A multi-layer composite pipe and its preparation method and application

Abstract

The present invention discloses a multi-layer composite pipe, its preparation method and application, and relates to the technical field of plastic pipes. A multi-layer composite pipe comprises, from the outside to the inside, an antifreeze outer layer, an adhesive layer, and a pressure-bearing inner layer. The antifreeze outer layer comprises PPR; nanoparticles; an elastomer; the adhesive layer comprises a gradient copolymer, and the raw materials for preparing the gradient copolymer include PP, PE, and a silane coupling agent; the gradient copolymer comprises a first gradient layer, a second gradient layer, and a third gradient layer in the thickness direction of the adhesive layer. The mass ratio of PP to PE in the first gradient layer is 7:3 to 8:2, the mass ratio of PP to PE in the second gradient layer is 4:6 to 5:5, and the mass ratio of PP to PE in the third gradient layer is 2:8 to 3:7; the pressure-bearing inner layer comprises PERT and peroxide. The multi-layer composite pipe of the present invention has excellent anti-freeze cracking performance and long-term hydrostatic stability.

Description

Technical Field

[0001] The present invention relates to the technical field of plastic pipes, and in particular to a multi-layer composite pipe and a preparation method and application thereof. Background Art

[0002] Traditional PPR pipes are prone to microcracks in low-temperature environments due to external impact or frost heave pressure, leading to leakage risks. Currently, three-layer composite pipes (such as PPR / bonding layer / PERT II) combine the advantages of the combined materials, but have the following problems:

[0003] 1. Delamination risk: Due to poor compatibility between PPR and PERT materials, the bonding layer is prone to failure;

[0004] 2. Insufficient antifreeze performance: The outer layer of PPR is not modified and has poor low-temperature impact resistance;

[0005] 3. Hydrostatic design flaw: The impact of stratification on long-term pressure bearing is not considered. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the prior art by providing a multilayer composite pipe, its preparation method, and its application. The multilayer composite pipe of the present invention exhibits excellent freeze-cracking resistance and long-term hydrostatic stability, resolving the issues of delamination risk, insufficient low-temperature impact resistance, and inappropriate hydrostatic design associated with prior multilayer composite pipes.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] In a first aspect, the present invention provides a multi-layer composite pipe, which comprises, from the outside to the inside, an antifreeze outer layer, an adhesive layer, and a pressure-bearing inner layer. The antifreeze outer layer comprises the following components by weight: 82-92% random copolymer polypropylene (PPR); 5-8% nanoparticles; 3-5% elastomer;

[0009] The adhesive layer includes a gradient copolymer (PP-g-PE), the raw materials for preparing the gradient copolymer include polypropylene (PP), polyethylene (PE) and a silane coupling agent, and the gradient copolymer includes a first gradient layer, a second gradient layer, and a third gradient layer in the thickness direction of the adhesive layer. The first gradient layer is close to the antifreeze outer layer, and the third gradient layer is close to the pressure-bearing inner layer. The mass ratio of PP to PE in the first gradient layer is 7:3-8:2, the mass ratio of PP to PE in the second gradient layer is 4:6-5:5, and the mass ratio of PP to PE in the third gradient layer is 2:8-3:7;

[0010] The pressure-bearing inner layer comprises the following components in percentage by mass: 96-99% heat-resistant polyethylene (PERT) and 1-2% peroxide.

[0011] It should be noted that the gradient copolymer is a block copolymer formed by chemical bonding of PP and PE, and its molecular chain segments change in a step-by-step manner along the thickness direction, which can achieve improved interface compatibility.

