A high-strength, tough, creep-resistant pipe and its preparation method

Abstract

This invention provides a high-strength, tough, and creep-resistant pipe and its preparation method, relating to the field of polymer composite materials technology. The high-strength, tough, and creep-resistant pipe includes an inner layer and an outer layer. The materials forming the layers of the high-strength, tough, and creep-resistant pipe independently include the following components by weight: 30-40 parts polyolefin matrix, 30-50 parts polylactic acid, 10-20 parts polyhydroxyalkanoates, 3-8 parts cellulose nanocrystals, and 2-4 parts crosslinking agent. The crosslinking agent includes citric acid and dicumyl peroxide, with a weight ratio of citric acid:dicumyl peroxide = (1-3):1. The degree of crosslinking in the high-strength, tough, and creep-resistant pipe decreases gradually from the outside to the inside, satisfying: α1-α3 ≥ 35%, where α1 represents the degree of crosslinking of the outer layer and α3 represents the degree of crosslinking of the inner layer. Through the synergistic effect of polylactic acid, polyhydroxyalkanoates, and cellulose nanocrystals, the microphase composition of the matrix is ​​adjusted, significantly improving the comprehensive mechanical properties of the pipe.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials technology, and more specifically to a high-strength, tough, creep-resistant pipe with synergistic mechanical property enhancement through a multiphase reinforcement system and a gradient crosslinking structure, and its preparation method. Background Technology

[0002] Traditional pipes mainly rely on petroleum-based polyethylene (HDPE) and peroxide crosslinking system, which have the following defects: (1) Strength-toughness imbalance: the addition of rigid particles (such as calcium carbonate) leads to an elongation at break of <100%; (2) Insufficient creep resistance: the creep modulus of a single crosslinking structure is <1500 MPa; (3) Poor high temperature performance: the creep modulus decreases by >50% at 80℃.

[0003] Therefore, this invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned deficiencies of existing pipes by providing a high-strength, high-toughness, and creep-resistant pipe. By selecting specific combinations of raw materials and optimizing the pipe structure, the overall mechanical properties of the pipe can be significantly improved.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a high-strength, high-toughness, creep-resistant pipe, comprising an inner layer and an outer layer, wherein the materials forming the layers of the high-strength, high-toughness, creep-resistant pipe independently comprise the following components in parts by weight:

[0007] 30-40 parts polyolefin matrix, 30-50 parts polylactic acid, 10-20 parts polyhydroxyalkanoate, 3-8 parts cellulose nanocrystals, 2-4 parts crosslinking agent; the crosslinking agent includes citric acid and dicumyl peroxide, with a weight ratio of citric acid: dicumyl peroxide = (1-3):1; the degree of crosslinking of the high-strength and tough creep-resistant pipe decreases from the outside to the inside, satisfying: α1-α3≥35%, where α1 represents the degree of crosslinking of the outer layer and α3 represents the degree of crosslinking of the inner layer.

[0008] As an embodiment of the present invention, the polyolefin matrix includes high-density polyethylene.

[0009] As an embodiment of the present invention, the intrinsic viscosity of the polylactic acid is denoted as η. A η A =0.3~1.2 dL / g.

[0010] As an embodiment of the present invention, the intrinsic viscosity of the polyhydroxy fatty acid ester is denoted as η. B η B =0.5~4.0 dL / g.

[0011] As an embodiment of the present invention, the difference between the intrinsic viscosity of polylactic acid and the intrinsic viscosity of polyhydroxyalkanoate is η. B -η A =0.2~0.8 dL / g.

[0012] As an embodiment of the present invention, the diameter of the cellulose nanocrystals is 10~30 nm.

[0013] As an embodiment of the present invention, the aspect ratio of the cellulose nanocrystals is 20~100.

[0014] As an embodiment of the present invention, the crystallinity of the cellulose nanocrystals is ≥80%.

[0015] As an embodiment of the present invention, the high-strength and tough creep-resistant pipe further includes an intermediate transition layer, which is located between the outer layer and the inner layer.

[0016] As an embodiment of the present invention, the degree of crosslinking of the outer layer of the high-strength and tough creep-resistant pipe is denoted as α1, the degree of crosslinking of the intermediate transition layer is denoted as α2, and the degree of crosslinking of the inner layer is denoted as α3, satisfying: α1-α2≥15%, and / or α2-α3≥20%.

[0017] As an embodiment of the present invention, α1 satisfies: 70%≤α1≤80%.

[0018] As an embodiment of the present invention, α2 satisfies: 50%≤α2≤60%.

[0019] As an embodiment of the present invention, α3 satisfies: 30%≤α3≤45%.

