Environment-friendly PVC-U plastic pipe fitting and manufacturing method thereof

Abstract

The invention discloses an environment-friendly PVC-U plastic pipe fitting and a manufacturing method thereof, and relates to the technical field of plastic waste recycling, and the environment-friendly PVC-U plastic pipe fitting is prepared from the following raw materials in parts by weight: 30-50 parts of regenerated PVC particles and 50-70 parts of raw PVC resin. Plasma-nano SiO2 co-activation is combined with a supercritical CO2 extraction technology, plasticizer residues in regenerated PVC are efficiently removed, the surface activity is improved, a multi-scale mechanical interlocking structure is constructed in cooperation with a four-level plant fiber reinforcing agent, gradient density design is combined, the ring stiffness larger than or equal to 8 kN / m < 2 > is achieved under the condition that no metal reinforcing layer exists, and the performance of the regenerated PVC is improved. Meanwhile, by utilizing the synergistic effect of maleic anhydride grafting modification and pulsed magnetic field orientation curing, the compatibility of the reclaimed material and primary resin is improved, a modulus gradient interface is formed, stress concentration is effectively relieved, the recovery rate of the waste PVC pipe is increased, and the product has the self-cleaning and electromagnetic shielding functions.

Description

Technical Field

[0001] The present invention relates to the technical field of plastic waste recycling, and particularly to an environment-friendly PVC-U plastic pipe fitting and a manufacturing method thereof. Background Art

[0002] U-PVC, also known as rigid PVC or PVC-U, is an amorphous thermoplastic resin made by polymerizing vinyl chloride monomers and adding certain additives. Traditional PVC-U pipes have low compressive strength and are prone to deformation in buried or pressurized scenarios. The upper limit of the long-term use temperature is about 60°C, and they are prone to softening at high temperatures, which limits their application in complex working conditions. Industrial waste PVC pipes have performance degradation due to photo-aging and oxidative degradation, and direct blending is likely to cause brittle cracking of products. It is necessary to rely on virgin resin to ensure performance, increasing carbon emissions and resource consumption.

[0003] The defects of existing PVC-U plastic pipe fittings are as follows:

[0004] 1. Patent document JPH04350606A discloses communication cables and plastic conduits. However, due to poor interfacial compatibility and insufficient thermal stability of the recycled materials in the conduits in the above document, it is necessary to rely on virgin resin, increasing carbon emissions. The performance of waste PVC pipes deteriorates severely after recycling, and they cannot cope with the technical problems of long-term use or use in harsh environments.

[0005] 2. Patent document US4568112A discloses pipe joint restrictor glands. However, the pipes prepared in the above document have the technical problems of low ring stiffness and incomplete recyclability.

[0006] 3. Patent document US4606558A discloses plastic pipe fittings. However, the plastic pipe fittings prepared in the above document have the technical problems of inability to self-clean and electromagnetic shielding.

[0007] 4. Patent document CN104631460A discloses a method for integrally breaking piles by the circumferential cutting and splitting method. However, the pipes in the above document have the technical problem of high residual stress. Summary of the Invention

[0008] The purpose of the present invention is to provide an environment-friendly PVC-U plastic pipe fitting and a manufacturing method thereof to solve the technical problems raised in the above background art.

[0009] To achieve the above purpose, the present invention provides the following technical solution: An environment-friendly PVC-U plastic pipe fitting is made from the following raw materials in parts by weight:

[0010] 30-50 parts of recycled PVC particles,

[0011] 50-70 parts of virgin PVC resin,

[0012] 2 - 5 parts of photocatalytic self - cleaning nano - TiO2—SiO2 composite particles,

[0013] 15 - 25 parts of modified nano - calcium carbonate,

[0014] 8 - 18 parts of four - layer plant fiber reinforcing agent,

[0015] 2 - 4 parts of interfacial phase solvent,

[0016] 0.3 - 0.8 parts of graphene quantum dot cross - linker;

[0017] wherein the recycled PVC particles are obtained by plasma - nano - SiO2 co - activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and its preparation includes:

[0018] Step a: The recycled PVC particles are subjected to alkaline cleaning and eddy current sorting to remove metal impurities, and after ultrasonic cleaning, a supercritical CO2 extraction step is added to remove plasticizer residues. The pressure during extraction is ≥10 MPa, the temperature is 40 - 60 °C, and the time is 30 min;

[0019] Step b: After being crushed to 80 - 120 mesh, they are added to a plasma activation device for activation treatment. During the activation process, a nitrogen / argon mixed gas with a volume ratio of 1:3 - 5 and silica aerogel are co - activated to form a "PVC surface - nano - SiO2 - plasma" three - phase reaction system. The power seal is 3 - 5 W / cm 2 and they are activated for 5 - 8 minutes under this condition, and after activation, vacuum ultraviolet irradiation is used to induce surface hydroxylation. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 50 - 100 mj / cm 2 ;

[0020] Step c: During the grafting process, ultrasonic waves of 20 kHz are applied using an ultrasonic vibration fluidized bed, and a maleic anhydride monomer ethanol solution with a concentration of 5 - 8 wt% is atomized and sprayed. The grafting reaction is carried out at 160 - 175 °C, and the grafting rate is controlled at ≥10 wt%;

[0021] Step d: After grafting, orientation curing treatment is carried out using a pulsed magnetic field. The intensity of the pulsed magnetic field is 0.8 - 1.2 T, and the frequency is 50 Hz;

[0022] The pipe fittings are of all - plastic structure without a metal reinforcement layer, and the melt flow rate is 0.5 - 1.5 g / 10 min, and the ring stiffness is ≥8 kN / m 2 。

[0023] Preferably, the interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH-g-PE) and epoxidized soybean oil at a mass ratio of 1:0.5 - 2, where the grafting rate of MAH-g-PE is 1.2 - 1.8 wt%, and the melt index is 2.5 - 4.0 g / 10 min.

[0024] Preferably, the plant fiber reinforcing agent adopts a four-level synergistic reinforcement structure:

[0025] Nanoscale: Add 0.5 - 1.0 wt% of cellulose nanocrystals;

[0026] Micron scale: A mixture composed of steam-exploded bamboo powder and rice husk powder at a mass ratio of 1 - 2:1, with a steam pressure of 2.5 - 3.5 MPa, a pressure holding time of 30 - 90 seconds, and the fiber diameter after blasting ≤ 20 μm and the aspect ratio ≥ 50:1 to improve the cellulose crystallinity;

[0027] Millimeter scale: Introduce aligned sisal for confinement;

[0028] Sub-millimeter scale: Embed three-dimensional network-shaped basalt fibers to form a multi-scale mechanical interlocking structure.

[0029] Preferably, the pipe wall has a functional gradient density structure, where:

[0030] The inner layer has a recycled PVC content of 40 - 50 wt% and a density of 1.38 - 1.42 g / cm 3 , contains a photocatalytic layer, and has a TiO2 concentration gradient distribution with a surface concentration reaching 8 wt%;

[0031] The outer layer has a recycled PVC content of 20 - 30 wt% and a density of 1.33 - 1.37 g / cm 3 , contains an electromagnetic shielding layer, and has a graphene quantum dot / nickel chain composite structure with an SE value ≥ 20 dB;

[0032] The density difference between layers is 0.05 - 0.15 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.2 - 1.5:1, establishing a modulus gradient interface transition zone with a width of 0.3 - 0.5 mm and a modulus change rate ≤ 5% / mm.

[0033] Preferably, the modified nano-calcium carbonate is calcium carbonate nanoparticles surface-modified with silane coupling agent KH-570, the modification temperature is 70 - 90 °C, the modifier addition amount is 1.5 - 3.0 wt% of the mass of nano-calcium carbonate, the surface contact angle of the modified calcium carbonate ≤ 20°, and the dispersion particle size in the PVC matrix ≤ 120 nm.