[0012] In the antifreeze outer layer of the present invention, the synergistic toughening is achieved by adding nanoparticles and elastomers to PPR, which is beneficial to improving the low-temperature impact resistance of the outer layer. The gradient copolymer includes three gradient layers, and the ratio of PP and PE in the molecular chain segments of the gradient copolymer gradually changes along the thickness direction of the adhesive layer. The interface is seamlessly transitioned by chemical bonding, which is beneficial to eliminating the risk of delamination between the inner and outer layers. The pressure-bearing inner layer is cross-linked by inducing PERT by adding peroxide, which is beneficial to improving the pressure resistance of the inner layer. Therefore, the present invention makes the multi-layer composite pipe have excellent anti-freeze cracking performance and long-term hydrostatic stability through the mutual cooperation of the antifreeze outer layer, the adhesive layer, and the pressure-bearing inner layer.

[0013] Preferably, the thickness ratio of the antifreeze outer layer, the adhesive layer and the pressure-bearing inner layer is (3-6):1:(4-7).

[0014] Preferably, the PPR has a weight average molecular weight (Mw) of 300,000-350,000 g / mol and a molecular weight distribution (PDI) of 2.0-3.0. The molecular weight is determined by gel permeation chromatography (GPC, ASTM D6474-20).

[0015] Molecular weight distribution (PDI): The ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (PDI = Mw / Mn), which characterizes the width of molecular weight distribution.

[0016] Preferably, the PPR has a melt index of 0.2-0.5 g / 10 min under the test conditions of 230° C. / 2.16 kg, and the melt index is measured according to ASTM D1238-01.

[0017] Preferably, the nanoparticles include at least one of nano-silicon dioxide (SiO2), nano-calcium carbonate (CaCO3), nano-montmorillonite (MMT), and nano-zinc oxide (ZnO).

[0018] Nano-calcium carbonate is low-cost and highly dispersible, and can work with elastomers to enhance interfacial adhesion and reduce the brittleness of the material. Nano-montmorillonite has a layered structure that enhances the barrier and mechanical properties of pipes, making it particularly suitable for chemical corrosion resistance. Nano-zinc oxide has both antibacterial and thermal conductivity, which can broaden the application of pipes in medical or geothermal systems. Therefore, the present invention improves the performance of pipes by selecting the aforementioned nanoparticles for synergistic toughening with elastomers.

[0019] Preferably, the nanoparticles have a particle size of 10-50 nm and a specific surface area of ​​150-300 m2 By controlling the size of nanoparticles, it is beneficial to improve dispersibility, strengthen the interface, avoid particle agglomeration, and thus improve the performance of the pipe.

[0020] Preferably, the elastomer includes at least one of styrene-ethylene-butylene-styrene copolymer (SEBS), hydrogenated styrene-isoprene-styrene block copolymer (SEPS), and polyolefin elastomer (POE).

[0021] SEPS is more heat-resistant than SEBS (up to 130°C for long-term use), making it suitable for high-temperature heating systems. POE exhibits excellent compatibility with PP, significantly improving impact strength at lower addition levels. Therefore, the present invention's choice of synergistically compounding these elastomers with nanoparticles helps address the creep issue of elastomers at high temperatures.

[0022] Preferably, the weight-average molecular weight (Mw) of the elastomer is 100,000-150,000 g / mol. Controlling the molecular weight of the elastomer can improve the toughening efficiency of the pipe. Molecular weight is determined using GPC (GPC) according to ASTM D6474-20.

[0023] Preferably, the elastomer has a melt index (MI) of 5-10 g / 10 min at 230°C / 5 kg. Controlling the MI of the elastomer can prevent thermal degradation of the pipe during processing. The MI is determined according to ASTM D1238-01.

[0024] Preferably, the PP in the gradient copolymer has a melt index of 25-35 g / 10 min at 230°C / 2.16 kg. High melt index PP ensures interpenetrating flow in the gradient copolymer. Melt index is measured using ASTM D1238-01.

[0025] Preferably, the melt index of the PE in the gradient copolymer matches that of the PP, and the melt index of the PE is 1-5 g / 10 min under the test conditions of 190°C / 2.16 kg. The melt index is determined according to ISO 1133-1:2022.

[0026] Preferably, the weight-average molecular weight (Mw) of PERT is 300,000-500,000 g / mol to balance pressure resistance and processability, and the molecular weight distribution (Mw / Mn) is 2.0-3.5 to improve processing stability. Molecular weight is determined by GPC using ASTM D6474-20.