[0020] As an embodiment of the present invention, the thickness of the outer layer of the high-strength and tough creep-resistant pipe is denoted as H1, the thickness of the intermediate transition layer is denoted as H2, and the thickness of the inner layer is denoted as H3, satisfying: H1:H2:H3=1:(1~2):(1~3).

[0021] A second aspect of the present invention provides a method for preparing the high-strength, high-toughness, creep-resistant pipe described in the first aspect of the present invention, comprising the following steps:

[0022] According to the stated weight proportions, the polyolefin matrix, polylactic acid, polyhydroxy fatty acid ester, and cellulose nanocrystals are mixed evenly, and then a crosslinking agent is added and mixed. The mixture is then subjected to a crosslinking reaction in a multilayer co-extruder at a gradient from the outside to the inside at 200~230℃, 180~200℃, and 160~180℃ to obtain the high-strength and tough creep-resistant pipe.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention utilizes the synergistic effect of PLA, PHA, and CNC to adjust the microphase composition of the matrix, enabling these three materials to form a balanced multiphase system within the polyolefin matrix. This further optimizes the nano-reinforcement effect, significantly improving the mechanical properties of the material. Furthermore, by combining this with a gradient of crosslinking degree, the overall mechanical properties of the pipe can be significantly enhanced. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the high-strength and tough creep-resistant pipe of Embodiment 1 of the present invention. In the figure, 1 represents the inner layer with a small degree of cross-linking, 2 represents the intermediate transition layer, and 3 represents the outer layer with a large degree of cross-linking. Detailed Implementation

[0026] To better illustrate the purpose, technical solution, and advantages of this invention, specific embodiments will be used to further describe the invention below. However, these embodiments do not limit the invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in this invention are conventional reagents, methods, and equipment in this technical field. Unless otherwise specified, all reagents and materials used in this invention are commercially available.

[0027] In this invention, the technical features described in an open-ended manner include both closed-ended technical solutions composed of the listed features and open-ended technical solutions that include the listed features.

[0028] In this invention, numerical ranges are involved. Unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe features or characteristics, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0029] Unless otherwise specified, all reagents or instruments used in this invention are commercially available products.

[0030] In a first aspect, an embodiment of the present invention provides a high-strength, high-toughness, creep-resistant pipe, comprising an inner layer and an outer layer, wherein the material forming the layers of the high-strength, high-toughness, creep-resistant pipe independently comprises the following components in parts by weight:

[0031] 30-40 parts polyolefin matrix, 30-50 parts polylactic acid, 10-20 parts polyhydroxyalkanoate, 3-8 parts cellulose nanocrystals, 2-4 parts crosslinking agent;

[0032] The crosslinking agent includes citric acid and dicumyl peroxide, with a weight ratio of citric acid: dicumyl peroxide = (1~3): 1;

[0033] The degree of crosslinking of the high-strength and tough creep-resistant pipe material decreases from the outside to the inside, satisfying: α1-α3≥35%, where α1 represents the degree of crosslinking of the outer layer and α3 represents the degree of crosslinking of the inner layer.

[0034] Polylactic acid (PLA) and polyhydroxyalkanoates (PHA) exhibit certain compatibility with polyolefins, forming a relatively uniform dispersed phase within the polyolefin matrix. Cellulose nanocrystals (CNCs) contain numerous active groups such as hydroxyl groups on their surface, which can interact with PLA, PHA, and polyolefin molecular chains through mechanisms like hydrogen bonding and van der Waals forces, thereby enhancing interphase bonding. This favorable interfacial bonding facilitates the effective transfer of stress between different phases, enabling the pipe to withstand stress more uniformly under external forces, preventing premature material failure due to interfacial debonding, and ultimately improving the material's mechanical strength and toughness.

[0035] PLA and PHA possess inherent crystallinity and can act as heterogeneous nucleating agents in polyolefin systems, inducing polyolefin crystallization. CNC also promotes crystallization; its nanoscale size provides more nucleation sites, further accelerating polyolefin crystallization. Through this induced crystallization, the crystallinity and morphology of polyolefins are improved, and the regularity of crystalline regions is enhanced, thereby strengthening the material's mechanical properties. Increased crystallinity can increase the material's hardness and strength. Simultaneously, suitable crystalline morphology and size distribution also contribute to improved toughness, as crystalline regions can hinder crack propagation and absorb more energy.