[0034] Preferably, after the surface graft modification treatment of the recycled PVC particles, the interfacial binding energy between them and the virgin PVC resin is ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 0.8 - 1.2 per nm 2 .

[0035] Preferably, the manufacturing steps of the environment-friendly PVC-U plastic pipe fittings are as follows:

[0036] Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 5 - 8 mm, finely crushed to 80 - 120 meshes, and then subjected to alkaline cleaning, where pH = 9 - 11, ultrasonic cleaning at 50 - 60 °C for 20 min, plasticizer desorption treatment in supercritical CO2 fluid with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor;

[0037] Step S2, Premixing of raw materials: The recycled PVC particles, virgin PVC resin, and modified nano-calcium carbonate are premixed in a high-speed mixer at 800 - 1200 rpm for 3 - 5 min, controlling the material temperature ≤ 50 °C, and introducing a trace amount of ozone for interfacial pre-oxidation during the premixing process, with the introduced ozone concentration being 5 ppm;

[0038] Step S3, Dynamic cross-linking and kneading: The premixed material, plant fiber reinforcing agent, and 0.1 - 0.3 phr dicumyl peroxide (DCP) are sequentially added to a kneader, kneaded at 160 - 175 °C for 8 - 15 minutes, with the rotor speed being 40 - 60 rpm, and an alternating electric field is applied during the kneading process to induce fiber orientation alignment, with the electric field strength being 3 kV / mm and the frequency being 1 kHz;

[0039] Step S4, Gradient co-extrusion molding: A double-channel extrusion die head is used, with the recycled PVC content in the inner layer material being 40 - 50 wt%, and 20 - 30 wt% in the outer layer. The die head temperature is zone-controlled as follows: the inner runner is 170 - 180 °C, the outer runner is 165 - 175 °C, and the extrusion pressure is 8 - 12 MPa;

[0040] Step S5, Annealing process: After the gradient co-extrusion molding in Step S4, a segmented microwave irradiation annealing process is used to reduce the residual stress of the pipe fittings;

[0041] Step S6, Laser marking: A 1064 nm wavelength fiber laser is used to engrave a two-dimensional code mark with a depth of 50 - 80 μm on the surface of the pipe fittings, including the recycled material ratio, production batch, and carbon footprint data.

[0042] Preferably, in the step S4, the gradient co-extrusion molding uses a counter-rotating conical twin-screw. The rotational speed of the inner screw is 12 - 15 rpm, the rotational speed of the outer screw is 15 - 18 rpm, the pressure in the inner die cavity is controlled at 12 - 15 MPa, and the melt flow rate ratio between the inner and outer layers is maintained at 1:1.2 - 1.5.

[0043] Preferably, in the step S5, the segmented microwave irradiation annealing process has a microwave frequency of 2.45 GHz and a power density of 0.5 - 1.2 W / cm 3 , and the irradiation time of the inner layer is 20 - 30% longer than that of the outer layer.

[0044] Preferably, the pipe fittings after the gradient co-extrusion molding in the step S4 have a temperature-resistant structure, with a glass transition temperature range of 75 - 85 °C, and the difference in linear expansion coefficients between the inner and outer layers ≤ 1.5×10 -5 / °C.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] 1. Through the plasma - nano-SiO2 co-activation combined with the supercritical CO2 extraction technology, the present invention efficiently removes the plasticizer residues in the recycled PVC and improves the surface activity. It cooperates with the four-level plant fiber reinforcing agent to construct a multi-scale mechanical interlocking structure, combined with the gradient density design, to achieve a ring stiffness ≥ 8 kN / m under the condition of no metal reinforcing layer 2 . At the same time, by utilizing the synergistic effect of maleic anhydride graft modification and pulsed magnetic field orientation curing, the compatibility between the recycled material and the virgin resin is improved, forming a modulus gradient interface, effectively alleviating stress concentration, being beneficial to improving the recovery rate of waste PVC pipes, and the product has both self-cleaning and electromagnetic shielding functions;

[0047] 2. The present invention forms a cross-scale mechanical interlock through a four-level reinforcement architecture. The nano-scale cellulose nanocrystals construct a three-dimensional hydrogen bond network, and the micro-scale steam explosion method is used to treat bamboo powder and rice husk powder to improve the cellulose crystallinity and aspect ratio, thereby improving the mechanical properties of the material. Cooperating with the orientation limiting effect of millimeter-scale sisal fibers and the space network of sub-millimeter-scale basalt fibers, under the cross-linking action of graphene quantum dots, a multi-level stress conduction path is formed, and through the synergistic effect of natural fibers and mineral fibers, while maintaining the complete recyclability of the material, it can ensure that the manufactured pipe fittings have excellent ring stiffness;

[0048] 3. Through the gradient regulation of the recycled PVC content in the wall thickness direction, the present invention constructs a "photocatalytic self-cleaning - electromagnetic shielding" dual-functional integrated structure. The gradient distribution of TiO2 concentration improves the photocatalytic efficiency of the inner surface layer, and the graphene quantum dot / nickel chain composite shielding layer achieves a broadband shielding effectiveness of SE ≥ 20 dB, endowing the pipe fittings with self-cleaning and electromagnetic shielding functions. At the same time, the stress concentration is alleviated through the modulus gradient interface, optimizing the overall impact resistance.

[0049] 4. The present invention realizes the uniform dispersion of multi-scale fibers and precise control between layers through dynamic cross-linking internal mixing and gradient co-extrusion molding, and cooperates with segmented microwave annealing to reduce residual stress, ensuring the stability of temperature change resistance. Brief Description of the Drawings

[0050] Figure 1 It is a three-dimensional structural schematic diagram of the present invention. Detailed Description of the Embodiments

[0051] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0052] Embodiment 1: An embodiment provided by the present invention: An environment-friendly PVC-U plastic pipe fitting is made of the following raw materials in parts by weight:

[0053] 40 parts of recycled PVC particles,

[0054] 60 parts of virgin PVC resin,

[0055] 3 parts of photocatalytic self-cleaning nano-TiO2—SiO2 composite particles,

[0056] 20 parts of modified nano-calcium carbonate,

[0057] 14 parts of four-layer plant fiber reinforcing agent,

[0058] 3 parts of interfacial phase solvent,

[0059] 0.6 part of graphene quantum dot cross-linking agent;

[0060] Among them, the recycled PVC particles are obtained by plasma-nano-SiO2 co-activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and its preparation includes:

[0061] Step a: The recycled PVC particles are subjected to alkaline cleaning, eddy current sorting to remove metal impurities, and after ultrasonic cleaning, a supercritical CO2 extraction step is added to remove the residual plasticizer. The pressure during extraction is ≥10 MPa, the temperature is 50 °C, and the time is 30 min;

[0062] Step b: After being pulverized to 100 meshes, it is added to a plasma activation device for activation treatment. During the activation process, a nitrogen / argon mixed gas with a volume ratio of 1:4 and silica aerogel are co-activated to form a "PVC surface-nano-SiO2-plasma" three-phase reaction system, and the power seal is 4 W / cm2 Activate for 6 minutes under the conditions, and use vacuum ultraviolet irradiation to induce surface hydroxylation after the activation is completed. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 75 mj / cm 2 ;

[0063] Step c: Apply ultrasonic waves of 20 kHz using an ultrasonic vibration fluidized bed during the grafting process, and atomize and spray a maleic anhydride monomer ethanol solution with a concentration of 6 wt%, and carry out the grafting reaction at 170 °C, and control the grafting rate to be ≥10 wt%;

[0064] Step d: After the grafting is completed, perform orientation curing treatment using a pulsed magnetic field, where the intensity of the pulsed magnetic field is 1 T and the frequency is 50 Hz;

