which is located between the fourth and fifth temperature zones. After adding the glass powder, adjust the screw speed to 180 rpm and continue to melt and blend for 2 minutes. Discharge the material, water-cool it into strips, and cut it into pellets to obtain the outer flame-retardant composite layer pellets.

[0085] S4: Perform three-layer co-extrusion molding, sizing and cooling, traction and cutting as described in Comparative Example 1 to obtain multi-layer polyethylene pipe finished product.

[0086] Performance testing

[0087] 1. Limiting oxygen index (LOI) test: Samples were prepared from the outer flame-retardant composite layer granules using a flat vulcanizing machine after hot pressing to form the outer layer into sheets. The samples were conditioned according to GB / T 2918-2018 before testing. The oxygen index was determined according to GB / T2406.2-2009, measuring the minimum oxygen volume fraction required to sustain combustion in an oxygen-nitrogen mixed atmosphere. Sample dimensions were 80 mm × 10 mm × 4 mm, with 5 samples per group, and the median value was used.

[0088] 2. Cone calorimetry tests were conducted on samples taken from the outer flame-retardant composite laminate. The sample dimensions were 100 mm × 100 mm × 3 mm, and the samples were conditioned according to GB / T 2918-2018 before testing. Cone calorimetry was performed according to GB / T 16172-2007, with an irradiance of 50 kW / m². 2 The device was placed horizontally and ignited externally. The duration of continuous flaming combustion, peak heat release rate, total heat release, total smoke generation, and residual rate at the end of the test were recorded.

[0089] 3. The internal pressure resistance test of the pipes shall be conducted using a constant internal water pressure test on pipe sections of the same specification. The medium shall be water, and the test temperature shall be 80 ℃. The test conditions and judgment shall be performed in accordance with GB / T 6111-2018. Three pipe sections shall be used in each group, and the failure time shall be recorded. If the pipes fail to fail by the set termination time, they shall be deemed to have passed, and the termination time shall be recorded.

[0090] 4. Mortar abrasion test targets the wear resistance contribution of the inner wall layer of the pipe. Ring-shaped specimens are cut from the inner wall of the pipe and processed into arc-shaped pieces of specified dimensions. The abrasive used is a mortar system prepared with quartz sand and water. The test apparatus and operating conditions are set up according to the mortar abrasion rate method in the appendix of CJ / T323-2010. The mass difference before and after the test is measured and converted to the abrasion amount per unit area. The results are expressed in mg / cm³. 2 This indicates the difference in wear under sand-bearing erosion conditions.

[0091] The test results are shown in Tables 1 and 2.

[0092] Table 1 Limiting oxygen index and cone calorimetry data of the outer flame-retardant composite layer

[0093]

[0094] Table 2 Data on internal pressure resistance and abrasion of inner wall mortar for pipes

[0095]

[0096] Conclusion: As shown in Table 1, the outer flame-retardant composite layer, after simultaneously introducing boron-containing polysiloxane ceramic precursor polymer and multifunctional olefin crosslinking agent, and employing a three-stage melt blending process followed by the side-feeding of glass powder in the later stage, exhibits a relatively high limiting oxygen index. Furthermore, the peak heat release rate and total heat release under cone calorimetry are significantly lower than those of comparatives 1–3 lacking key components. This result indicates that the outer flame-retardant system not only relies on the endothermic decomposition of magnesium hydroxide but also forms a more stable interfacial structure within the melt blending thermal history through precursor and crosslinking triggering. This allows the glass powder, added later, to more effectively fill gaps and provide shielding, thereby more effectively suppressing the heat release process with the same or similar filler dosage.

[0097] Examples 2 and 3 maintain the same system structure and process. Even with single-variable substitutions only in the precursor or crosslinking triggering component, the flame retardant index is still superior to the comparative group, but shows a decline to varying degrees compared to Example 1. This indicates that the outer layer system is sensitive to the precursor structure and crosslinking triggering method. The combination of polyborosiloxane and divinylbenzene is more conducive to forming a repeatable interface state during processing, resulting in more stable combustion suppression performance. In Example 4, reducing the total amount of glass powder coating layer leads to a decrease in the flame retardant index, reflecting the substantial influence of the glass powder surface state on its dispersion and participation in the shielding structure formation during the short-term mixing process. In Example 5, moving the glass powder addition position forward and increasing the mixing intensity in the later stage further reduces the flame retardant index, indicating that the later-stage side-feeding, short-time, and low-speed settings are more conducive to preventing glass powder agglomeration or surface state deterioration under long thermal histories and high shear, thus more fully utilizing the viscous flow and gap-filling effect of low-softening-point glass powder.