[0036] CNC (Cellular Necklace) possesses extremely high strength and modulus, and its nanoscale size allows it to be uniformly dispersed within a polyolefin matrix. When the material is subjected to external forces, CNC can bear part of the load, playing a role in stress transfer and dispersion, thereby improving the overall strength of the material. Furthermore, CNC can restrict the movement of polyolefin molecular chains, increasing the interaction between molecular chains and making it more difficult for molecular chain slippage and deformation to occur under stress, thus improving the material's creep resistance. PLA and PHA contain flexible segments, which can act as bridges between polyolefin molecular chains, increasing the degree of entanglement between molecular chains and improving the material's creep resistance.

[0037] Polylactic acid (PLA), polyhydroxyalkanoates (PHA), and cellulose nanocrystals (CNC) each possess distinct properties and advantages. Their combined action within a polyolefin matrix can achieve complementary and synergistic performance. The synergistic effect of PLA, PHA, and CNC, by adjusting the microphase composition of the matrix, allows these three materials to form a balanced multiphase system within the polyolefin matrix, further optimizing the nano-reinforcement effect and significantly enhancing the material's mechanical properties.

[0038] In some embodiments of the present invention, the polyolefin matrix includes high-density polyethylene.

[0039] In some embodiments of the present invention, the intrinsic viscosity of the polylactic acid is 0.3~1.2 dL / g.

[0040] In this invention, a fully automatic viscometer is used to determine the viscosity of polylactic acid according to the steps described in YY / T 0661-2017. The preferred solvent is chloroform, and the test temperature is 30°C.

[0041] In some embodiments of the present invention, the intrinsic viscosity of the polyhydroxy fatty acid ester is 0.5~4.0 dL / g. Common polyhydroxy fatty acid esters in the art can be used in the present invention, including but not limited to at least one of poly-3-hydroxybutyrate (PHB), poly-3-hydroxy-butyrate-valerate (PHBV), poly-3-hydroxy-butyrate-hexanoate (PHBH), and poly-3-hydroxy-butyrate-4-hydroxybutyrate (P(3,4HB)).

[0042] In this invention, the intrinsic viscosity of PHA is determined using an Ubbelohde viscometer. Dichloromethane is used as the solvent, the test temperature is 25℃±0.1℃, and an Ubbelohde viscometer with a diameter of 0.8 to 0.9 mm is used. The intrinsic viscosity value determined by this method can more intuitively show the performance of PHA materials.

[0043] In some embodiments of the present invention, the difference between the intrinsic viscosity of the polylactic acid and the intrinsic viscosity of the polyhydroxyalkanoate is η. B -η A =0.2~0.8 dL / g. The difference in intrinsic viscosity also affects the dispersion performance of PLA and PHA in the polyolefin matrix, as well as the interfacial interaction between them and the polyolefin matrix. A certain viscosity difference exists between the two; lower viscosity slurries often result in smaller particle sizes of the dispersed phase. PLA dispersed phases have relatively high mechanical strength; when uniformly dispersed in the polyolefin matrix as fine particles, they can improve the tensile strength and modulus of the polyolefin. PHA dispersed phases have relatively good flexibility; when dispersed in a appropriately larger form, they can improve the impact toughness of polyolefin pipes. However, if the viscosity difference is too large, it may lead to inconsistent molecular chain entanglement at the phase interface, weakening the interfacial bonding force and reducing the dispersion stability of the dispersed phase in the polyolefin matrix, making phase separation more likely. An appropriate viscosity difference may allow the molecular chain interactions at the phase interface to reach a better balance, which is beneficial for improving the dispersion uniformity and stability of the dispersed phase in the polyolefin matrix.

[0044] In some embodiments of the present invention, the cellulose nanocrystals (CNCs) have a diameter of 10-30 nm, an aspect ratio of 20 ≤ 100, and a crystallinity ≥ 80%. Highly crystallinity CNCs possess higher hardness and modulus, and adding them to a polyolefin matrix can improve the strength of polyolefin pipes. However, excessively highly crystallinous CNCs can reduce the toughness of polyolefin pipes because highly crystallinous CNCs are inherently brittle and prone to cracking under external force. Furthermore, cracks propagate relatively easily within the crystalline region, leading to reduced toughness. In this invention, CNCs with a suitable aspect ratio are selected to work in conjunction with PLA and PHA, forming a specific dispersed phase within the polyolefin matrix. The combined effect of these three components improves the interfacial bonding between the CNCs and the polyolefin matrix, maintaining a certain level of toughness while increasing strength.

[0045] It should be noted that the aspect ratio of cellulose nanocrystals can be measured using techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Crystallinity can be determined using X-ray diffraction.