[0065] The pipe fitting is a fully plastic structure without a metal reinforcement layer, and the melt flow rate is 1 g / 10 min, and the ring stiffness ≥8 kN / m 2 ;

[0066] The interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH-g-PE) and epoxy soybean oil in a mass ratio of 1:1.5, where the grafting rate of MAH-g-PE is 1.5 wt% and the melt index is 3.3 g / 10 min;

[0067] The plant fiber reinforcing agent adopts a four-level cooperative reinforcement structure:

[0068] Nanoscale: Add 0.7 wt% of cellulose nanocrystals;

[0069] Micron scale: It is a mixture composed of bamboo powder and rice husk powder treated by steam explosion in a mass ratio of 1.5:1. The steam pressure is 3 MPa, the pressure holding time is 60 seconds, and the fiber diameter after explosion is ≤20 μm, and the aspect ratio ≥50:1 to improve the cellulose crystallinity;

[0070] Millimeter scale: Introduce oriented sisal limit;

[0071] Sub-millimeter scale: Embed three-dimensional network-shaped basalt fibers to form a multi-scale mechanical interlocking structure;

[0072] The pipe fitting has a functional gradient density structure in the wall thickness direction, where:

[0073] The inner layer of recycled PVC has a content of 45 wt% and a density of 1.40 g / cm 3 , contains a photocatalytic layer, and has a TiO2 concentration gradient distribution, and the surface layer concentration reaches 8 wt%;

[0074] The outer layer of recycled PVC has a content of 25 wt% and a density of 1.35 g / cm 3 , contains an electromagnetic shielding layer, and has a graphene quantum dot / nickel chain composite structure, and the SE value ≥20 dB;

[0075] The interlayer density difference is 0.10 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.4:1. A modulus gradient interface transition zone is established with a width of 0.4 mm and a modulus change rate ≤ 5% / mm;

[0076] The modified nano calcium carbonate is calcium carbonate nanoparticles surface-modified with silane coupling agent KH-570. The modification treatment temperature is 80 °C, the modifier addition amount is 2.5 wt% of the mass of nano calcium carbonate, the contact angle of the modified calcium carbonate surface ≤ 20°, and the dispersed particle size in the PVC matrix ≤ 120 nm;

[0077] After the recycled PVC particles are surface graft-modified, the interfacial binding energy with the virgin PVC resin ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 1 per nm 2 ;

[0078] The manufacturing steps of this environmentally friendly PVC-U plastic pipe fitting are as follows:

[0079] Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 6 mm, finely crushed to 100 mesh, and then alkaline cleaned with pH = 10, ultrasonically cleaned at 55 °C for 20 min, the plasticizer is desorbed in supercritical CO2 fluid with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation is carried out in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor;

[0080] Step S2, Premixing of raw materials: The recycled PVC particles, virgin PVC resin and modified nano calcium carbonate are premixed in a high-speed mixer at 1000 rpm for 4 min, controlling the material temperature ≤ 50 °C, and a trace amount of ozone is introduced during premixing for interfacial pre-oxidation, and the introduced ozone concentration is 5 ppm;

[0081] Step S3, Dynamic crosslinking and mixing: The premixed material, plant fiber reinforcing agent and 0.2 phr dicumyl peroxide (DCP) are sequentially added to a mixer, mixed at 170 °C for 13 minutes, the rotor speed is 50 rpm, and an alternating electric field is applied during mixing to induce fiber orientation, and the electric field strength is 3 kV / mm and the frequency is 1 kHz;

[0082] Step S4, Gradient co-extrusion molding: A two-channel extrusion die head is used, the content of recycled PVC in the inner layer material is 45 wt%, the outer layer is 25 wt%, the die head temperature is zone-controlled as: the inner layer runner is 175 °C, the outer layer runner is 170 °C, and the extrusion pressure is 10 MPa;

[0083] Step S5, Annealing Process: After the gradient co-extrusion molding in Step S4, a segmented microwave irradiation annealing process is adopted to reduce the residual stress of the pipe fittings.

[0084] Step S6, Laser Marking: Use a 1064nm wavelength fiber laser to engrave a QR code mark with a depth of 65μm on the surface of the pipe fittings, including the proportion of recycled materials, production batch, and carbon footprint data.

[0085] In Step S4, a counter-rotating conical twin-screw is used for gradient co-extrusion molding. The inner screw rotates at 13 rpm, the outer screw rotates at 17 rpm, the inner die cavity pressure is controlled at 14 MPa, and the melt flow rate ratio of the inner and outer layers is maintained at 1:1.3.

[0086] For the segmented microwave irradiation annealing process in Step S5, the microwave frequency is 2.45 GHz and the power density is 0.9 W / cm 3 , and the irradiation time of the inner layer is 25% longer than that of the outer layer.

[0087] After the gradient co-extrusion molding in Step S4, the pipe fittings have a temperature-resistant structure with a glass transition temperature range of 80°C, and the difference in linear expansion coefficients between the inner and outer layers is ≤1.5×10 -5 / °C.

[0088] Example 2: An example provided by the present invention: An environmentally friendly PVC-U plastic pipe fitting is made from the following raw materials in parts by weight:

[0089] 30 parts of recycled PVC particles,

[0090] 70 parts of virgin PVC resin,

[0091] 2 parts of photocatalytic self-cleaning nano TiO2—SiO2 composite particles,

[0092] 25 parts of modified nano calcium carbonate,

[0093] 8 parts of four-layer plant fiber reinforcing agent,

[0094] 4 parts of interfacial phase solvent,

[0095] 0.3 part of graphene quantum dot crosslinking agent;

[0096] Among them, the recycled PVC particles are obtained by plasma—nano SiO2 co-activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and its preparation includes:

[0097] Step a: Wash the recycled PVC particles with alkali, perform eddy current sorting to remove metal impurities, and add a supercritical CO2 extraction step after ultrasonic cleaning to remove plasticizer residues. The pressure during extraction is ≥10 MPa, the temperature is 60°C, and the time is 30 min.

[0098] Step b: After crushing to 80 mesh, it is added into a plasma activation device for activation treatment. During the activation process, a nitrogen / argon mixed gas with a volume ratio of 1:5 is introduced to co-activate with the silica aerogel, forming a "PVC surface - nano-SiO2 - plasma" three-phase reaction system. The power seal is 3 W / cm 2 Under this condition, it is activated for 8 minutes, and after the activation is completed, vacuum ultraviolet irradiation is used to induce surface hydroxylation. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 50 mj / cm 2 ;

[0099] Step c: During the grafting process, ultrasonic vibration fluidized bed is used to apply ultrasonic waves of 20 kHz, and a maleic anhydride monomer ethanol solution with a concentration of 8 wt% is atomized and sprayed. The grafting reaction is carried out at 160 °C, and the grafting rate is controlled to be ≥ 10 wt%;

[0100] Step d: After the grafting is completed, pulse magnetic field is used for orientation curing treatment. The intensity of the pulse magnetic field is 1.2 T, and the frequency is 50 Hz;

[0101] The pipe fitting is a fully plastic structure without a metal reinforcement layer, and the melt flow rate is 0.5 g / 10 min, and the ring stiffness ≥ 8 kN / m 2 ;

[0102] The interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH-g-PE) and epoxy soybean oil with a mass ratio of 1:2. The grafting rate of MAH-g-PE is 1.2 wt%, and the melt index is 4.0 g / 10 min;

[0103] The plant fiber reinforcing agent adopts a four-level collaborative reinforcement structure:

[0104] Nanoscale: Add 0.5 wt% of cellulose nanocrystals;

