A precise temperature control forming process for hot melt connection of a regenerated base super-large caliber HDPE pipe

Abstract

This invention discloses a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes in the field of polymer pipe processing technology. After milling and cleaning the pipe end face, a stable interface layer is prepared by melt extrusion using recycled high-density polyethylene granules, pipe granules, maleic anhydride-grafted high-density polyethylene, black masterbatch, antioxidants, and calcium-magnesium-boron-silicon thermally conductive and stable micro-powder, and then adhered to the pipe end face. The joint is then obtained by heating the plate of a PE pipe hot-melt butt welding machine, moving the plate for butt welding, applying pressure for fusion, and cooling for shaping. The calcium-magnesium-boron-silicon thermally conductive and stable micro-powder is prepared through borosilicate activated sol, calcium-magnesium composite deposition, drying and calcination, and surface modification. This invention can improve the melt stability, temperature control uniformity, and joint forming quality of recycled-based pipe end faces.

Description

Technical Field

[0001] This invention belongs to the field of polymer pipe processing technology, specifically relating to a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes. Background Technology

[0002] With the continuous expansion of municipal water supply and drainage, industrial transportation, farmland irrigation, sponge city construction, and underground integrated pipe network construction, high-density polyethylene (HDPE) pipes have become an important choice for large-diameter plastic pipes due to their advantages such as corrosion resistance, light weight, convenient construction, smooth inner wall, good flexibility, and strong integrity of heat fusion connection. Especially in ultra-large diameter pipe applications, pipe connections not only need to withstand the combined effects of long-term internal pressure, external soil pressure, temperature changes, and foundation settlement, but also need to maintain stable and reliable joint quality under on-site construction conditions. Heat fusion connection is a commonly used connection method for HDPE pipes. It involves heating, melting, butt-jointing, and cooling the pipe ends to form a continuous joint, theoretically achieving an integral connection effect similar to that of the parent material. However, ultra-large diameter pipes have large end face areas, high wall thickness, and uneven circumferential temperature distribution. Differences in heat transfer between the inner and outer walls of the pipe ends can easily occur, leading to insufficient local melting, uneven flanges, inadequate interface fusion, and inconsistent cooling shrinkage, thus affecting the long-term service safety of the joint.

[0003] In recent years, with the increasing demands for resource recycling and green manufacturing, recycled high-density polyethylene (HDPE) pipes have gained growing attention. By rationally utilizing recycled HDPE granules in pipe raw materials, raw material costs can be reduced, plastic waste emissions decreased, and the resource utilization level of the pipe industry improved. However, the source, degree of thermo-oxidative aging, molecular weight distribution, residual impurity content, and melt flow stability of recycled granules often fluctuate. When processed together with virgin pipe-grade materials, the melting response at the pipe ends during hot-melt joining is not easily kept completely consistent. Especially in the hot-melt joining of ultra-large diameter pipes, the pipe ends are heated for a long time, and the heat-affected zone is wide. This easily amplifies local viscoelastic differences in the recycled material, leading to problems such as uneven melt layer thickness, unstable melt flow, insufficient interfacial chain segment diffusion, and stress concentration in the weld area. Therefore, relying solely on conventional pipe end milling, hot plate heating, and pressure fusion processes is insufficient to fully meet the requirements for precise temperature control and stable molding of recycled ultra-large diameter pipes.

[0004] In existing technologies, to improve the quality of hot-melt connections of high-density polyethylene (HDPE) pipes, process control is typically implemented by adjusting hot plate temperature, heat absorption time, butt joint pressure, and cooling time. Some solutions also aim to improve joint performance by modifying pipe formulations or enhancing the precision of construction equipment. However, for recycled-based ultra-large diameter pipes, simply increasing the hot plate temperature can easily lead to localized overmelting, oxidation, or abnormal edge flanging at the end face. Simply extending the heat absorption time increases the construction cycle and exacerbates the impact of material thermal history. Furthermore, conventional pipe end interfaces lack an auxiliary layer that can simultaneously ensure uniform heat conduction, melt stability, and polyethylene compatibility, making it difficult to stably form a uniform molten interface under fluctuating conditions of recycled-based materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes.

[0006] In a first aspect, the present invention provides a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes, comprising the following steps: S1. By weight, place the recycled ultra-large diameter HDPE pipe in the clamp of a hydraulic butt welding machine, mill the two pipe ends, and then wipe the end faces and inner and outer walls of the two pipe ends with anhydrous ethanol; mix 55-75 parts of recycled HDPE granules, 25-45 parts of PE100 grade HDPE pipe granules, 2-5 parts of maleic anhydride grafted high-density polyethylene, 1-3 parts of HDPE pipe grade black masterbatch, 0.10-0.30 parts of antioxidant 1010, 0.10-0.30 parts of antioxidant 168 and 0.6-2.5 parts of calcium magnesium borosilicate thermally conductive and saturated micro powder, and melt extrude at 170-205℃ to obtain HDPE saturated sheet; cut the HDPE saturated sheet to obtain a saturated interface layer; attach the saturated interface layer to the end faces of the two pipe ends to obtain two pretreated pipe ends; S2. Preheat the heating plate of the PE pipe hot melt butt welding machine to 210-225℃, place the heating plate between the two pre-treated pipe ends, so that the pre-treated pipe ends contact the heating plate to form an initial flange; then reduce the clamping force and continue to absorb heat to obtain two molten pipe ends. S3. Remove the heating plate of the PE pipe hot melt butt welding machine from between the two molten pipe ends, butt the two molten pipe ends together, apply pressure to fuse them together, and obtain a fusion joint; S4. Cool the fusion joint while it is fixed in the fixture; then remove the fixture and let it cool to 40-50℃.