[0027] Preferably, the PERT has a melt index (MI) of 0.3-0.8 g / 10 min under test conditions of 190° C. and 2.16 kg, which is beneficial for balancing fluidity and crosslinking efficiency. The melt index is measured according to ASTM D1238-01.

[0028] Preferably, the density of the PERT is 0.945-0.950 g / cm 3 .

[0029] Preferably, the cross-linking degree of PERT is ≥75%, preferably 75-80%.

[0030] Preferably, the antifreeze outer layer further comprises 0-1 wt % of an antioxidant.

[0031] Preferably, the antioxidant includes at least one of antioxidant 1010, antioxidant 168 (tris(2,4-di-tert-butylphenyl) phosphite), and antioxidant 1076 (β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate).

[0032] Preferably, the silane coupling agent includes KH-550 (aminosilane) or KH-570 (methacryloyloxysilane). KH-570 contains double bonds and can undergo a grafting reaction with PP / PE molecular chains, which helps to enhance the interfacial chemical bonding strength and improve the peel strength of the pipe.

[0033] Preferably, the pressure-bearing inner layer further comprises 0-2 wt % of a cross-linking aid.

[0034] Preferably, the auxiliary cross-linking agent is triallyl isocyanurate (TAIC), which is beneficial to improving the cross-linking efficiency.

[0035] Preferably, the peroxide comprises dicumyl peroxide (DCP) or 2,5-dimethyl-2,5-di-tert-butylperoxide (DBPH).

[0036] DCP has a low decomposition temperature (130°C), making it suitable for use with the co-crosslinking agent TAIC to improve crosslinking efficiency. DBPH has a high decomposition temperature (180°C), making it suitable for high-temperature extrusion processes and helping to reduce pre-crosslinking byproducts.

[0037] In a second aspect, the present invention further provides a method for preparing a multilayer composite pipe, comprising the following steps:

[0038] (1) The components of the antifreeze outer layer are mixed uniformly to obtain the outer layer mixed ingredients; PP and PE are added into a twin-screw extruder in a gradient ratio, and then a silane coupling agent is added, and a gradient copolymer is obtained after reaction extrusion; the components of the pressure-bearing inner layer are mixed uniformly to obtain the inner layer mixed ingredients;

[0039] (2) The outer layer mixed ingredients, the gradient copolymer, and the inner layer mixed ingredients are subjected to multi-layer co-extrusion molding to obtain a multi-layer composite tube blank, and then the multi-layer composite tube blank is expanded, sized, and cooled to obtain the multi-layer composite pipe.

[0040] The present invention dynamically expands and sizing the tube blank, thereby facilitating elimination of interface stress and preventing delamination of the multi-layer composite tube.

[0041] Preferably, the screw speed of the twin-screw extruder in step (1) is 200-400 rpm, preferably 300 rpm.

[0042] Preferably, in step (2), the diameter expansion rate is 8-12%, and the cooling rate is 5-20°C / min.

[0043] It should be noted that the expansion rate refers to the percentage increase in the outer diameter of the pipe after expansion, and the calculation formula is:

[0044] Expansion rate = (outer diameter of the pipe after expansion - original outer diameter) / original outer diameter × 100%.

[0045] In a third aspect, the present invention also provides an application of a multi-layer composite pipe in a water supply and heating system.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] In the antifreeze outer layer of the present invention, the synergistic toughening is achieved by adding nanoparticles and elastomers to PPR, which is beneficial to improving the low-temperature impact resistance of the outer layer. The gradient copolymer includes three gradient layers, and the ratio of PP and PE in the molecular chain segments of the gradient copolymer gradually changes along the thickness direction of the adhesive layer. The interface is seamlessly transitioned by chemical bonding, which is beneficial to eliminating the risk of delamination between the inner and outer layers. The pressure-bearing inner layer is cross-linked by inducing PERT by adding peroxide, which is beneficial to improving the pressure resistance of the inner layer. Therefore, the present invention makes the multi-layer composite pipe have excellent anti-freeze cracking performance and long-term hydrostatic stability through the mutual cooperation of the antifreeze outer layer, the adhesive layer, and the pressure-bearing inner layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 1 is a schematic structural diagram of the multi-layer composite pipe of the present invention, wherein 1 is a pressure-bearing inner layer; 2 is an adhesive layer; and 3 is an antifreeze outer layer. DETAILED DESCRIPTION