[0046] In some embodiments of the present invention, the high-strength, high-toughness, and creep-resistant pipe further includes an intermediate transition layer located between the outer layer and the inner layer. In the high-strength, high-toughness, and creep-resistant pipe, the degree of cross-linking among the outer layer, intermediate transition layer, and inner layer decreases in a gradient.

[0047] In some embodiments of the present invention, the degree of crosslinking of the outer layer of the high-strength and creep-resistant pipe is denoted as α1, the degree of crosslinking of the intermediate transition layer is denoted as α2, and the degree of crosslinking of the inner layer is denoted as α3, satisfying: α1-α2≥15%, and / or α2-α3≥20%. It should be noted that in the present invention, the degree of crosslinking of each layer of the polyolefin pipe can be determined according to the standard GB / T 18474-2001 Test method for crosslinking degree of crosslinked polyethylene (PE-X) pipes and fittings.

[0048] In some embodiments of the present invention, the value of α1 satisfies: 70%≤α1≤80%, specifically it can be any value among 70%, 75%, 78%, 80% or an interval range formed between any two values.

[0049] In some embodiments of the present invention, the value of α2 satisfies: 50%≤α2≤60%, specifically it can be any value among 50%, 55%, 58%, and 60%, or an interval range formed between any two values.

[0050] In some embodiments of the present invention, the value of α3 satisfies: 30%≤α3≤45%, specifically it can be any value among 30%, 35%, 38%, 40%, and 45%, or an interval range formed between any two values.

[0051] In some embodiments of the present invention, the thickness of the outer layer of the high-strength and tough creep-resistant pipe is denoted as H1, the thickness of the intermediate transition layer is denoted as H2, and the thickness of the inner layer is denoted as H3, satisfying: H1:H2:H3=1:(1~2):(1~3).

[0052] In some embodiments of the present invention, functional additives may be added as needed. These functional additives include, but are not limited to, compatibilizers, and the compatibilizer may be polypropylene grafted with maleic anhydride (PP-g-MAH).

[0053] A second aspect of the present invention provides a method for preparing the high-strength, high-toughness, creep-resistant pipe described in the first aspect of the present invention, comprising the following steps:

[0054] According to the stated weight proportions, the polyolefin matrix, polylactic acid, polyhydroxy fatty acid ester, and cellulose nanocrystals are mixed evenly, and then a crosslinking agent is added and mixed. The mixture is then subjected to a crosslinking reaction in a multilayer co-extruder at a gradient from the outside to the inside at 200~230℃, 180~200℃, and 160~180℃ to obtain the high-strength and tough creep-resistant pipe.

[0055] In some embodiments of the present invention, the crosslinking agent includes citric acid or its derivatives and peroxide, wherein the mass ratio of citric acid or its derivatives and peroxide is (1~3):1.

[0056] It should be noted that, in this invention, by changing the ratio of the two crosslinking agents (one is citric acid or its derivative, and the other is a peroxide), the reaction temperature, and the screw speed in the extruder corresponding to different feed ports, the degree of crosslinking in the inner layer, intermediate transition layer, and outer layer of the high-strength and tough creep-resistant pipe can be adjusted; by changing the amount added at different feed ports in the co-extruder, the thickness of the inner layer, intermediate transition layer, and outer layer of the high-strength and tough creep-resistant pipe can be adjusted.

[0057] In the extruder: cross-linking with a peroxide cross-linking agent is initiated at 200~230℃ to obtain an outer layer with a high-density cross-linking network; cross-linking with a mixture of peroxide and citric acid cross-linking agents is initiated at 180~200℃ to obtain an intermediate transition layer with a medium-density cross-linking network; cross-linking with a citric acid cross-linking agent is initiated at 160~180℃ to obtain an inner layer with a low-density cross-linking network.

[0058] The following are specific embodiments of the present invention.

[0059] Information on some of the raw materials used in the embodiments of the present invention is listed below. Unless otherwise specified, all raw materials are commercially available products or are prepared by conventional means in the art:

[0060] High-density polyethylene (HDPE): density is 0.948 g / cm³ 3 ENRON 5112, purchased from ExxonMobil, USA;

[0061] Polylactic acid:

[0062] PLA-1: Intrinsic viscosity is 0.3 dL / g;

[0063] PLA-2: Intrinsic viscosity is 0.5 dL / g;

[0064] PLA-3: Intrinsic viscosity is 1.2 dL / g;

[0065] It should be noted that the polylactic acid (PLA) used in the embodiments of the present invention is prepared from lactide using conventional ring-opening polymerization methods in the art. Taking PLA-3 as an example, the specific preparation process is as follows:

[0066] (1) 20 mol of commercially available L-lactide and 0.3 mol of commercially available D-lactide were dissolved in cyclohexane solvent to obtain a reaction solution with a monomer concentration of 15% w / v (L-lactide + D-lactide). Catalyst (0.050 mol of stannous octoate and 0.250 mol of zinc oxide) was added and transferred to a 20 L stainless steel reactor. The pressure was reduced to 50 Pa, and the mixture was slowly heated to 145 °C. The mixture was stirred for 1 hour to completely dissolve L-lactide and D-lactide. The organic solvent vapor generated by heating was removed, and the reaction pressure was maintained at 50 Pa. The reaction was stopped after 16 hours at 145 °C.