[0007] In this invention, the mechanism of the precise temperature-controlled molding process for hot-melt connection of recycled ultra-large diameter HDPE pipes is that after the recycled ultra-large diameter HDPE pipes are fixed by the hydraulic butt welding machine clamp, the oxide layer, contaminants and uneven areas on the end faces of the two pipes are removed by milling, and then the end faces and inner and outer walls of the two pipes are wiped with anhydrous ethanol to keep the interface to be connected clean and flat. HDPE recycled granules, PE100 grade HDPE pipe granules, maleic anhydride-grafted high-density polyethylene, HDPE pipe-grade black masterbatch, antioxidant 1010, antioxidant 168, and calcium magnesium borosilicate thermally conductive and melt-stabilizing micropowder form a continuous melt during melt extrusion. The HDPE recycled granules and PE100 grade HDPE pipe granules constitute the main forming phase. Maleic anhydride-grafted high-density polyethylene improves the interfacial bonding between the calcium magnesium borosilicate thermally conductive and melt-stabilizing micropowder and the main forming phase. The HDPE pipe-grade black masterbatch enhances the material's light-shielding and weather-resistant properties. Antioxidants 1010 and 168 inhibit oxidative degradation during hot working. The calcium magnesium borosilicate thermally conductive and melt-stabilizing micropowder is dispersed in the melt, improving the thermal uniformity and melt stability of the HDPE melt-stabilizing sheet. After the HDPE melt-stabilizing sheet is cut to form a melt-stabilizing interface layer and adhered to the end faces of two pipes, a transition layer compatible with the pipe matrix is ​​formed between the two pipe ends. When the heating plate of the PE pipe hot-melt butt welding machine contacts the two pre-treated pipe ends, the surface layer and the stabilizing interface layer of the two pipe ends soften and melt upon heating. The initial flanging removes residual micro-defects and local impurities from the end face, and continued heat absorption makes the thickness and flow state of the molten layer tend to be uniform. After the heating plate of the PE pipe hot-melt butt welding machine is removed, the two molten pipe ends contact under pressure fusion. The molten chain segments diffuse, entangle, and fuse with each other. The stabilizing interface layer participates in the melt flow and becomes a continuous component of the fusion joint. When the fusion joint is cooled under the fixation of the fixture, the molten zone gradually crystallizes and solidifies. Calcium magnesium borosilicate thermally conductive and stabilizing micro-powder mitigates local temperature differences, and maleic anhydride-grafted high-density polyethylene enhances the interface continuity, enabling the fusion joint to form a stable connection structure.

[0008] According to a preferred embodiment of the present invention, in step S1, the melt extrusion time at 170-205°C is 3-8 minutes.

[0009] According to a preferred embodiment of the present invention, in step S2, the heat absorption time continues for 60-120 minutes.

[0010] In step S3 of the preferred embodiment of the present invention, the pressure fusion time is 20-45 min.

[0011] According to a preferred embodiment of the present invention, in step S4, the cooling time is 30-60 minutes.

[0012] According to a preferred embodiment of the present invention, the preparation steps of the calcium-magnesium-boron-silicon thermally conductive and melt-stabilized micro powder include: A1. By weight, add 80-120 parts of deionized water to the reactor and stir at 25-35℃. Add 18-30 parts of liquid sodium silicate, 4-8 parts of boric acid, and 1.5-3.5 parts of sodium tetraborate decahydrate and continue stirring. Then adjust the pH to 8.8-9.5, raise the temperature to 55-65℃, add 0.10-0.30 parts of sodium dodecylbenzenesulfonate, and stir to obtain borosilicate activated sol. A2. Add 45-65 parts of light calcium carbonate, 12-24 parts of light magnesium hydroxide powder, and 0.3-0.8 parts of sodium lignosulfonate to 90-130 parts of deionized water, stir, and obtain a suspension slurry; add borosilicate activated sol dropwise to the suspension slurry, react at 60-75℃, and adjust the pH to 8.8-9.5; filter, wash with deionized water, and obtain a wet calcium-magnesium-boron-silicon composite precursor; A3. The wet calcium-magnesium-borosilicate composite precursor is dried at 105-115℃ to obtain the dried product; the dried product is heated to 320-360℃ for treatment, and then calcined at 540-620℃; after natural cooling, it is ball-milled and sieved to obtain calcium-magnesium-borosilicate multiphase micro powder. A4. Add 90-110 parts of calcium magnesium borosilicate multiphase micro powder to a mixer and preheat at 80-95℃; mix 25-40 parts of anhydrous ethanol, 0.8-1.8 parts of isopropyl distearate aluminate, 0.6-1.5 parts of stearic acid and 0.05-0.15 parts of antioxidant 1010 to obtain a surface treatment solution; add the surface treatment solution to a mixer, then add 2.0-5.0 parts of maleic anhydride grafted polyethylene wax, heat to 120-135℃ and mix, cool, pulverize and sieve.