[0049] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below with reference to specific embodiments, but the protection scope and implementation methods of the present invention are not limited thereto.

[0050] Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0051] Example 1

[0052] This embodiment discloses a multi-layer composite pipe, such as Figure 1 As shown, from the outside to the inside, it includes an antifreeze outer layer 3, an adhesive layer 2, and a pressure-bearing inner layer 1. The thickness ratio of the antifreeze outer layer 3, the adhesive layer 2, and the pressure-bearing inner layer 1 is 5:1:4.

[0053] The raw materials of the antifreeze outer layer include the following components: PPR 89.8wt%; nanoparticles 6wt%; elastomer 4wt%; and antioxidant 0.2wt%.

[0054] The PPR was purchased from Yangzi Petrochemical 503, with a weight average molecular weight Mw of 320,000 g / mol and a melt index of 0.3 g / 10 min.

[0055] The nanoparticles are nanosilica, which are purchased from Evonik AEROSIL® 200 (fumed nanosilica), have a particle size of 12 nm (primary particles) and a specific surface area of ​​200 ± 25 m 2 / g.

[0056] The elastomer is SEBS purchased from Kraton G1652, with a weight average molecular weight of 120,000 g / mol and a melt index of 7 g / 10 min.

[0057] The antioxidant is BASF's antioxidant 1010.

[0058] The adhesive layer is made of a gradient copolymer made from PP, PE, and a silane coupling agent. The gradient copolymer comprises, along the thickness of the adhesive layer, a first gradient layer, a second gradient layer, and a third gradient layer, with a thickness ratio of 1:1:1. The first gradient layer is located adjacent to the antifreeze outer layer, while the third gradient layer is located adjacent to the pressure-bearing inner layer. The mass ratio of PP to PE in the first gradient layer is 7:3, the mass ratio of PP to PE in the second gradient layer is 5:5, and the mass ratio of PP to PE in the third gradient layer is 3:7.

[0059] In the gradient copolymer, the PP was purchased from ExxonMobil PP5341E1, with a weight-average molecular weight of 250,000 g / mol and a melt index of 30 g / 10 min. The PE was linear low-density PE purchased from Mitsui Chemicals SP0540, with a weight-average molecular weight of 100,000 g / mol and a melt index of 1.0 g / 10 min.

[0060] The raw materials of the pressure-bearing inner layer include 97 wt % of PERT, 2 wt % of peroxide, and 1 wt % of triallyl isocyanurate. The triallyl isocyanurate is purchased from Showa Denko (Japan) TAICROS®TAIC, model M-60 (powdered, purity ≥99%).

[0061] The PERT was purchased from Total XRT70, and the density of PERT was 0.945-0.950 g / cm 3 , the weight average molecular weight (Mw) is 350,000 g / mol, and the melt index is 0.5 g / 10min.

[0062] The peroxide is dicumyl peroxide, purchased from AkzoNobel (AkzoNobel) Perkadox® BC-FF, with a purity of ≥98% and a decomposition temperature of 130°C.

[0063] This embodiment discloses a method for preparing a multi-layer composite pipe, comprising the following steps:

[0064] (1) The raw materials of the antifreeze outer layer were added to a twin-screw extruder (Coperion ZSK 58). The temperature of the twin-screw extruder was set to 180-200 °C and the screw speed was 300 rpm. After mixing evenly, the outer layer mixed ingredients were obtained.