[0067] (2) After the pressure in the reactor reaches atmospheric pressure, add chloroform to dissolve the solid in the reactor;

[0068] (3) Add methanol reagent to the product obtained in step (2), filter, wash with methanol, and dry at 90°C for 4 hours to obtain the polylactic acid resin PLA-3.

[0069] The preparation methods for PLA-1 and PLA-2 are similar to those for PLA-3. By conventionally adjusting the proportions of raw materials, reaction temperature, and time, polylactic acid with different intrinsic viscosities can be obtained.

[0070] The synthesis conditions for PLA-1 were as follows: monomer ratio: L-lactide:D-lactide = 19:1 (molar ratio); reaction temperature: 135℃ (±5℃); reaction time: 10 hours; monomer concentration: 15% w / v.

[0071] The synthesis conditions for PLA-2 were as follows: monomer ratio: L-lactide:D-lactide = 18:2 (molar ratio); reaction temperature: 140℃ (±5℃); reaction time: 14 hours; the molar ratio of stannous octoate to zinc oxide in the catalyst was 1:4; and the monomer concentration was 20% w / v.

[0072] The intrinsic viscosity of polylactic acid is tested using a fully automatic viscometer, following the steps described in YY / T 0661-2017. The solvent is chloroform, and the test temperature is 30℃±0.1℃.

[0073] Polyhydroxy fatty acid esters:

[0074] PHB (poly-3-hydroxybutyrate): The intrinsic viscosity was measured to be 0.5 dL / g;

[0075] PHBV (poly-3-hydroxy-butyrate-valerate): The intrinsic viscosity was measured to be 1.0 dL / g;

[0076] P(3,4HB) (poly-3-hydroxybutyrate-4-hydroxybutyrate): The intrinsic viscosity was measured to be 2.0 dL / g;

[0077] It should be noted that the polyhydroxy fatty acid esters used in the embodiments of the present invention are all self-made. Taking PHB (poly-3-hydroxybutyrate) as an example, the preparation method is as follows:

[0078] Using (R,S)-β-butyrolactone (99% purity, Tokyo Kasei Corporation) as the reactant and cycloalkoxytin as the initiator (molar ratio of reactant (R,S)-β-butyrolactone to initiator cycloalkoxytin = 600:1), the monomer and initiator were added to a dry glass polymerization ampoule under nitrogen protection with a magnetic stirrer. The ampoule was connected to a high vacuum system, and the mixture was degassed by liquid nitrogen three times and then evacuated to 10°C. -3 After Pa, the ampoule is sealed with a flame under liquid nitrogen cryoprotection; the ampoule is then placed in a constant temperature oil bath for polymerization, and reacted at 80°C for 48 hours. After completion, the polymer is dissolved in chloroform, coagulated with a coagulant, and dried in a vacuum oven to constant weight to obtain PHB.

[0079] PHBV (formed by copolymerization of 3-hydroxybutyric acid (3HB) and 3-hydroxyvalerate (3HV) monomers in a 1:1 molar ratio) and P(3,4HB) (formed by copolymerization of 3-hydroxybutyric acid and 4-hydroxybutyric acid in a 9:1 molar ratio), are prepared using the same process as PHB. By adjusting process parameters such as reaction temperature, time, type and amount of initiator, the following can be obtained:

[0080] The process parameters for PHBV are as follows: reaction temperature is 75℃; reaction time is 48 hours; the molar ratio of the reaction monomer (3-hydroxybutyric acid + 3-hydroxyvalerate) to the initiator cycloalkoxide tin is 500:1; the comonomer 3HV is added in the middle of the reaction (20 hours) to ensure uniform distribution; other parameters not listed are consistent with the preparation process of PHB.

[0081] The process parameters for P(3,4HB) are as follows: reaction temperature is 90℃; reaction time is 42 hours; stannous octoate is selected as the initiator; the molar ratio of the reactant monomer (3-hydroxybutyric acid + 4-hydroxybutyric acid) to the initiator stannous octoate is 400:1; other parameters not listed are consistent with the preparation process of PHB; among them, 4-hydroxybutyric acid (4HB) needs to be pre-activated under nitrogen protection (60℃, 2 hours) to improve the reaction activity; other parameters not listed are consistent with the preparation process of PHB.