[0013] In this invention, the formation mechanism of the calcium-magnesium-borosilicate thermally conductive and fusibly stable micro-powder lies in the following: Liquid sodium silicate is added to deionized water to form a soluble silicate system. Boric acid and sodium tetraborate decahydrate together form a buffer environment, allowing the silicate system to gradually condense and form a borosilicate-containing activated sol with a borosilicate-linked structure under mild alkaline conditions. Sodium dodecylbenzenesulfonate is distributed in the borosilicate-activated sol, improving the wetting and dispersion state of the system formed by liquid sodium silicate, boric acid, and sodium tetraborate decahydrate, reducing local agglomeration, and maintaining a relatively uniform flow and deposition state in the borosilicate-activated sol. Light calcium carbonate, light magnesium hydroxide powder, and sodium lignosulfonate are added to deionized water to form a suspension slurry. Sodium lignosulfonate is adsorbed onto the surface of the light calcium carbonate and light magnesium hydroxide powder, keeping the particles dispersed in the aqueous phase. After the borosilicate activated sol is added dropwise to the suspension slurry, the buffer environment formed by boric acid and sodium tetraborate decahydrate can reduce the alkalinity fluctuations brought by the light magnesium hydroxide powder. This allows the borosilicate structure to deposit, adsorb, and continue to condense on the surface of the light calcium carbonate and light magnesium hydroxide powder, forming a wet calcium-magnesium-boron-silicon composite precursor. After drying to remove free and adsorbed water, the wet calcium-magnesium-boron-silicon composite precursor undergoes heat treatment and calcination, further solidifying the borosilicate structure. The light magnesium hydroxide powder undergoes dehydration transformation and, together with the light calcium carbonate and borosilicate structure, forms a calcium-magnesium-boron-silicon multiphase micro powder. After ball milling and sieving, the calcium-magnesium-borosilicate (CMP) multiphase micro powder has a more uniform particle distribution. Anhydrous ethanol carries isopropyl distearate, stearic acid, and antioxidant 1010 to the surface of the CMP multiphase micro powder. Isopropyl distearate binds to the active sites on the particle surface, and stearic acid synergistically forms a hydrophobic organic layer. Maleic anhydride-grafted polyethylene wax softens and covers the particle surface during heating and mixing, transforming the CMP multiphase micro powder from a hydrophilic inorganic surface to an interface state suitable for the dispersion of HDPE recycled particles and PE100 grade HDPE pipe particles in the melt, ultimately yielding CMP thermally conductive and melt-stable micro powder.

[0014] According to a preferred embodiment of the present invention, in step A1, the reaction time at 55-65°C is 30-60 minutes.

[0015] According to a preferred embodiment of the present invention, in step A2, the reaction time at 60-75°C is 2-4 hours.

[0016] According to a preferred embodiment of the present invention, in step A3, the heating time to 320-360°C is 1-2 hours, and the calcination time to 540-620°C is 2-4 hours.

[0017] According to a preferred embodiment of the present invention, in step A4, the mixing time at 120-135°C is 30-60 minutes.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention sets a stabilizing interface layer before hot-melt connection of recycled ultra-large diameter pipes, so that the two pipe ends form a transition layer with stable composition and good interfacial compatibility before being heated. The stabilizing interface layer is made by melt extrusion of recycled HDPE granules, PE100 grade HDPE pipe granules, maleic anhydride grafted high-density polyethylene, HDPE pipe grade black masterbatch, antioxidant 1010, antioxidant 168 and calcium magnesium borosilicate thermally conductive stabilizing micro powder. It can mitigate the influence of batch differences of recycled materials on the melting state of the pipe ends, improve the continuity of the melt and the stability of interface fusion after the end face is heated, and reduce the abnormal melting, cold joint and edge turning caused by insufficient local melting, impurity residue or uneven material flow at the end face.

[0019] (2) The calcium-magnesium-borosilicate thermally conductive and stable micro powder is prepared from liquid sodium silicate, boric acid, sodium tetraborate decahydrate, sodium dodecylbenzene sulfonate, light calcium carbonate, light magnesium hydroxide powder, sodium lignosulfonate, anhydrous ethanol, isopropyl distearate aluminate, stearic acid, antioxidant 1010, and maleic anhydride grafted polyethylene wax. Its inorganic multiphase structure can improve the heat transfer uniformity of the stable interface layer, and its surface organic modified layer can improve the dispersion and compatibility of the calcium-magnesium-borosilicate thermally conductive and stable micro powder in the pipe melt, so that the stable interface layer can transfer heat more evenly when heated by the heating plate of the PE pipe hot melt butt welding machine, reducing the risk of local overheating or local undermelting at the pipe end.