[0098] As shown in Table 2, all groups of pipes reached the termination time without damage under constant internal water pressure at 80 ℃, indicating that changes in the outer flame-retardant system did not weaken the internal pressure resistance reliability provided by the middle pressure-bearing layer. The three-layer structure effectively isolates the pressure-bearing function from the flame-retardant filling system. The wear of the inner wall mortar showed very little difference among the groups, indicating that the wear resistance contribution provided by the inner composite layer remained stable and did not fluctuate significantly due to changes in the outer flame-retardant system. Combining Tables 1 and 2, it can be concluded that the multi-layer structure, coupled with the outer layer formulation and process, ensures that the improvement in flame-retardant performance is mainly achieved by the outer layer, while maintaining the stability of pressure resistance and inner wall wear resistance. Furthermore, it exhibits a clear window dependence on the glass powder surface treatment and the subsequent side-feeding process.

[0099] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A flame-retardant and wear-resistant multilayer polyethylene pipe, characterized in that, The pipe consists of an inner composite layer, a middle pressure-bearing layer, and an outer flame-retardant composite layer from the inside out. The outer flame-retardant composite layer comprises, based on 100 parts by weight of polyethylene matrix, 70-110 parts of magnesium hydroxide, 10-40 parts of glass powder, 0.5-6 parts of boron-containing polysiloxane ceramic precursor polymer, 0.2-3 parts of polyfunctional olefin crosslinking agent, 2-8 parts of maleic anhydride-grafted polyethylene, 0.20-0.50 parts of antioxidant, and 0.20-0.80 parts of lubricant; The glass powder is selected from low melting point glass powder D235; The glass powder undergoes a composite surface treatment to form a coating layer, which includes an inorganic base coating and a pre-ceramicized outer coating. The glass powder surface is sequentially formed with a zirconium phosphate inorganic base coating and a pre-ceramicized outer coating formed by curing polysilazane. The mass of the inorganic base coating is 0.1 to 1.0% of the mass of the glass powder, and the mass of the pre-ceramicized outer coating is 0.2 to 2.0% of the mass of the glass powder.

2. The flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 1, characterized in that, The boron-containing polysiloxane ceramic precursor polymer is polyborosiloxane.

3. The flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 1, characterized in that, The multifunctional olefin crosslinking agent is divinylbenzene.

4. The flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 1, characterized in that, The mass of the coating layer is 0.3 to 3.0% of the mass of the glass powder.

5. A method for preparing a flame-retardant and wear-resistant multilayer polyethylene pipe according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1: Magnesium hydroxide, boron-containing polysiloxane ceramic precursor polymer and multifunctional olefin crosslinking agent are mixed and then melt-blended in the first stage. During the first stage melt-blending process, negative pressure is vented to obtain pretreated filler masterbatch. S2. Polyethylene matrix and maleic anhydride-grafted polyethylene are added to the pretreated filler masterbatch obtained in step S1 for a second stage of melt blending. S3. After step S2, glass powder is added for the third stage of melt blending and granulation to obtain the outer flame-retardant composite layer granules. S4. The inner composite layer granules, the middle pressure-bearing layer granules and the outer flame-retardant composite layer granules are co-extruded, sized and cooled to obtain the pipe.

6. The method for preparing the flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 5, characterized in that, The first stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperature settings of each temperature zone of the twin-screw extruder are 170℃, 180℃, 185℃, 185℃ and 180℃ respectively, the screw speed is 300 rpm, the material residence time in the machine is 7 minutes, and negative pressure exhaust is set in the second to fourth temperature zones, with an exhaust pressure of -0.085MPa.

7. The method for preparing the flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 5, characterized in that, The second stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperature settings of each temperature zone of the twin-screw extruder are 165℃, 175℃, 180℃, 185℃, 185℃, and 180℃ respectively. The screw speed is 240 rpm, the material residence time in the machine is 5 minutes, and a negative pressure exhaust is set in the third temperature zone with an exhaust pressure of -0.070MPa.

8. The method for preparing flame-retardant and wear-resistant multilayer polyethylene pipe according to claim 5, characterized in that, The third stage of melt blending is carried out in a co-rotating twin-screw extruder, and the temperature settings of each temperature zone of the twin-screw extruder are 165℃, 175℃, 180℃, 185℃, 185℃, and 180℃ respectively. Glass powder is added to the molten material in step S2 through the side feed port. The glass powder is added between the fourth and fifth temperature zones. After the glass powder is added, melt blending continues for 2 minutes, and the screw speed is 180 rpm.

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