[0082] The intrinsic viscosity of polyhydroxyalkanoates was obtained through testing. The testing method was as follows: the viscosity was measured using an Ubbelohde viscometer with a diameter of 0.8–0.9 mm, the test temperature was 25℃ ± 0.1℃, and the solvent was dichloromethane.

[0083] Polytrimethylene carbonate (PTMC): The intrinsic viscosity was measured to be 0.5 dL / g, and it was purchased from Hubei Qiansheng Biotechnology Co., Ltd.

[0084] Polycaprolactone (PCL): 104673JB, with an intrinsic viscosity of 0.5 dL / g, purchased from Adamas.

[0085] Cellulose nanocrystals (CNC):

[0086] CNC-1: Diameter 10nm, aspect ratio 20, crystallinity 80%;

[0087] CNC-2: Diameter 20nm, aspect ratio 60, crystallinity 85%;

[0088] CNC-3: Diameter 30nm, aspect ratio 100, crystallinity 90%;

[0089] Cellulose nanofibers (CNF): purchased from Shengquan Group, with a diameter of 20 nm, an aspect ratio of 900, and a crystallinity of 60% as measured.

[0090] Nano-sized calcium carbonate whiskers: purchased from Jiangxi Fengzhu New Material Technology Co., Ltd., with a diameter of 10nm, an aspect ratio of 500, and a crystallinity of 30% as measured.

[0091] It should be noted that the cellulose nanocrystals, cellulose nanofibers, and nano-calcium carbonate whiskers in the embodiments of the present invention are all commercially available products (CNF with an aspect ratio of 900 can be prepared by high-intensity mechanical shearing, and the aspect ratio can be further screened by shearing; the specific diameter and aspect ratio can be obtained by scanning electron microscopy (SEM); the crystallinity is obtained by XRD testing).

[0092] Crosslinking agent:

[0093] Citric acid: commercially available;

[0094] Dicumyl peroxide (DCP): Commercially available;

[0095] Compatibilizer: PP-g-MAH, ADMER™ PP QF460, purchased from Mitsui Chemicals, Japan.

[0096] Example 1

[0097] This embodiment provides a high-strength, tough, creep-resistant pipe, the preparation method of which includes the following steps:

[0098] According to the raw material formula described in Table 1 (numbers represent dosages, units are parts by weight), the polyolefin matrix, polylactic acid, polyhydroxyalkanoate, and cellulose nanocrystals are mixed evenly, then a crosslinking agent is added and the mixture is transferred to different feed ports (inner layer feed port, intermediate transition layer feed port, and outer layer feed port) of a co-extruder. The extrusion temperature corresponding to the inner layer feed port is set to 160℃ and the screw speed to 100 r / min; the extrusion temperature corresponding to the intermediate transition layer feed port is set to 180℃ and the screw speed to 100 r / min; and the extrusion temperature corresponding to the outer layer feed port is set to 200℃ and the screw speed to 100 r / min. Multi-layer co-extrusion is then performed to obtain the high-strength, tough, creep-resistant pipe. A schematic diagram of the pipe structure is shown below. Figure 1 As shown, the specific parameters of the obtained pipe are detailed in Table 1.

[0099] Examples 2-14, Comparative Examples 1-7

[0100] A series of pipes are provided, which are prepared according to the raw material formulations described in Tables 1-4 and with reference to the method of Example 1. By changing the co-extrusion processing conditions (e.g., the reaction temperature in the extruder, the screw speed, the mass ratio of the mixture at different feed ports, etc.), pipes with different structural parameters can be obtained.

[0101] For example, the amounts of raw materials used in Examples 2 to 4 are different, and the preparation process is consistent with that in Example 1. However, the ratio of crosslinking agent DCP to citric acid is different, which will further lead to different degrees of crosslinking α1 of the outer layer, α2 of the intermediate transition layer, and α3 of the inner layer.

[0102] Compared with Example 1, Examples 5-10 only changed the types of similar raw materials, while the other preparation processes remained the same as in Example 1;

[0103] In Examples 11 and 12, the difference from Example 1 is that the degree of crosslinking of different layers of the pipe is adjusted by changing the screw speed at the corresponding feed port of the different layers of the pipe in the extruder to change the crosslinking time of polyethylene in the extruder; the intermediate transition layer can also be further adjusted by adjusting the temperature in the extruder barrel to change the activity of the two crosslinking agents, thereby changing the ratio of the two crosslinking agents with crosslinking activity; both of the above methods can achieve the effect of changing the degree of crosslinking of polyethylene in different layers. For specific process parameters, those skilled in the art can obtain suitable process parameters through a limited number of orthogonal experiments based on the process parameters of Example 1 and the correlation between screw speed and degree of crosslinking in the art.