[0020] (3) In this invention, the heating plate of the PE pipe hot-melt butt welding machine absorbs heat from the two pre-treated pipe ends. After the initial flange is formed, the heat absorption continues. Then, the heating plate of the PE pipe hot-melt butt welding machine is removed, the two molten pipe ends are joined, pressure is applied for fusion, and the joint is fixed and cooled by a clamp. This allows the fusion joint to be formed under stable thermal history and pressure. This process can promote the diffusion of molten chain segments and interface entanglement, so that the stable fusion interface layer can fully participate in the formation of the fusion joint. At the same time, it reduces the local shrinkage difference during the cooling process and improves the forming consistency, weld density, long-term load-bearing stability and construction quality control of the hot-melt connection joint of recycled ultra-large diameter pipe. Detailed Implementation

[0021] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0022] Example 1 This embodiment provides a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes, including the following steps: S1. Select two recycled-based extra-large diameter HDPE pipes. Place the two recycled-based extra-large diameter HDPE pipes in the hydraulic butt welding machine fixture, adjust the two pipe ends to a coaxial state, and mill the two pipe ends until the end faces of the two pipe ends are continuous, flat, and can fit together. Then wipe the end faces and inner and outer walls of the two pipe ends with 20g of anhydrous ethanol respectively, and set aside after the anhydrous ethanol has completely evaporated. Combine 65g of recycled HDPE granules, 35g of PE100 grade HDPE pipe granules, 3.5g of maleic anhydride grafted high-density polyethylene, 2g of HDPE pipe-grade black masterbatch, and 0.20g of antioxidant. 1010, 0.20g antioxidant 168, and 1.55g calcium magnesium borosilicate thermally conductive and melt-stabilized micro powder were added to a mixer and mixed at 25℃ for 10min. Then, the mixture was added to an extruder and melt-extruded at 175℃, 185℃, 187.5℃, and 187.5℃ for 5.5min to obtain HDPE melt-stabilized sheets. The HDPE melt-stabilized sheets were cut into annular sheets that matched the end faces of the pipes to obtain melt-stabilized interface layers. After softening the melt-stabilized interface layers at 170℃ for 30s, they were bonded to the end faces of two pipes to cover the areas to be connected, resulting in two pre-treated pipe ends. S2. Preheat the heating plate of the PE pipe hot melt butt welding machine to 217.5℃ and keep the temperature stable. Then place the heating plate between the two pre-treated pipe ends so that the pre-treated pipe ends are in contact with the heating plate. Initial flanging is formed at 0.18MPa for 180s. Then reduce the clamping force to reduce the contact pressure to 0.03MPa and continue to absorb heat for 90min to obtain two molten pipe ends. S3. Within 15 seconds, remove the heating plate of the PE pipe hot melt butt welding machine from between the two molten pipe ends, butt the two molten pipe ends together, and apply pressure at 0.20MPa for 32.5 minutes to fuse and obtain a fusion joint; S4. Cool the fusion joint for 45 minutes while it is fixed in the fixture, with the ambient temperature at 25°C; then remove the fixture and allow it to cool to 45°C.

[0023] Preparation steps of calcium magnesium borosilicate thermally conductive and melt-stabilized micro powder: A1. Add 100g of deionized water to a reactor equipped with stirring, heating, and pH detection functions. Stir at 300r / min for 10min at 30℃. Add 24g of liquid sodium silicate, 5.5g of boric acid, and 2.3g of sodium tetraborate decahydrate, and continue stirring for 30min. Then, adjust the pH to 9.15 with the reserved 0.5g of boric acid and 0.2g of sodium tetraborate decahydrate, so that the total amount of boric acid is 6g and the total amount of sodium tetraborate decahydrate is 2.5g. Heat to 60℃ and react for 45min. Add 0.20g of sodium dodecylbenzenesulfonate and continue stirring at 300r / min for 20min to obtain borosilicate activated sol. A2. Add 55g of light calcium carbonate, 18g of light magnesium hydroxide powder, and 0.55g of sodium lignosulfonate to 110g of deionized water and stir at 500r / min for 30min at 30℃ to obtain a suspension slurry. Add the borosilicate activated sol obtained in A1 dropwise to the suspension slurry over 60min, maintaining a stirring speed of 500r / min during the dropwise addition. After the dropwise addition is complete, react at 67.5℃ for 3h, and stabilize the pH of the system to 9.15 through the buffering effect of the borosilicate activated sol itself. After the reaction is completed, filter the mixture through a 100μm filter cloth to obtain a filter cake. Wash the filter cake three times with 300g of deionized water, using 100g of deionized water each time, to obtain a wet calcium-magnesium-borosilicate composite precursor. A3. The wet calcium-magnesium-boron-silicon composite precursor was spread on a stainless steel tray and dried at 110°C for 12 hours to obtain the dried product. The dried product was placed in a furnace and heated to 340°C for 1.5 hours, and then heated to 580°C for calcination for 3 hours. After naturally cooling to 25°C, the calcined product was placed in a ball mill jar and ball-milled for 2 hours. After ball milling, it was sieved with a 48μm sieve to obtain calcium-magnesium-boron-silicon multiphase micro powder. A4. Add 100g of calcium magnesium borosilicate multiphase micro powder to a mixer and preheat at 87.5℃ for 30min. Mix 32.5g of anhydrous ethanol, 1.3g of isopropyl distearate aluminate, 1.05g of stearic acid and 0.10g of antioxidant 1010 and stir for 10min to obtain a surface treatment solution. Add the surface treatment solution to the mixer and mix at 600r / min for 20min to ensure that the surface treatment solution is evenly distributed on the surface of the calcium magnesium borosilicate multiphase micro powder and remove the volatilized anhydrous ethanol. Then add 3.5g of maleic anhydride grafted polyethylene wax, heat to 127.5℃ and mix for 45min. Cool to 25℃, pulverize and sieve using an 80μm sieve to obtain calcium magnesium borosilicate thermally conductive and fusibly stable micro powder.