[0105] Micron scale: It is a mixture composed of steam-exploded bamboo powder and rice husk powder with a mass ratio of 2:1. The steam pressure is 2.5 MPa, and the pressure holding time is 90 seconds. After blasting, the fiber diameter ≤ 20 μm, and the aspect ratio ≥ 50:1, improving the cellulose crystallinity;

[0106] Millimeter scale: Introduce oriented sisal limit;

[0107] Sub-millimeter scale: Embed three-dimensional network-shaped basalt fibers to form a multi-scale mechanical interlocking structure;

[0108] The pipe fitting has a functional gradient density structure in the wall thickness direction, where:

[0109] The content of recycled PVC in the inner layer is 40 wt%, and the density is 1.42 g / cm 3, containing a photocatalytic layer with a TiO₂ concentration gradient distribution, and the surface concentration reaches 8 wt%;

[0110] The outer layer contains 20 wt% of recycled PVC with a density of 1.37 g / cm 3 , containing an electromagnetic shielding layer with a graphene quantum dot / nickel chain composite structure, and the SE value ≥ 20 dB;

[0111] The density difference between layers is 0.05 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.5:1, establishing a modulus gradient interface transition zone with a width of 0.3 mm and a modulus change rate ≤ 5% / mm;

[0112] The modified nano calcium carbonate is calcium carbonate nanoparticles surface-modified with silane coupling agent KH-570. The modification treatment temperature is 90 °C, the additive amount of the modifier is 1.5 wt% of the mass of nano calcium carbonate. After modification, the surface contact angle of calcium carbonate ≤ 20°, and the dispersion particle size in the PVC matrix ≤ 120 nm;

[0113] After the recycled PVC particles are surface graft-modified, the interfacial binding energy with the virgin PVC resin ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 1.2 per nm 2 ;

[0114] The manufacturing steps of this environmentally friendly PVC-U plastic pipe fitting are as follows:

[0115] Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 5 mm, finely crushed to 120 mesh, and then subjected to alkaline cleaning with pH = 9, ultrasonic cleaning at 60 °C for 20 min, plasticizer desorption treatment in supercritical CO₂ fluid with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor;

[0116] Step S2, Premixing of raw materials: The recycled PVC particles, virgin PVC resin, and modified nano calcium carbonate are premixed in a high-speed mixer at 800 rpm for 5 min, controlling the material temperature ≤ 50 °C, and a trace amount of ozone is introduced during premixing for interfacial pre-oxidation, and the introduced ozone concentration is 5 ppm;

[0117] Step S3, Dynamic cross-linking and mixing: The premixed material, plant fiber reinforcing agent, and 0.1 phr of dicumyl peroxide (DCP) are added to the mixer in sequence, mixed at 175 °C for 8 minutes, with a rotor speed of 60 rpm, and an alternating electric field is applied during mixing to induce fiber orientation arrangement, and the electric field strength is 3 kV / mm and the frequency is 1 kHz;

[0118] Step S4, gradient co-extrusion molding: A double-channel extrusion die head is used. The recycled PVC content in the inner layer material is 40 wt%, and that in the outer layer is 30 wt%. The die head temperature is controlled in zones as follows: the inner channel is 170 °C, the outer channel is 175 °C, and the extrusion pressure is 8 MPa;

[0119] Step S5, annealing process: After the gradient co-extrusion molding in Step S4, a segmented microwave irradiation annealing process is adopted to reduce the residual stress of the pipe fittings;

[0120] Step S6, laser marking: A 1064 nm wavelength fiber laser is used to engrave a two-dimensional code mark with a depth of 80 μm on the surface of the pipe fittings, which includes the recycled material ratio, production batch, and carbon footprint data;

[0121] In Step S4, a counter-rotating conical twin-screw is used for gradient co-extrusion molding. The rotational speed of the inner screw is 12 rpm, and that of the outer screw is 18 rpm. The pressure in the inner die cavity is controlled at 12 MPa, and the melt flow rate ratio between the inner and outer layers is maintained at 1:1.5;

[0122] In the segmented microwave irradiation annealing process of Step S5, the microwave frequency is 2.45 GHz, and the power density is 0.5 W / cm 3 , and the irradiation time of the inner layer is 30% longer than that of the outer layer;

[0123] The pipe fittings after the gradient co-extrusion molding in Step S4 have a temperature-resistant structure, with a glass transition temperature range of 75 °C, and the difference in linear expansion coefficient between the inner and outer layers is ≤ 1.5×10 -5 / °C.

[0124] Example 3: An example provided by the present invention: An environmentally friendly PVC-U plastic pipe fitting is made from the following raw materials in parts by weight:

[0125] 50 parts of recycled PVC particles,

[0126] 50 parts of virgin PVC resin,

[0127] 5 parts of photocatalytic self-cleaning nano TiO2—SiO2 composite particles,

[0128] 15 parts of modified nano calcium carbonate,

[0129] 18 parts of four-layer plant fiber reinforcing agent,

[0130] 2 parts of interfacial phase solvent,

[0131] 0.8 part of graphene quantum dot crosslinking agent;

[0132] Among them, the recycled PVC particles are obtained by plasma—nano SiO2 co-activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and their preparation includes:

[0133] Step a: Subject the recycled PVC particles to alkaline cleaning and eddy current separation to remove metal impurities. After ultrasonic cleaning, add a supercritical CO2 extraction step to remove residual plasticizers. The pressure during extraction is ≥10 MPa, the temperature is 40 °C, and the time is 30 min.

[0134] Step b: After pulverizing to 120 mesh, add them to a plasma activation device for activation treatment. During the activation process, introduce a nitrogen / argon mixed gas with a volume ratio of 1:3 and silica aerogel for co-activation to form a "PVC surface - nano-SiO2 - plasma" three-phase reaction system. The power seal is 5 W / cm 2 Activate for 5 minutes under these conditions, and use vacuum ultraviolet irradiation to induce surface hydroxylation after activation. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 100 mj / cm 2 ;

[0135] Step c: Apply ultrasonic waves of 20 kHz using an ultrasonic vibration fluidized bed during the grafting process, and atomize and spray a 5 wt% maleic anhydride monomer ethanol solution. Conduct the grafting reaction at 175 °C, and control the grafting rate to be ≥10 wt%.

[0136] Step d: After grafting, perform orientation curing treatment using a pulsed magnetic field. The intensity of the pulsed magnetic field is 0.8 T, and the frequency is 50 Hz.

[0137] The pipe fitting is a fully plastic structure without a metal reinforcement layer, and its melt flow rate is 1.5 g / 10 min, and the ring stiffness is ≥8 kN / m 2 ;

[0138] The interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH-g-PE) and epoxy soybean oil in a mass ratio of 1:0.5. The grafting rate of MAH-g-PE is 1.8 wt%, and the melt index is 2.5 g / 10 min.

[0139] The plant fiber reinforcing agent adopts a four-level cooperative reinforcement structure:

[0140] Nanoscale: Add 1.0 wt% cellulose nanocrystals;

[0141] Micron scale: A mixture composed of steam-exploded bamboo powder and rice husk powder in a mass ratio of 1:1. The steam pressure is 3.5 MPa, the pressure holding time is 30 seconds. After blasting, the fiber diameter is ≤20 μm, and the aspect ratio is ≥50:1 to improve the cellulose crystallinity.