[0104] It needs to be further explained that:

[0105] The empirical correlation between screw speed and the degree of crosslinking of polyethylene is as follows: In multilayer co-extrusion, screw speed (r / min) directly affects the residence time (t) of the material in the extruder, thereby regulating the degree of crosslinking (α). The specific empirical formula for this correlation is: t = L / (N*D), where L is the effective length of the screw, N is the screw speed, and D is the screw diameter. Increasing the screw speed (N↑) will shorten the residence time (t↓), resulting in insufficient crosslinking reaction time, thus reducing the degree of crosslinking (α↓).

[0106] Furthermore, the relationship between the degree of crosslinking (α) and temperature (T) and time (t) can be simplified to the following empirical formula: α = k * T * k is a material constant, which is related to the type and concentration of the crosslinking agent; the crosslinking reaction rate (material constant k) follows the Arrhenius equation, and at a fixed crosslinking agent concentration, the degree of crosslinking is linearly related to the square root of the reaction time.

[0107] In Examples 13 and 14, the difference from Example 1 is that the thickness of different layers of the pipe is different, which is controlled by changing the mass of the mixed raw materials added to the corresponding feed ports of different layers in the co-extruder.

[0108] Table 1

[0109]

[0110] Table 2

[0111]

[0112] Table 3

[0113]

[0114] Note: The components in Table 3 are the same as in Example 1.

[0115] Table 4

[0116]

[0117] It should be noted that:

[0118] (1) Unless otherwise specified, the raw material composition (including type and amount) of the pipes in Table 3 is the same as that in Example 1;

[0119] (2) The thickness of each layer can be obtained by scanning the cross section of the pipe using a scanning electron microscope. The degree of cross-linking of the pipe is tested according to the standard GB / T 18474-2001 Test method for cross-linking degree of cross-linked polyethylene (PE-X) pipes and fittings. The sample is placed in a sieve, wrapped into a bag and weighed. Then the sieve bag is suspended in xylene solvent in a round bottom flask for extraction and the mass before and after extraction is weighed. The mass percentage of the residue that is not dissolved after extraction (i.e., gel content) is used as the degree of cross-linking of the sample.

[0120] Performance testing

[0121] The performance of the pipes prepared in the above embodiments and comparative examples was tested as follows:

[0122] 1. Creep performance test:

[0123] The test was conducted according to the method in GB / T 41061-2021. The specimen type was Type I, the test conditions were 25±1℃ for 100h, the tensile load was 20MPa, and the test results were expressed as creep modulus (E1) in MPa.

[0124] The present invention also tested the high-temperature creep modulus after 1000 hours under a load of 20 MPa at 80℃, denoted as E2. The retention rate of the creep modulus of the pipe after 80℃ / 1000 hours of loading is calculated as (E2 / E1)×100%.

[0125] The test results are detailed in Table 5;

[0126] 2. Tensile property test:

[0127] A universal tensile testing machine was used to perform tensile testing at 25±2℃ and a rate of 50 mm / min. The tensile strength (MPa), elongation at break (%), and yield elongation (%) were obtained from the stress-strain curves. The test results are detailed in Table 5.

[0128] 3. Impact strength test

[0129] The test was conducted according to the method in standard GB / T1843-2008, wherein: the notch type of the specimen was type A; the test environment was 23±2℃, 50±5 RH.%; the pendulum energy was 5±1 J; the pendulum speed was 3.5±0.1 m / s; the support span was 64 mm; the end curvature radius was 3.0±0.5 mm; the impact direction was perpendicular to the specimen surface; and the impact strength (kJ / m²) was [not specified]. 2 The test results are detailed in Table 5.

[0130] Table 5

[0131]

[0132] The above results indicate that:

[0133] This invention utilizes the synergistic effect of PLA, PHA, and CNC to adjust the microphase composition of the matrix, enabling these three materials to form a balanced multiphase system within the polyolefin matrix. This further optimizes the nano-reinforcement effect, significantly improving the mechanical properties of the material. Furthermore, by combining this with a gradient of crosslinking degree, the overall mechanical properties of the pipe can be significantly enhanced.