[0024] Example 2 The difference between this embodiment and Embodiment 1 is that this embodiment provides a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes, including the following steps: S1. Place the recycled ultra-large diameter HDPE pipe in the hydraulic butt welding machine fixture, and mill the two pipe ends to make the end faces of the two pipe ends continuous and flat. Then wipe the end faces and inner and outer walls of the two pipe ends with anhydrous ethanol. After the anhydrous ethanol evaporates, set aside for later use. Mix 55g of recycled HDPE granules, 25g of PE100 grade HDPE pipe granules, 2g of maleic anhydride grafted high-density polyethylene, 1g of HDPE pipe grade black masterbatch, 0.10g of antioxidant 1010, 0.10g of antioxidant 168 and 0.6g of calcium magnesium borosilicate thermally conductive and saturated micro powder, and melt extrude at 170℃ for 3min to obtain HDPE saturated sheet. Cut the HDPE saturated sheet to obtain the saturated interface layer. Adhere the saturated interface layer to the end faces of the two pipe ends so that the saturated interface layer covers the area to be connected on the two pipe ends to obtain two pre-treated pipe ends. S2. Preheat the heating plate of the PE pipe hot melt butt welding machine to 210℃, place the heating plate between the two pre-treated pipe ends, so that the pre-treated pipe ends contact the heating plate of the PE pipe hot melt butt welding machine to form an initial flange; then reduce the clamp pushing force and continue to absorb heat for 60 minutes to obtain two molten pipe ends. S3. Remove the heating plate of the PE pipe hot melt butt welding machine from between the two molten pipe ends, butt the two molten pipe ends together, apply pressure and fuse for 20 minutes to obtain a fusion joint; S4. Cool the fusion joint for 30 minutes while it is fixed in the fixture; then remove the fixture and let it cool to 40°C.

[0025] Preparation steps of calcium magnesium borosilicate thermally conductive and melt-stabilized micro powder: A1. Add 80g of deionized water to the reactor and stir at 25°C. Add 18g of liquid sodium silicate, 4g of boric acid, and 1.5g of sodium tetraborate decahydrate. Continue stirring to ensure that the liquid sodium silicate, boric acid, and sodium tetraborate decahydrate are uniformly dispersed in the aqueous phase. Then adjust the pH to 8.8, raise the temperature to 55°C and react for 30 minutes. Add 0.10g of sodium dodecylbenzenesulfonate and continue stirring to form a uniform and stable sol state, thus obtaining borosilicate activated sol. A2. Add 45g of light calcium carbonate, 12g of light magnesium hydroxide powder, and 0.3g of sodium lignosulfonate to 90g of deionized water, stir, and fully wet and uniformly suspend the light calcium carbonate and light magnesium hydroxide powder to obtain a suspension slurry; add the borosilicate activated sol obtained in A1 to the suspension slurry, react at 60℃ for 2h, and adjust the pH to 8.8; after the reaction is completed, filter, wash the filter cake with deionized water, and obtain the wet calcium magnesium borosilicate composite precursor; A3. The wet calcium-magnesium-borosilicate composite precursor was dried at 105℃ to obtain the dried product; the dried product was heated to 320℃ for 1 hour, and then heated to 540℃ for 2 hours; after natural cooling, the calcined product was ball-milled and sieved to obtain calcium-magnesium-borosilicate multiphase micro powder. A4. Add 90g of calcium magnesium borosilicate multiphase micro powder to a mixer and preheat at 80℃ to ensure the powder is in a dry and dispersed state. Mix 25g of anhydrous ethanol, 0.8g of isopropyl distearate aluminate, 0.6g of stearic acid, and 0.05g of antioxidant 1010 to obtain a surface treatment solution. Add the surface treatment solution to the mixer to ensure it is evenly distributed on the surface of the calcium magnesium borosilicate multiphase micro powder. Then add 2.0g of maleic anhydride-grafted polyethylene wax, heat to 120℃ and mix for 30 minutes. Cool, pulverize, and sieve to obtain calcium magnesium borosilicate thermally conductive and fusibly stable micro powder.