[0142] Millimeter scale: Introduce oriented sisal limiters;

[0143] Sub-millimeter scale: Embed three-dimensional network-shaped basalt fibers to form a multi-scale mechanical interlocking structure;

[0144] The pipe fitting has a functionally graded density structure in the wall thickness direction, where:

[0145] The inner layer has a recycled PVC content of 50 wt%, a density of 1.38 g / cm 3 , contains a photocatalytic layer, and has a TiO2 concentration gradient distribution, with the surface concentration reaching 8 wt%;

[0146] The outer layer has a recycled PVC content of 30 wt%, a density of 1.33 g / cm 3 , contains an electromagnetic shielding layer, and has a graphene quantum dot / nickel chain composite structure, with an SE value ≥ 20 dB;

[0147] The density difference between layers is 0.15 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.2:1. A modulus gradient interface transition zone is established, with a width of 0.5 mm and a modulus change rate ≤ 5% / mm;

[0148] The modified nano calcium carbonate is calcium carbonate nanoparticles surface-modified with silane coupling agent KH-570. The modification treatment temperature is 70 °C, the modifier addition amount is 3.0 wt% of the mass of nano calcium carbonate, the surface contact angle of the modified calcium carbonate ≤ 20°, and the dispersion particle size in the PVC matrix ≤ 120 nm;

[0149] After the recycled PVC particles are surface graft-modified, their interfacial binding energy with the virgin PVC resin ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 0.8 per nm 2 ;

[0150] The manufacturing steps of this environmentally friendly PVC-U plastic pipe fitting are as follows:

[0151] Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 8 mm, finely crushed to 80 mesh, then alkaline cleaned, where pH = 11, ultrasonically cleaned at 50 °C for 20 min, plasticizer desorption treatment is carried out in supercritical CO2 fluid, with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation is carried out in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor;

[0152] Step S2, Premixing of raw materials: The recycled PVC particles, virgin PVC resin, and modified nano calcium carbonate are premixed in a high-speed mixer at 1200 rpm for 3 min, controlling the material temperature ≤ 50 °C, and a trace amount of ozone is introduced during premixing for interfacial pre-oxidation, with the introduced ozone concentration of 5 ppm;

[0153] Step S3, Dynamic Crosslinking Internal Mixing: Add the premix, plant fiber reinforcing agent, and 0.3 phr of dicumyl peroxide (DCP) into the internal mixer in sequence, mix at 160 °C for 15 minutes, with the rotor speed of 40 rpm, and apply an alternating electric field during the internal mixing process to induce fiber orientation alignment, with the electric field strength of 3 kV / mm and the frequency of 1 kHz;

[0154] Step S4, Gradient Coextrusion Molding: Use a dual-channel extrusion die head. The content of recycled PVC in the inner layer material is 50 wt%, and that in the outer layer is 20 wt%. The die head temperature is controlled in zones as follows: the inner channel is 180 °C, the outer channel is 165 °C, and the extrusion pressure is 12 MPa;

[0155] Step S5, Annealing Process: After the gradient coextrusion molding in Step S4, adopt a segmented microwave irradiation annealing process to reduce the residual stress of the pipe fittings;

[0156] Step S6, Laser Marking: Use a 1064 nm wavelength fiber laser to engrave a two-dimensional code mark with a depth of 50 μm on the surface of the pipe fittings, including the recycled material ratio, production batch, and carbon footprint data;

[0157] In Step S4, the gradient coextrusion molding uses a counter-rotating conical twin screw. The rotation speed of the inner screw is 15 rpm, the rotation speed of the outer screw is 15 rpm, the inner die cavity pressure is controlled at 15 MPa, and the melt flow rate ratio of the inner and outer layers is maintained at 1:1.2;

[0158] Step S5 segmented microwave irradiation annealing process, with the microwave frequency of 2.45 GHz and the power density of 1.2 W / cm 3 , and the irradiation time of the inner layer is 20% longer than that of the outer layer;

[0159] The pipe fittings after the gradient coextrusion molding in Step S4 have a temperature-resistant structure, with the glass transition temperature range of 85 °C, and the difference in linear expansion coefficients between the inner and outer layers ≤ 1.5×10 -5 / °C.

[0160] Example 4: An example provided by the present invention: An environmentally friendly PVC-U plastic pipe fitting is made from the following raw materials in parts by weight:

[0161] 35 parts of recycled PVC particles,

[0162] 63 parts of virgin PVC resin,

[0163] 5 parts of photocatalytic self-cleaning nano TiO2—SiO2 composite particles,

[0164] 18 parts of modified nano calcium carbonate,

[0165] 14 parts of four-layer plant fiber reinforcing agent,

[0166] 3 parts of interfacial phase solvent,

[0167] 0.3 parts of graphene quantum dot crosslinker;

[0168] Among them, the recycled PVC particles are obtained by plasma - nano SiO2 co - activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and its preparation includes:

[0169] Step a: The recycled PVC particles are subjected to alkaline cleaning and eddy current sorting to remove metal impurities, and after ultrasonic cleaning, a supercritical CO2 extraction step is added to remove plasticizer residues. The pressure during extraction is ≥10 MPa, the temperature is 43 °C, and the time is 30 min;

[0170] Step b: After being crushed to 112 meshes, it is added to a plasma activation device for activation treatment. During the activation process, a nitrogen / argon mixed gas with a volume ratio of 1:3 and silica aerogel are co - activated to form a "PVC surface - nano SiO2 - plasma" three - phase reaction system. The power seal is 4 W / cm 2 Under this condition, it is activated for 8 minutes, and after activation, vacuum ultraviolet irradiation is used to induce surface hydroxylation. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 100 mj / cm 2 ;

[0171] Step c: During the grafting process, ultrasonic vibration fluidized bed is used to apply ultrasonic waves of 20 kHz, and a 7 wt% maleic anhydride monomer ethanol solution is atomized and sprayed. The grafting reaction is carried out at 172 °C, and the grafting rate is controlled to be ≥10 wt%;

[0172] Step d: After grafting, pulse magnetic field is used for orientation curing treatment, where the intensity of the pulse magnetic field is 1.1 T and the frequency is 50 Hz;

[0173] The pipe fitting is a fully plastic structure without a metal reinforcement layer, and its melt flow rate is 0.9 g / 10 min, and the ring stiffness ≥8 kN / m 2 ;

[0174] The interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH - g - PE) and epoxy soybean oil in a mass ratio of 1:0.9. The grafting rate of MAH - g - PE is 1.4 wt%, and the melt index is 2.9 g / 10 min;

[0175] The plant fiber reinforcing agent adopts a four - level cooperative reinforcement structure:

[0176] Nanoscale: Add 0.8 wt% of cellulose nanocrystals;

[0177] Micron scale: A mixture composed of bamboo powder and rice husk powder treated by steam explosion in a mass ratio of 1.2:1, with a steam pressure of 2.8 MPa, a pressure holding time of 45 seconds, a fiber diameter ≤ 20 μm and a length-diameter ratio ≥ 50:1 after explosion, which improves the cellulose crystallinity;

[0178] Millimeter scale: Introduce sisal with oriented arrangement for limitation;

[0179] Sub-millimeter scale: Embed three-dimensional network-shaped basalt fibers to form a multi-scale mechanical interlocking structure;

[0180] The pipe wall thickness direction has a functional gradient density structure, where:

[0181] The inner layer has a recycled PVC content of 45 wt%, a density of 1.40 g / cm 3 , contains a photocatalytic layer, and has a TiO2 concentration gradient distribution, with the surface concentration reaching 8 wt%;

[0182] The outer layer has a recycled PVC content of 23 wt%, a density of 1.34 g / cm 3 , contains an electromagnetic shielding layer, and has a graphene quantum dot / nickel chain composite structure, with an SE value ≥ 20 dB;

[0183] The density difference between layers is 0.15 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.5:1, establishing a modulus gradient interface transition zone with a width of 0.5 mm and a modulus change rate ≤ 5% / mm;

[0184] The modified nano calcium carbonate is calcium carbonate nanoparticles surface-modified by silane coupling agent KH-570, with a modification treatment temperature of 78 °C, a modifier addition amount of 1.6 wt% of the mass of nano calcium carbonate, a surface contact angle of the modified calcium carbonate ≤ 20°, and a dispersion particle size ≤ 120 nm in the PVC matrix;