[0065] (2) Add PP and PE into the twin-screw reaction extruder in a gradient ratio and feed them in layers: multiple feed ports are set in the twin-screw reaction extruder, and the feed port of the first gradient layer is injected with a mixture of PP and PE with a mass ratio of 7:3; the feed port of the second gradient layer is injected with a mixture of PP and PE with a mass ratio of 5:5; and the feed port of the third gradient layer is injected with a mixture of PP and PE with a mass ratio of 3:7.

[0066] The twin-screw extruder was set to the following temperature settings: feed section: 180°C; mixing section: three temperature-controlled zones (zone 1: 200°C, zone 2: 210°C, and zone 3: 220°C); die head temperature: 230°C; screw speed: 300 rpm. High temperature and high shear forces promote the copolymerization of PP and PE while preventing material degradation. A silane coupling agent (KH-570) was injected into the mixing section at a mass of 1% of the total mass of the raw materials used to prepare the gradient copolymer. The reaction time was 3 minutes, resulting in a gradient copolymer.

[0067] (3) The raw materials in the pressure-bearing inner layer were added to a single-screw extruder (BATTERNFELD BEX2-135) at a temperature of 220°C and a pressure of 15 MPa. After the cross-linking reaction, the mixed ingredients for the inner layer were obtained.

[0068] (4) The outer layer mixed ingredients, the gradient copolymer, and the inner layer mixed ingredients are co-extruded in a spiral stacking die (Coperion, Germany) to obtain a multi-layer composite tube blank; finally, the multi-layer composite tube is obtained after diameter expansion, sizing, and cooling.

[0069] Among them, the temperature gradient of co-extrusion is set as: antifreeze outer layer 180℃ → adhesive layer 190℃ → pressure-bearing inner layer 220℃; it can achieve interpenetration of molecular chains at the interface of each layer and enhance the bonding force.

[0070] The specific steps of expansion and sizing cooling are as follows:

[0071] S1: Pre-expansion treatment: preheat the tube to 100℃ to soften the material.

[0072] S2: Compressed air injection: 0.8 MPa compressed air is injected through the inner cavity of the tube blank to expand the outer diameter of the tube to 110% of the outer diameter.

[0073] S3: Synchronous cooling: Spray cooling water on the outer surface and circulate water in the inner cavity for cooling, with a cooling rate of 20℃ / min.

[0074] S4: Sizing: Calibrate the size using a sizing sleeve (German Unicor mold) to ensure roundness error ≤ 0.1mm.

[0075] Example 2

[0076] A multi-layer composite pipe comprises, from outside to inside, an antifreeze outer layer 3, an adhesive layer 2, and a pressure-bearing inner layer 1, wherein the thickness ratio of the antifreeze outer layer 3, the adhesive layer 2, and the pressure-bearing inner layer 1 is 5:1:4.

[0077] The raw materials of the antifreeze outer layer include the following components: PPR 89.8wt%; nanoparticles 6wt%; elastomer 4wt%; and antioxidant 0.2wt%.

[0078] The PPR was purchased from Borealis RA130E, with a weight average molecular weight of 320,000 g / mol and a melt index of 0.4 g / 10 min.

[0079] The nanoparticles are nano calcium carbonate, Guangxi Warner New Materials, HN-601.

[0080] The elastomer is SEPS, purchased from Kuraray Septon® 2006, with a weight average molecular weight of 130,000 g / mol and a melt index of 6 g / 10 min.

[0081] The antioxidant is BASF's antioxidant 1010.

[0082] The adhesive layer is made of a gradient copolymer made from PP, PE, and a silane coupling agent. The gradient copolymer comprises, along the thickness of the adhesive layer, a first gradient layer, a second gradient layer, and a third gradient layer, with a thickness ratio of 1:1:1. The first gradient layer is located adjacent to the antifreeze outer layer, while the third gradient layer is located adjacent to the pressure-bearing inner layer. The mass ratio of PP to PE in the first gradient layer is 7:3, the mass ratio of PP to PE in the second gradient layer is 5:5, and the mass ratio of PP to PE in the third gradient layer is 3:7.