[0026] Example 3 The difference between this embodiment and Embodiment 1 is that this embodiment provides a precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes, including the following steps: S1. Place the recycled ultra-large diameter HDPE pipe in the hydraulic butt welding machine fixture, and mill the two pipe ends to make the end faces of the two pipe ends continuous and flat. Then wipe the end faces and inner and outer walls of the two pipe ends with anhydrous ethanol. After the anhydrous ethanol evaporates, set aside for later use. Mix 75g of recycled HDPE granules, 45g of PE100 grade HDPE pipe granules, 5g of maleic anhydride grafted high-density polyethylene, 3g of HDPE pipe grade black masterbatch, 0.30g of antioxidant 1010, 0.30g of antioxidant 168 and 2.5g of calcium magnesium borosilicate thermally conductive and saturated micro powder, and melt extrude at 205℃ for 8min to obtain HDPE saturated sheet. Cut the HDPE saturated sheet to obtain the saturated interface layer. Adhere the saturated interface layer to the end faces of the two pipe ends so that the saturated interface layer covers the area to be connected on the two pipe ends to obtain two pre-treated pipe ends. S2. Preheat the heating plate of the PE pipe hot melt butt welding machine to 225°C. Place the heating plate between the two pre-treated pipe ends so that the pre-treated pipe ends contact the heating plate to form an initial flange. Then reduce the clamping force and continue to absorb heat for 120 minutes to obtain two molten pipe ends. S3. Remove the heating plate of the PE pipe hot melt butt welding machine from between the two molten pipe ends, butt the two molten pipe ends together, apply pressure and fuse for 45 minutes to obtain a fusion joint; S4. Cool the fusion joint for 60 minutes while it is fixed in the fixture; then remove the fixture and let it cool to 50°C.

[0027] Preparation steps of calcium magnesium borosilicate thermally conductive and melt-stabilized micro powder: A1. Add 120g of deionized water to the reactor and stir at 35℃. Add 30g of liquid sodium silicate, 8g of boric acid and 3.5g of sodium tetraborate decahydrate, and continue stirring to uniformly disperse the liquid sodium silicate, boric acid and sodium tetraborate decahydrate in the aqueous phase. Then adjust the pH to 9.5, raise the temperature to 65℃ and react for 60min. Add 0.30g of sodium dodecylbenzenesulfonate and continue stirring to form a uniform and stable sol state, thus obtaining borosilicate activated sol. A2. Add 65g of light calcium carbonate, 24g of light magnesium hydroxide powder, and 0.8g of sodium lignosulfonate to 130g of deionized water, stir, and fully wet and uniformly suspend the light calcium carbonate and light magnesium hydroxide powder to obtain a suspension slurry; add the borosilicate activated sol obtained in A1 dropwise to the suspension slurry, react at 75℃ for 4h, and adjust the pH to 9.5; after the reaction is completed, filter, wash the filter cake with deionized water, and obtain the wet calcium magnesium borosilicate composite precursor; A3. The wet calcium-magnesium-borosilicate composite precursor was dried at 115℃ to obtain the dried product; the dried product was heated to 360℃ for 2 hours, and then calcined at 620℃ for 4 hours; after natural cooling, the calcined product was ball-milled and sieved to obtain calcium-magnesium-borosilicate multiphase micro powder. A4. Add 110g of calcium magnesium borosilicate multiphase micro powder to a mixer and preheat at 95℃ to ensure the powder is in a dry and dispersed state. Mix 40g of anhydrous ethanol, 1.8g of isopropyl distearate aluminate, 1.5g of stearic acid, and 0.15g of antioxidant 1010 to obtain a surface treatment solution. Add the surface treatment solution to the mixer to ensure it is evenly distributed on the surface of the calcium magnesium borosilicate multiphase micro powder. Then add 5.0g of maleic anhydride-grafted polyethylene wax, heat to 135℃, mix for 60min, cool, pulverize, and sieve to obtain calcium magnesium borosilicate thermally conductive and fusibly stable micro powder.

[0028] Comparative Example 1 The difference between this comparative example and Example 1 is that calcium magnesium borosilicate thermally conductive and susceptible micro powder is not added in S1, and the amount of HDPE recycled particles is adjusted from 65g to 66.55g. The rest is the same as in Example 1.

[0029] Comparative Example 2 The difference between this comparative example and Example 1 is that calcium magnesium borosilicate multiphase micro powder without surface treatment in step A4 is used to replace calcium magnesium borosilicate thermally conductive and fusibly stable micro powder. The calcium magnesium borosilicate multiphase micro powder is prepared according to A1-A3 in Example 1, without the A4 step treatment, and the rest is the same as in Example 1.

[0030] Comparative Example 3 The difference between this comparative example and Example 1 is that maleic anhydride-grafted high-density polyethylene is not added in S1, and the amount of PE100 grade HDPE pipe granules is adjusted from 35g to 38.5g. The rest is the same as in Example 1.

[0031] A series of standardized tests were conducted on the precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes described in Examples 1-3 and Comparative Examples 1-3.