[0185] After the recycled PVC particles are surface grafted and modified, their interfacial binding energy with the virgin PVC resin ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 0.9 per nm 2 ;

[0186] The manufacturing steps of this environmentally friendly PVC-U plastic pipe fitting are as follows:

[0187] Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 6 mm, finely crushed to 80 mesh, then alkaline cleaned, where pH = 9, ultrasonic cleaned at 60 °C for 20 min, plasticizer desorption treatment is carried out in supercritical CO2 fluid with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation is carried out in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor;

[0188] Step S2, raw material premixing: The recycled PVC particles, virgin PVC resin, and modified nano-calcium carbonate are premixed in a high-speed mixer at 850 rpm for 5 min, controlling the material temperature ≤ 50°C, and introducing a trace amount of ozone for interfacial pre-oxidation during the premixing process, with the introduced ozone concentration being 5 ppm;

[0189] Step S3, dynamic cross-linking and kneading: The premixed material, plant fiber reinforcing agent, and 0.3 phr of dicumyl peroxide (DCP) are sequentially added into a kneader, kneaded at 167°C for 12 minutes, with the rotor speed being 55 rpm, and an alternating electric field is applied during the kneading process to induce fiber orientation, with the electric field strength being 3 kV / mm and the frequency being 1 kHz;

[0190] Step S4, gradient co-extrusion molding: A two-channel extrusion die head is used, with the recycled PVC content in the inner layer material being 44 wt% and in the outer layer being 25 wt%. The die head temperature is zone-controlled as follows: the inner channel is 177°C and the outer channel is 170°C, and the extrusion pressure is 10 MPa;

[0191] Step S5, annealing process: After the gradient co-extrusion molding in Step S4, a segmented microwave irradiation annealing process is adopted to reduce the residual stress of the pipe fittings;

[0192] Step S6, laser marking: A 1064 nm wavelength fiber laser is used to engrave a two-dimensional code mark with a depth of 70 μm on the surface of the pipe fittings, including the recycled material ratio, production batch, and carbon footprint data;

[0193] In Step S4, the gradient co-extrusion molding uses a counter-rotating conical twin-screw. The rotational speed of the inner screw is 14 rpm, the rotational speed of the outer screw is 16 rpm, the inner die cavity pressure is controlled at 14 MPa, and the melt flow rate ratio between the inner and outer layers is maintained at 1:1.3;

[0194] In Step S5, the segmented microwave irradiation annealing process has a microwave frequency of 2.45 GHz and a power density of 0.9 W / cm 3 , and the irradiation time of the inner layer is 25% longer than that of the outer layer;

[0195] The pipe fittings after the gradient co-extrusion molding in Step S4 have a temperature-resistant structure, with a glass transition temperature range of 80°C, and the difference in linear expansion coefficients between the inner and outer layers ≤ 1.5×10 -5 / °C.

[0196] Comparative Example 1; Prepared by referring to the method of Example 1. The difference from Example 1 is that the plasma activation step is omitted, and other raw materials, steps, and parameters are the same as those in Example 1.

[0197] Comparative Example 2; Prepared by referring to the method of Example 1, the difference from Example 1 is that: the maleic anhydride graft modification step is omitted, and other raw materials, steps and parameters are the same as those in Example 1.

[0198] Comparative Example 3; Prepared by referring to the method of Example 1, the difference from Example 1 is that: a single-screw extruder is used in Step S4, and other raw materials, steps and parameters are the same as those in Example 1.

[0199] Comparative Example 4; Prepared by referring to the method of Example 1, the difference from Example 1 is that: the die head temperature is uniformly 170 °C, and there is no layer control, and other raw materials, steps and parameters are the same as those in Example 1.

[0200] Comparative Example 5; Prepared by referring to the method of Example 1, the difference from Example 1 is that: the annealing process is changed to conventional hot air oven annealing, and it is kept at 120 °C for 2 hours, and other raw materials, steps and parameters are the same as those in Example 1.

[0201] Comparative Example 6; Prepared by referring to the method of Example 1, the difference from Example 1 is that: the steam explosion treatment step is omitted, and other raw materials, steps and parameters are the same as those in Example 1.

[0202] Comparative Example 7; Prepared by referring to the method of Example 1, the difference from Example 1 is that: only a single-layer micron-level plant fiber reinforcing agent is used, and other raw materials, steps and parameters are the same as those in Example 1.

[0203] Comparative Example 8; Prepared by referring to the method of Example 1, the difference from Example 1 is that: the ultrasonic cleaning step is omitted, and other raw materials, steps and parameters are the same as those in Example 1.

[0204] Performance test:

[0205] Test 1, Ring stiffness test:

[0206] The test standard is: GB / T 9647—2015 "Determination of Ring Stiffness of Thermoplastic Pipe";

[0207] Test method: Cut the pipe fitting specimen, place it on the ring stiffness testing machine, apply a radial load at a speed of 6 mm / min, record the load-deformation curve, and obtain the ring stiffness.

[0208] Test 2, Interfacial binding energy test:

[0209] The test standard is: Improved method of ASTM D3167—2010 "Test for Tensile Lap Shear Strength of Adhesives";

[0210] Test method: Prepare a regenerated PVC / raw PVC double-layer composite specimen, conduct a 180° peel test using a universal testing machine at a speed of 50 mm / min, and obtain the interfacial binding energy.

[0211] Test 3. Melt flow rate (MFR) test:

[0212] The test standard is: GB / T 3682—2018 "Determination of Melt Mass-Flow Rate of Thermoplastics";

[0213] Test method: Put 5 g of pellets into a melt indexer (temperature 190 °C, load 5 kg), cut the melt extruded within 10 minutes, and weigh to obtain the MFR value.

[0214] Test 4. Coefficient of linear expansion test:

[0215] The test standard is: ISO 11359—2:2021 "Plastics - Thermomechanical Analysis (TMA)";

[0216] Test method: Cut the inner layer / outer layer specimens respectively, and use a thermomechanical analyzer to measure the dimensional changes in the range of 20 - 80 °C at a heating rate of 3 °C / min to obtain the coefficient of linear expansion.

[0217] Test 5. Residual stress:

[0218] Test method: Place the pipe fitting slice in a stress detection device to obtain the residual stress.

[0219] Table 1 Performance tests of environmentally friendly PVC-U plastic pipe fittings

[0220]

[0221]

[0222] Furthermore, through the synergistic effect of nitrogen / argon plasma etching and maleic anhydride grafting, it is beneficial to increase the carboxyl density on the surface of regenerated PVC, and the interfacial binding energy reaches 162 mJ / m 2 , which is increased by 82% and 116% respectively compared with the non-activated (Comparative Example 1: 89 mJ / m 2 ) and non-grafted (Comparative Example 2: 75 mJ / m 2 );

[0223] Through the gradient distribution of the regenerated PVC content in the inner and outer layers (inner layer 45 wt% vs outer layer 25 wt%) and the regulation of the elastic modulus ratio (1.4:1) in the double-channel co-extrusion process, the ring stiffness of the pipe fitting reaches 9.2 kN / m 2 (Example 1), which is higher than that of the homogeneous structure (Comparative Example 4: 6.3 kN / m 2) increased by 46%, and the linear expansion coefficient difference was optimized to 1.2×10 —5 / ℃, meeting the engineering requirement of deformation rate <0.15% under temperature difference of -20~60℃, achieving synergistic enhancement of rigidity and toughness;