[0083] In the gradient copolymer, PP was purchased from LyondellBasell, model number was Hostalen PPH5416, melt index was 28 g / 10 min, and molecular weight was 220,000 g / mol.

[0084] The PE was purchased from Dow Chemical, model DOWLEX TM 2045G, melt index is 1.0 g / 10min (190℃ / 2.16kg), density is 0.920 g / cm 3 .

[0085] The raw materials of the pressure-bearing inner layer include 97 wt% PERT, 2 wt% peroxide, and 1 wt% triallyl isocyanurate. The triallyl isocyanurate was purchased from Showa Denko (Japan) as TAICROS®TAIC, model M-60 (powdered, purity ≥99%).

[0086] The PERT was purchased from Borealis, model number Borealis HE3490-LS, with a weight average molecular weight of 380,000 g / mol and a melt index of 0.6 g / 10 min.

[0087] The peroxide is 2,5-dimethyl-2,5-di-tert-butyl peroxyhexane, purchased from AkzoNobel Trigonox® 101, with a purity of ≥95% and a decomposition temperature of 180°C.

[0088] The preparation method of a multi-layer composite pipe is the same as that in Example 1.

[0089] Example 3

[0090] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 88.8wt%; nanoparticles 7wt%; elastomer 4wt%; and antioxidant 0.2wt%.

[0091] Example 4

[0092] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 87.8wt%; nanoparticles 7wt%; elastomer 5wt%; and antioxidant 0.2wt%.

[0093] Example 5

[0094] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 87.8wt%; nanoparticles 8wt%; elastomer 4wt%; and antioxidant 0.2wt%.

[0095] Example 6

[0096] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 86.8wt%; nanoparticles 8wt%; elastomer 5wt%; and antioxidant 0.2wt%.

[0097] Example 7

[0098] A multi-layer composite pipe, which is different from Example 1 in that the elastomer is a polyolefin elastomer purchased from Dow Chemical under the model ENGAGE TM 8180, melt index is 5 g / 10min, weight average molecular weight is 110,000 g / mol, density is 0.870 g / cm 3 .

[0099] Example 8

[0100] A multilayer composite pipe, which differs from Example 1 in that the elastomer is ethylene propylene diene monomer (EPDM) rubber, purchased from ExxonMobil of the United States, model Vistalon 2504, with a weight-average molecular weight of 180,000 g / mol and a melt index of 8 g / 10 min (230°C / 5kg).

[0101] Comparative Example 1

[0102] A multi-layer composite pipe, which is different from Example 1 in that no nanoparticles are added to the raw materials of the antifreeze outer layer.

[0103] Comparative Example 2

[0104] A multi-layer composite pipe, which is different from Example 1 in that no elastomer is added to the raw materials of the antifreeze outer layer.

[0105] Comparative Example 3

[0106] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 89.8wt%; nanoparticles 4wt%; elastomer 6wt%; and antioxidant 0.2wt%.

[0107] Comparative Example 4

[0108] A multi-layer composite pipe, which differs from Example 1 in that the raw materials of the antifreeze outer layer include the following components: PPR 88.8wt%; nanoparticles 9wt%; elastomer 2wt%; and antioxidant 0.2wt%.

[0109] Comparative Example 5

[0110] A multi-layer composite pipe differs from Example 1 in that the raw material of the bonding layer uses an equal mass of PP / PE blend (non-gradient structure) instead of the gradient copolymer, that is, in the raw material of the bonding layer: the mass percentage of PP and PE are both 50% (uniformly mixed).

[0111] Comparative Example 6

[0112] A multi-layer composite pipe is different from Example 1 in that the raw material of the pressure-bearing inner layer is PERT, that is, no peroxide is added to PERT for cross-linking.

[0113] Comparative Example 7

[0114] A multi-layer composite pipe, which differs from Example 1 in that, in the gradient copolymer, the mass ratio of PP to PE in the first gradient layer is 3:7, the mass ratio of PP to PE in the second gradient layer is 5:5, and the mass ratio of PP to PE in the third gradient layer is 7:3.