[0032] The recycled-based ultra-large diameter HDPE pipe hot-melt connection joints prepared in Examples 1-3 and Comparative Examples 1-3 were placed in an environment of 23°C and 50% relative humidity for 24 hours before being tested.

[0033] During the end-face heat absorption temperature difference test, before the heating plate of the PE pipe hot melt butt welding machine finishes absorbing heat, eight measurement positions are evenly selected in the circumferential direction of the joint. The surface temperature near the outer wall, the middle and the inner wall of each position are measured. A total of 24 temperature values ​​are recorded for each joint. After removing abnormal values ​​that are obviously caused by poor probe contact, the difference between the highest and lowest temperatures is taken as the end-face heat absorption temperature difference. The smaller the value, the more uniform the heat absorption at the pipe end.

[0034] During the flange height difference test, 12 positions are evenly selected in the circumferential direction of the outer flange of the fusion joint. The outer flange height is measured with a vernier caliper with a resolution of 0.01mm. Each position is measured 3 times and the average value is taken. Then, the maximum value of the average value of the 12 positions is subtracted from the minimum value to obtain the flange height difference. The smaller the value, the more uniform the melt flow and butt joint formation.

[0035] During the weld tensile strength test, a strip specimen spanning the center of the weld is cut along the axial direction of the fusion joint. Five specimens are prepared for each group. The middle part of the specimen contains a complete fusion interface. After the specimen is placed in an environment of 23°C for 4 hours, it is clamped in a tensile testing machine and stretched at a speed of 50 mm / min until it breaks. The maximum tensile stress of each specimen is recorded and the average value is taken to obtain the weld tensile strength.

[0036] During the joint strength retention rate test, a base material sample was cut from the same batch of unwelded recycled ultra-large diameter HDPE pipes. The tensile strength of the base material was measured under the same conditions as the tensile strength of the weld. The tensile strength of the weld was divided by the tensile strength of the base material and then multiplied by 100% to obtain the joint strength retention rate. The higher the value, the closer the fusion joint is to the strength of the base material.

[0037] During the weld defect area ratio test, a weld section sample is cut from the circumference of the fusion joint. The section is then ground and polished until the weld area is clearly visible. The weld area is observed under a stereomicroscope, and the defect areas corresponding to non-fusion white lines, pores, inclusion enrichment, and interface cracks are counted. The defect area is divided by the total observed area of ​​the weld and then multiplied by 100% to obtain the weld defect area ratio. The smaller the value, the better the weld compactness.

[0038] During the hydrostatic holding time test, both ends of the fusion joint are sealed, the inside of the joint is filled with water and the air is expelled, and then it is placed in a 23°C water bath. A pressure of 1.6MPa is applied to the inside of the pipe and the pressure is maintained continuously. The holding time before the joint leaks, bulges, cracks or breaks is recorded. The upper limit of the test is 1000h. The sample that has not broken after 1000h is recorded as 1000h.

[0039] The performance test data above are shown in Table 1.

[0040] Table 1 Performance Test Results

[0041] As can be seen from the above, the end-face heat absorption temperature difference of Examples 1-3 is 3.1-4.0°C, which is significantly lower than that of Comparative Example 1 (8.7°C), Comparative Example 2 (6.5°C), and Comparative Example 3 (4.8°C). This indicates that the calcium magnesium borosilicate thermally conductive and susceptible micro powder, its surface compatibility structure, and maleic anhydride-grafted high-density polyethylene together improve the problem of uneven heating at the pipe end of the recycled ultra-large diameter HDPE pipe, making the molten layer at the pipe end more uniform during the heating process of the PE pipe hot melt butt welding machine heating plate.

[0042] The difference in flange height in Examples 1-3 was 0.42-0.56 mm, which was significantly lower than that in Comparative Example 1 (1.35 mm), Comparative Example 2 (1.02 mm), and Comparative Example 3 (0.88 mm). This indicates that the present invention can alleviate the problems of discontinuous flange and asymmetrical forming caused by the flow fluctuation of recycled material melt.

[0043] The weld tensile strength of Examples 1-3 was 24.9-25.8 MPa, and the joint strength retention rate was 93.1-96.3%, which were significantly higher than those of Comparative Example 1 (22.1 MPa and 82.5%), Comparative Example 2 (23.0 MPa and 85.8%), and Comparative Example 3 (22.6 MPa and 84.3%). This indicates that the calcium magnesium borosilicate thermally conductive and susceptible micropowder improved the uniformity of heat transfer. The surface-treated calcium magnesium borosilicate thermally conductive and susceptible micropowder improved its dispersion and interfacial bonding effect in polyethylene melt. Maleic anhydride-grafted high-density polyethylene further improved the compatibility between HDPE recycled particles, PE100 grade HDPE pipe particles and inorganic micropowder, thereby promoting molten chain segment diffusion and weld interface fusion.

[0044] The weld defect area ratio of Examples 1-3 was only 0.18-0.26%, which is much lower than that of Comparative Example 1 (1.12%), Comparative Example 2 (0.84%) and Comparative Example 3 (0.73%), indicating that the present invention reduces defects such as incomplete fusion lines, voids, inclusion enrichment and interface cracks.