[0224] Dynamic crosslinking with DCP (0.2 phr, 170 °C) combined with microwave gradient annealing (power density 0.9 W / cm 3 , inner layer irradiation time + 25%), so that the plant fiber / PVC interface forms a three-dimensional interpenetrating network, and the residual stress is reduced to 12.3MPa (Example 1), which is 55.4% lower than the traditional hot air annealing (Comparative Example 5: 27.6MPa);

[0225] Through plasma activation (3-5W / cm 2 ) synergistic with maleic anhydride grafting (1.5-2.3wt%), the surface carboxyl density of regenerated PVC reaches 0.8-1.2 / nm 2 , the interface bonding energy with the original resin is ≥150mJ / m 2 (Actual value: 162mJ / m 2 ), when the proportion of recycled PVC added is increased to 30-50%, the tensile strength is still maintained at ≥40MPa (Example 1: 42MPa);

[0226] Calcium zinc composite stabilizer (3-6 parts) and bio-based plasticizer (5-10 parts) are used, and the heavy metal content is less than 1ppm (ICP-MS detection), which complies with the requirements of the RoHS directive. The fully plasticized gradient structure design replaces the metal reinforcement layer, making the pipe 100% recyclable, achieving the effect of reducing carbon emissions. At the same time, laser marking technology (engraving depth 50-80μm) realizes the full traceability of the recycled material ratio and carbon footprint data, meeting the requirements of green product certification;

[0227] Through plasma-nano SiO2 co-activation combined with supercritical CO2 extraction technology, the plasticizer residue in the recycled PVC is efficiently removed and the surface activity is improved. A multi-scale mechanical interlocking structure is constructed with a four-layer plant fiber reinforcement. Combined with a gradient density design, a ring stiffness of ≥8kN / m is achieved without a metal reinforcement layer. 2 At the same time, the synergistic effect of maleic anhydride grafting modification and pulse magnetic field orientation curing is used to improve the compatibility of recycled materials and virgin resins, form a modulus gradient interface, effectively alleviate stress concentration, and help improve the recovery rate of waste PVC pipes. The product also has both self-cleaning and electromagnetic shielding functions.

[0228] Form a cross-scale mechanical interlock through a four-level enhancement architecture. Nanoscale cellulose nanocrystals construct a three-dimensional hydrogen bond network, and microscale steam explosion treatment is used for bamboo powder and rice husk powder to improve the cellulose crystallinity and aspect ratio, thereby enhancing the mechanical properties of the material. In combination with the orientation limiting effect of millimeter-scale sisal fibers and the spatial network of sub-millimeter-scale basalt fibers, under the cross-linking action of 0.3-0.8 wt% graphene quantum dots, a multi-level stress conduction path is formed. And through the synergistic effect of natural fibers and mineral fibers, while maintaining the complete recyclability of the material, it can ensure that the manufactured pipe fittings have excellent ring stiffness;

[0229] Regulate the content gradient of recycled PVC in the wall thickness direction to construct a "photocatalytic self-cleaning - electromagnetic shielding" dual-functional integrated structure. The concentration gradient distribution of TiO2 improves the photocatalytic efficiency of the inner surface layer, and the graphene quantum dot / nickel chain composite shielding layer achieves a broadband shielding effectiveness of SE≥20dB;

[0230] The density difference and modulus gradient transition zone of 0.05-0.15 g / cm 3 in the layer can synergistically form a self-buffering interface layer to improve the shear strength between layers. KH-570 modified nano-calcium carbonate is uniformly dispersed at the 120nm level in the PVC matrix. In combination with the interfacial binding energy of 150 mJ / m 2 on the surface of the recycled particles and the carboxyl density of 0.8-1.2 per / nm 2 form a multiple chemical anchoring effect, making the impact toughness of the composite material 70% higher than that of traditional recycled plastics;

[0231] This design breaks through the limitations of single-performance recycled materials and realizes the dual optimization of "structural load-bearing - surface function" through gradient functionalization, providing a technical paradigm for the preparation of high-end functional products from plastic waste and promoting the formation of a closed-loop value chain of "waste plastics - high-performance composite materials";

[0232] Dynamic cross-linking and mixing (alternating electric field of 3 kV / mm induces fiber orientation) and gradient co-extrusion molding (counter-rotating twin screws, flow rate ratio of 1:1.2-1.5) achieve uniform dispersion of multi-scale fibers and precise control between layers. In combination with segmented microwave annealing (2.45 GHz, inner layer irradiation extended by 20-30%), the residual stress is reduced. The glass transition temperature is 75-85 °C, and the difference in expansion coefficient is ≤1.5×10 -5 / °C to ensure temperature change resistance stability.

[0233] Working principle: Through the co-activation of plasma - nano-SiO2 and the supercritical CO2 extraction technology, efficiently remove the plasticizer residues in recycled PVC and improve the surface activity. In combination with a four-level plant fiber reinforcing agent to construct a multi-scale mechanical interlock structure, combined with the gradient density design, achieve a ring stiffness ≥8 kN / m under the condition of no metal reinforcing layer 2, while utilizing the synergistic effect of maleic anhydride graft modification and pulsed magnetic field orientation curing to enhance the compatibility between the recycled material and the virgin resin, form a modulus gradient interface, effectively relieve stress concentration, which is beneficial to improving the recovery rate of waste PVC pipes, and the product has both self-cleaning and electromagnetic shielding functions. A cross-scale mechanical interlock is formed through a four-level reinforcement structure. Nanoscale cellulose nanocrystals construct a three-dimensional hydrogen bond network, and microscale steam explosion method is used to treat bamboo powder and rice husk powder to improve the crystallinity and aspect ratio of cellulose, thereby improving the mechanical properties of the material. With the orientation limiting effect of millimeter-scale sisal fibers and the spatial network of sub-millimeter-scale basalt fibers, under the cross-linking effect of graphene quantum dots, a multi-level stress conduction path is formed, and through the synergistic effect of natural fibers and mineral fibers, while maintaining the complete recyclability of the material, it can ensure that the manufactured pipe fittings have excellent ring stiffness. By regulating the gradient of the recycled PVC content in the wall thickness direction, a "photocatalytic self-cleaning - electromagnetic shielding" dual-functional integrated structure is constructed, in which the concentration gradient distribution of TiO2 improves the photocatalytic efficiency of the inner surface layer, and the graphene quantum dot / nickel chain composite shielding layer achieves a broadband shielding effectiveness of SE≥20dB, endowing the pipe fittings with self-cleaning and electromagnetic shielding functions. At the same time, stress concentration is relieved through the modulus gradient interface, and the overall impact resistance is optimized. The uniform dispersion of multi-scale fibers and the precise control between layers are realized through dynamic cross-linking and gradient co-extrusion molding, and the residual stress is reduced by segmental microwave annealing to ensure the temperature change stability.