[0115] Performance testing

[0116] 1. Cyclic freeze-thaw test

[0117] Room temperature hydrolysis conditions: temperature 25±2℃, deionized water (conductivity ≤ 5μS / cm);

[0118] Freeze-thaw cycle process: immerse the pipe in a -20°C environment and freeze it for 2 hours; then transfer it to 25°C deionized water to thaw for 2 hours; repeat 100 times and inspect the pipe surface for cracks and the condition of the bonding layer.

[0119] 2. Peel strength test

[0120] Cutting direction: Cut along the pipe axis. Specimen size: width 10mm, length 150mm. Bonding area: Peeling area ≥ 50mm × 10mm. Test standard: ASTM D1876-08(2020). Peeling speed: 50mm / min. Average value of the peeling force during the stable section (≥5N / mm is acceptable) is taken to evaluate the peeling resistance of the adhesive layer under tensile load.

[0121] 3. Residual stress after expansion: Residual stress is measured according to ASTM D695-23 standard.

[0122] 4. Impact resistance: drop hammer impact test at -40℃

[0123] Test standard: ISO 179-1:2010, Specimen type: V-notch specimen; Temperature: -40°C; Impact energy: 15J (falling weight height adjustable); Qualified index: Impact strength ≥ 20 kJ / m 2 .

[0124] 5. Hydrostatic strength (50 years): Tested in accordance with ISO 17456:2006.

[0125] The above test results are shown in Table 1.

[0126] Table 1

[0127]

[0128] It can be seen from Table 1 that the multi-layer composite pipe of the present invention has excellent anti-freeze cracking performance and long-term hydrostatic stability.

[0129] By comparing Example 8 with Example 1, it can be seen that the elastomer in Example 8 is ethylene propylene diene monomer rubber (EPDM). Since EPDM has poor compatibility with PPR, the impact strength and peel strength of the antifreeze outer layer are reduced. The preferred elastomer of the present invention is SEBS, SEPS or POE, which has high compatibility with PPR, which is beneficial to further improve the peel strength and impact strength of the pipe.

[0130] By comparing Comparative Examples 1-2 with Example 1, it can be seen that no nanoparticles or elastomers are added to the raw materials of the antifreeze outer layer, and the rigidity and low-temperature toughness of the multi-layer composite pipe will be affected. This shows that by adding nanoparticles and elastomers to PPR for synergistic toughening, it is beneficial to improve the low-temperature impact resistance of the multi-layer composite pipe.

[0131] By comparing Comparative Examples 3-4 with Example 1, it can be seen that if the content of nanoparticles and elastomers in the raw materials of the antifreeze outer layer is too low or too high, the impact strength of the pipe will be affected. This shows that by controlling the content of nanoparticles and elastomers within the range defined by the present invention, it is beneficial to improve the low-temperature impact performance of the multi-layer composite pipe, and it is also beneficial to improve the long-term hydraulic stability of the pipe.

[0132] By comparing Comparative Example 5 with Example 1, it can be seen that in Comparative Example 5, the PP / PE blend is used instead of the gradient copolymer, and the peel strength is significantly reduced and the interface delamination is obvious. This shows that only by using the gradient copolymer as the raw material of the adhesive layer can the multilayer composite pipe be less likely to delaminate.

[0133] By comparing Comparative Example 6 with Example 1, it can be seen that no peroxide was added in Comparative Example 6, PERT was not cross-linked, and the hydrostatic strength was significantly reduced, indicating that the present invention is beneficial to improving the long-term hydrostatic stability of the pipe by inducing PERT cross-linking by adding peroxide.