[0045] The hydrostatic holding time of Examples 1-3 all reached 1000h, while that of Comparative Examples 1-3 was only 612h, 728h and 695h respectively, indicating that the fusion joint prepared by the present invention has better long-term pressure resistance stability.

[0046] In summary, Examples 1-3, compared with Comparative Examples 1-3, solved the technical problems of uneven end-face temperature distribution, insufficient stability of the molten layer, uneven flanging, numerous weld defects, low joint strength retention rate, and insufficient long-term pressure-bearing reliability in the hot-melt connection of recycled ultra-large diameter HDPE pipes.

Claims

1. A precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes, characterized in that, Includes the following steps: S1. By weight, place the recycled ultra-large diameter HDPE pipe in the clamp of a hydraulic butt welding machine, mill the two pipe ends, and then wipe the end faces and inner and outer walls of the two pipe ends with anhydrous ethanol; mix 55-75 parts of recycled HDPE granules, 25-45 parts of PE100 grade HDPE pipe granules, 2-5 parts of maleic anhydride grafted high-density polyethylene, 1-3 parts of HDPE pipe grade black masterbatch, 0.10-0.30 parts of antioxidant 1010, 0.10-0.30 parts of antioxidant 168 and 0.6-2.5 parts of calcium magnesium borosilicate thermally conductive and saturated micro powder, and melt extrude at 170-205℃ to obtain HDPE saturated sheet; cut the HDPE saturated sheet to obtain a saturated interface layer; attach the saturated interface layer to the end faces of the two pipe ends to obtain two pretreated pipe ends; S2. Preheat the heating plate of the PE pipe hot melt butt welding machine to 210-225℃, place the heating plate between the two pre-treated pipe ends, so that the pre-treated pipe ends contact the heating plate to form an initial flange; then reduce the clamping force and continue to absorb heat to obtain two molten pipe ends. S3. Remove the heating plate of the PE pipe hot melt butt welding machine from between the two molten pipe ends, butt the two molten pipe ends together, apply pressure to fuse them together, and obtain a fusion joint; S4. Cool the fusion joint while it is fixed in the fixture; then remove the fixture and let it cool to 40-50℃.

2. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 1, characterized in that, In step S1, the melt extrusion time at 170-205℃ is 3-8 minutes.

3. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 1, characterized in that, In step S2, the heat absorption time continues for 60-120 minutes.

4. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 1, characterized in that, In step S3, the pressure fusion time is 20-45 minutes.

5. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 1, characterized in that, In step S4, the cooling time is 30-60 minutes.

6. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to any one of claims 1-5, characterized in that, The preparation steps of the calcium-magnesium-borosilicate thermally conductive and melt-stabilized micro powder include: A1. By weight, add 80-120 parts of deionized water to the reactor and stir at 25-35℃. Add 18-30 parts of liquid sodium silicate, 4-8 parts of boric acid, and 1.5-3.5 parts of sodium tetraborate decahydrate and continue stirring. Then adjust the pH to 8.8-9.5, raise the temperature to 55-65℃, add 0.10-0.30 parts of sodium dodecylbenzenesulfonate, and stir to obtain borosilicate activated sol. A2. Add 45-65 parts of light calcium carbonate, 12-24 parts of light magnesium hydroxide powder, and 0.3-0.8 parts of sodium lignosulfonate to 90-130 parts of deionized water, stir, and obtain a suspension slurry; add borosilicate activated sol dropwise to the suspension slurry, react at 60-75℃, and adjust the pH to 8.8-9.5; filter, wash with deionized water, and obtain a wet calcium-magnesium-boron-silicon composite precursor; A3. The wet calcium-magnesium-borosilicate composite precursor is dried at 105-115℃ to obtain the dried product; the dried product is heated to 320-360℃ for treatment, and then calcined at 540-620℃; after natural cooling, it is ball-milled and sieved to obtain calcium-magnesium-borosilicate multiphase micro powder. A4. Add 90-110 parts of calcium magnesium borosilicate multiphase micro powder to a mixer and preheat at 80-95℃; mix 25-40 parts of anhydrous ethanol, 0.8-1.8 parts of isopropyl distearate aluminate, 0.6-1.5 parts of stearic acid and 0.05-0.15 parts of antioxidant 1010 to obtain a surface treatment solution; add the surface treatment solution to a mixer, then add 2.0-5.0 parts of maleic anhydride grafted polyethylene wax, heat to 120-135℃ and mix, cool, pulverize and sieve.

7. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 6, characterized in that, In step A1, the reaction time is 30-60 minutes after heating to 55-65℃.

8. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 6, characterized in that, In step A2, the reaction time is 2-4 hours at 60-75℃.

9. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 6, characterized in that, In step A3, the heating time to 320-360℃ is 1-2 hours, and the calcination time to 540-620℃ is 2-4 hours.

10. The precise temperature-controlled molding process for hot-melt connection of recycled-based ultra-large diameter HDPE pipes according to claim 6, characterized in that, In step A4, the mixing time at 120-135℃ is 30-60 minutes.

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