[0234] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

Claims

1. An environment-friendly PVC-U plastic pipe fitting, characterized in that: It is made from the following raw materials by weight: 30 - 50 parts of recycled PVC particles, 50 - 70 parts of virgin PVC resin, 2 - 5 parts of photocatalytic self-cleaning nano TiO2 - SiO2 composite particles, 15 - 25 parts of modified nano calcium carbonate, 8 - 18 parts of four-layer plant fiber reinforcing agent, 2 - 4 parts of interfacial phase solvent, 0.3 - 0.8 part of graphene quantum dot crosslinking agent; Among them, the recycled PVC particles are obtained by plasma - nano SiO2 co-activation and ultrasonic vibration fluidized bed grafting modification treatment of waste PVC pipes, and its preparation includes: Step a: The recycled PVC particles are subjected to alkaline cleaning, vortex sorting to remove metal impurities, and after ultrasonic cleaning, a supercritical CO2 extraction step is added to remove plasticizer residues. The pressure during extraction is ≥10 MPa, the temperature is 40 - 60 °C, and the time is 30 min; Step b: After pulverizing to 80 - 120 mesh, add it into the plasma activation device for activation treatment. During the activation process, introduce a nitrogen / argon mixed gas with a volume ratio of 1:3 - 5 for co-activation with silica aerogel to form a "PVC surface - nano-SiO2 - plasma" three-phase reaction system, and activate for 5 - 8 minutes under a power seal of 3 - 5 W / cm 2 conditions, and use vacuum ultraviolet irradiation to induce surface hydroxylation after activation is completed. The wavelength of the vacuum ultraviolet irradiation is 172 nm, and the dose is 50 - 100 mj / cm 2 ; Step c: During the grafting process, ultrasonic waves of 20 kHz are applied using an ultrasonic vibration fluidized bed, and a maleic anhydride monomer ethanol solution with a concentration of 5 - 8 wt% is atomized and sprayed, and the grafting reaction is carried out at 160 - 175 °C, and the grafting rate is controlled at ≥10 wt%; Step d: After grafting, orientation curing treatment is carried out using a pulsed magnetic field, where the intensity of the pulsed magnetic field is 0.8 - 1.2 T and the frequency is 50 Hz; The pipe fitting is a fully plastic structure without a metal reinforcement layer, and has a melt flow rate of 0.5 - 1.5 g / 10 min and a ring stiffness ≥ 8 kN / m 2 .

2. An environment-friendly PVC-U plastic pipe fitting according to claim 1, characterized in that: The interfacial phase solvent is composed of maleic anhydride grafted polyethylene (MAH-g-PE) and epoxy soybean oil in a mass ratio of 1:0.5 - 2, where the grafting rate of MAH-g-PE is 1.2 - 1.8 wt% and the melt index is 2.5 - 4.0 g / 10 min.

3. An environment-friendly PVC-U plastic pipe fitting according to claim 1, characterized in that: The plant fiber reinforcing agent adopts a four-level cooperative reinforcement structure: Nanoscale: Add 0.5 - 1.0 wt% of cellulose nanocrystals; Micron scale: A mixture composed of steam-exploded bamboo powder and rice husk powder in a mass ratio of 1 - 2:1, with a steam pressure of 2.5 - 3.5 MPa, a holding pressure time of 30 - 90 seconds, and the fiber diameter after explosion ≤20 μm and the aspect ratio ≥50:1 to improve cellulose crystallinity; Millimeter scale: Introduce oriented sisal for limiting; Sub-millimeter scale: Embed three-dimensional networked basalt fibers to form a multi-scale mechanical interlocking structure.

4. An environment-friendly PVC-U plastic pipe fitting according to claim 1, characterized in that: The pipe wall thickness direction of the pipe fitting has a functional gradient density structure, where: The inner layer has a recycled PVC content of 40 - 50 wt% and a density of 1.38 - 1.42 g / cm 3 , contains a photocatalytic layer, and has a TiO2 concentration gradient distribution with a surface concentration reaching 8 wt%; The outer layer has a recycled PVC content of 20 - 30 wt% and a density of 1.33 - 1.37 g / cm 3 , includes an electromagnetic shielding layer, and has a graphene quantum dot / nickel chain composite structure with an SE value ≥ 20 dB; The density difference between layers is 0.05 - 0.15 g / cm 3 , and the elastic modulus ratio of the inner layer to the outer layer is 1.2 - 1.5:1 to establish a modulus gradient interface transition zone with a width of 0.3 - 0.5 mm and a modulus change rate ≤ 5% / mm.

5. An environment-friendly PVC-U plastic pipe fitting according to claim 1, characterized in that: The modified nano calcium carbonate is calcium carbonate nanoparticles surface-modified with silane coupling agent KH - 570. The modification treatment temperature is 70 - 90 °C, the modifier addition amount is 1.5 - 3.0 wt% of the mass of nano calcium carbonate, the contact angle on the surface of the modified calcium carbonate ≤20°, and the dispersion particle size in the PVC matrix ≤120 nm.

6. An environment-friendly PVC-U plastic pipe fitting according to claim 1, characterized in that: After the recycled PVC particles are treated by surface graft modification, the interfacial binding energy between them and the virgin PVC resin is ≥ 150 mJ / m 2 , and the carboxyl density on the surface of the recycled particles is 0.8 - 1.2 per nm 2 .

7. A manufacturing method of an environment-friendly PVC-U plastic pipe fitting according to any one of claims 1-6, characterized in that: The manufacturing steps of this environmentally friendly PVC-U plastic pipe fitting are as follows: Step S1, Pretreatment of recycled PVC: The recycled PVC pipes are crushed in two stages, coarsely crushed to 5 - 8 mm, finely crushed to 80 - 120 mesh, then subjected to alkaline cleaning, where pH = 9 - 11, ultrasonic cleaning at 50 - 60 °C for 20 min, plasticizer desorption treatment in supercritical CO2 fluid with a pressure of 12 MPa, a temperature of 80 °C, and a time of 60 min, surface activation in a plasma treatment device, and then maleic anhydride grafting is completed in a fluidized bed reactor; Step S2, Raw material premixing: Premix recycled PVC particles, virgin PVC resin and modified nano calcium carbonate in a high-speed mixer at 800 - 1200 rpm for 3 - 5 minutes, control the material temperature ≤ 50 °C, and introduce a trace amount of ozone for interfacial pre-oxidation during the premixing process, and the introduced ozone concentration is 5 ppm; Step S3, Dynamic cross-linking and internal mixing: Add the premixed material, plant fiber reinforcing agent and 0.1 - 0.3 phr dicumyl peroxide (DCP) into the internal mixer in sequence, mix at 160 - 175 °C for 8 - 15 minutes, the rotor speed is 40 - 60 rpm, and apply an alternating electric field during the internal mixing process to induce fiber orientation, and the electric field strength is 3 kV / mm and the frequency is 1 kHz; Step S4, Gradient co-extrusion molding: Use a double-channel extrusion die head, the recycled PVC content in the inner layer material is 40 - 50 wt%, and 20 - 30 wt% in the outer layer. The die head temperature is zone-controlled as follows: the inner channel is 170 - 180 °C, the outer channel is 165 - 175 °C, and the extrusion pressure is 8 - 12 MPa; Step S5, Annealing process: After the gradient co-extrusion molding in Step S4, adopt a segmented microwave irradiation annealing process to reduce the residual stress of the pipe fittings; Step S6, Laser marking: Use a 1064 nm wavelength fiber laser to engrave a two-dimensional code mark with a depth of 50 - 80 μm on the surface of the pipe fittings, including the recycled material ratio, production batch and carbon footprint data.

8. The manufacturing method of an environment-friendly PVC-U plastic pipe fitting according to claim 7, characterized in that: In the gradient co-extrusion molding in Step S4, an oppositely rotating conical twin-screw is adopted. The rotation speed of the inner screw is 12 - 15 rpm, the rotation speed of the outer screw is 15 - 18 rpm, the inner die cavity pressure is controlled at 12 - 15 MPa, and the melt flow rate ratio of the inner and outer layers is maintained at 1:1.2 - 1.

5.

9. The manufacturing method of an environment-friendly PVC-U plastic pipe fitting according to claim 7, characterized in that: The step S5 is a segmented microwave irradiation annealing process with a microwave frequency of 2.45 GHz and a power density of 0.5 - 1.2 W / cm 3 , and the inner layer irradiation time is 20 - 30% longer than that of the outer layer.

10. The manufacturing method of an environment-friendly PVC-U plastic pipe fitting according to claim 7, characterized in that: The pipe fittings after the gradient co-extrusion molding in step S4 have a temperature-resistant structure, with a glass transition temperature range of 75-85 °C, and the difference in linear expansion coefficients between the inner layer and the outer layer is ≤ 1.5×10 -5 / °C.

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