[0134] The outer layer is highly cross-linked (α1=70~80%), forming a dense network through the synergistic effect of peroxide and citric acid, significantly improving creep resistance with a creep modulus ≥2000MPa; the inner layer is low-cross-linked (α3=30~45%), maintaining the toughness of the pipe. Specifically, the pipe of this invention has an elongation at break ≥140% and an impact strength greater than or equal to 21kJ / m. 2 In contrast, Example 7, which did not employ gradient crosslinking design, showed a significant reduction in creep modulus (to only 800 MPa) and impact strength decreased to 5 kJ / m. 2 This demonstrates the crucial role of gradient design in mechanical performance.

[0135] Pipe layer thickness: A thin outer layer allows for rapid cross-linking, while a thick inner layer improves the pipe's toughness. A thickness ratio of 1:1.5:2 is optimal, balancing rigidity and toughness mechanical properties.

[0136] The tensile strength of Examples 1-14 (≥30MPa) is significantly better than that of the comparative examples (tensile strength ≤30MPa), demonstrating the innovativeness of the PLA / PHA / CNC synergistic effect and gradient structure.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A high-strength, high-toughness, creep-resistant pipe, characterized in that, Including the inner and outer layers, the materials forming the layers of the high-strength, tough, creep-resistant tubing are independently composed of the following parts by weight: 30-40 parts polyolefin matrix, 30-50 parts polylactic acid, 10-20 parts polyhydroxyalkanoate, 3-8 parts cellulose nanocrystals, 2-4 parts crosslinking agent; The crosslinking agent includes citric acid and dicumyl peroxide, with a weight ratio of citric acid: dicumyl peroxide = (1~3): 1; The degree of crosslinking of the high-strength and tough creep-resistant pipe material decreases from the outside to the inside, satisfying: α1-α3≥35%, where α1 represents the degree of crosslinking of the outer layer and α3 represents the degree of crosslinking of the inner layer.

2. The high-strength, high-toughness, creep-resistant pipe according to claim 1, characterized in that, The polyolefin matrix includes high-density polyethylene.

3. The high-strength, high-toughness, creep-resistant pipe according to claim 1, characterized in that, The intrinsic viscosity of the polylactic acid is denoted as η. A η A =0.3~1.2 dL / g; the intrinsic viscosity of the polyhydroxyalkanoate is denoted as η. B η B =0.5~4.0 dL / g.

4. The high-strength, high-toughness, creep-resistant pipe according to claim 3, characterized in that, The difference between the intrinsic viscosity of polylactic acid and the intrinsic viscosity of polyhydroxyalkanoate is η. B -η A =0.2~0.8 dL / g.

5. The high-strength, high-toughness, creep-resistant pipe according to claim 1, characterized in that, The cellulose nanocrystals satisfy at least one of the following characteristics: (1) The diameter of the cellulose nanocrystals is 10~30 nm; (2) The aspect ratio of the cellulose nanocrystals is 20~100; (3) The crystallinity of the cellulose nanocrystals is ≥80%.

6. The high-strength, high-toughness, creep-resistant pipe according to claim 1, characterized in that, The high-strength and creep-resistant pipe also includes an intermediate transition layer, which is located between the outer layer and the inner layer.

7. The high-strength, high-toughness, creep-resistant pipe according to claim 6, characterized in that, The degree of crosslinking of the outer layer of the high-strength, tough, and creep-resistant pipe is denoted as α1, the degree of crosslinking of the intermediate transition layer is denoted as α2, and the degree of crosslinking of the inner layer is denoted as α3, and it satisfies at least one of the following characteristics: （1）α1-α2≥15%； （2）α2-α3≥20%。 8. The high-strength, high-toughness, creep-resistant pipe according to claim 7, characterized in that, Satisfying at least one of the following characteristics: （1）70%≤α1≤80%； （2）50%≤α2≤60%； （3）30%≤α3≤45%。 9. The high-strength, high-toughness, creep-resistant pipe according to claim 6, characterized in that, The thickness of the outer layer of the high-strength and tough creep-resistant pipe is denoted as H1, the thickness of the intermediate transition layer is denoted as H2, and the thickness of the inner layer is denoted as H3, satisfying: H1:H2:H3=1:(1~2):(1~3).

10. The method for preparing the high-strength, tough, creep-resistant pipe according to any one of claims 6 to 9, characterized in that, Includes the following steps: According to the stated weight proportions, the polyolefin matrix, polylactic acid, polyhydroxy fatty acid ester, and cellulose nanocrystals are mixed evenly, and then a crosslinking agent is added and mixed. In a multilayer co-extruder, the crosslinking reaction is carried out in a gradient from the outside to the inside at 200~230℃, 180~200℃, and 160~180℃ to obtain the high-strength and tough creep-resistant pipe.

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