[0134] Comparing Comparative Example 7 with Example 1 reveals that the low PP content and high PE content in the first gradient layer of Comparative Example 7 reduce the impact resistance of the first gradient layer, making it less able to withstand frost heave stress. Simultaneously, the high PP content and low PE content in the third gradient layer reduce its high-temperature resistance, resulting in insufficient rigidity in the pressure-bearing inner layer, making it unable to meet the requirements of the heating system. Therefore, by controlling the ratios of PP and PE in the first, second, and third gradient layers, the present invention improves the frost crack resistance and long-term hydrostatic stability of the multilayer composite pipe.

[0135] 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 the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A multi-layer composite pipe, characterized in that: From the outside to the inside, it includes an antifreeze outer layer, an adhesive layer, and a pressure-bearing inner layer. The antifreeze outer layer includes the following components by mass percentage: PPR 87.8-92%; nanoparticles 5-8%; elastomer 3-5%; The adhesive layer includes a gradient copolymer, and the raw materials for preparing the gradient copolymer include PP, PE and a silane coupling agent; the gradient copolymer includes a first gradient layer, a second gradient layer, and a third gradient layer in the thickness direction of the adhesive layer, the first gradient layer is close to the antifreeze outer layer, the third gradient layer is close to the pressure-bearing inner layer, the mass ratio of PP to PE in the first gradient layer is 7:3-8:2, the mass ratio of PP to PE in the second gradient layer is 4:6-5:5, and the mass ratio of PP to PE in the third gradient layer is 2:8-3:7; The pressure-bearing inner layer comprises the following components in percentage by weight: PERT 96-99%, peroxide 1-2%; and a cross-linking agent 0-2%. The nanoparticles include at least one of nano-silicon dioxide, nano-calcium carbonate, nano-montmorillonite, and nano-zinc oxide; The elastomer includes at least one of styrene-ethylene-butylene-styrene copolymer, hydrogenated styrene-isoprene-styrene block copolymer, and polyolefin elastomer.

2. The multi-layer composite pipe according to claim 1, characterized in that: The weight average molecular weight of the PPR is 300,000-350,000 g / mol; And / or, the PPR has a melt index of 0.2-0.5 g / 10 min under the test conditions of 230° C. / 2.16 kg.

3. The multi-layer composite pipe according to claim 1, characterized in that: The weight average molecular weight of the elastomer is 100,000-150,000 g / mol; And / or, the elastomer has a melt index of 5-10 g / 10 min under the test conditions of 230° C. / 5 kg.

4. The multi-layer composite pipe according to claim 1, wherein: The melt index of PP in the gradient copolymer is 25-35 g / 10min under the test conditions of 230°C / 2.16kg; And / or, the melt index of PE in the gradient copolymer under the test conditions of 190° C. / 2.16 kg is 1-5 g / 10 min.

5. The multi-layer composite pipe according to claim 1, wherein: The weight average molecular weight of the PERT is 300,000-500,000 g / mol; And / or, the PERT has a melt index of 0.3-0.8 g / 10 min under the test conditions of 190° C. / 2.16 kg.

6. The multi-layer composite pipe according to claim 1, wherein: The raw materials of the antifreeze outer layer also include 0-1wt% of antioxidant.

7. The multi-layer composite pipe according to claim 1, wherein: The peroxide includes dicumyl peroxide or 2,5-dimethyl-2,5-di-tert-butyl peroxide hexane.

8. A method for preparing a multi-layer composite pipe according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) The components of the antifreeze outer layer are mixed uniformly to obtain the outer layer mixed ingredients; PP and PE are added into a twin-screw extruder in a gradient ratio, and then a silane coupling agent is added, and a gradient copolymer is obtained after reaction extrusion; the components of the pressure-bearing inner layer are mixed uniformly to obtain the inner layer mixed ingredients; (2) The outer layer mixed ingredients, the gradient copolymer, and the inner layer mixed ingredients are subjected to multi-layer co-extrusion molding to obtain a multi-layer composite tube blank, and then the multi-layer composite tube blank is expanded, sized, and cooled to obtain the multi-layer composite pipe.

9. Use of the multi-layer composite pipe according to any one of claims 1 to 7 in a water supply or heating system.

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