A water-proof and salt-blocking composite geomembrane and its preparation method

Abstract

The present invention discloses a water-barrier and salt-blocking composite geomembrane and a preparation method thereof, wherein the water-barrier and salt-blocking composite geomembrane comprises, from the substrate to the surface, a high-density polyethylene substrate, a nanoclay modified layer, and an anti-ultraviolet coating. With the HDPE substrate as the core structural layer, its high tensile strength and tear resistance are fully utilized, giving the geomembrane excellent mechanical bearing capacity, which can resist soil deformation, external loads, and construction wear in engineering applications; nanoclay is evenly dispersed in the HDPE substrate through a melt blending process to form a three-dimensional nano-barrier structure, which greatly reduces the water vapor permeability, and the barrier efficiency is increased by more than 80% compared with traditional geomembranes, effectively inhibiting the penetration and migration of moisture, salt ions, and harmful substances; the surface polyurethane coating significantly reduces the photooxidative degradation rate by absorbing and reflecting ultraviolet rays; the synergistic effect of the coating and the substrate enables the material to maintain flexibility in the extreme temperature range of 50℃~80℃, avoiding failure caused by low-temperature brittle cracking or high-temperature softening.

Description

Technical Field

[0001] The invention belongs to the technical field of salt expansion prevention and control of saline soil roadbed, and particularly relates to a water-isolating and salt-blocking composite geomembrane and a preparation method thereof. Background Art

[0002] The widespread distribution of saline soil in Northwest China, coupled with its unique engineering geological and climatic conditions, leads to significant coupling between multiple physical fields within the soil (such as temperature, moisture, salt, and mechanical fields), which in turn triggers a variety of engineering diseases. During the large-scale cooling process in the cold season, the temperature is transferred downward from the surface, and the interaction between the temperature, moisture, salt, and mechanical fields within the soil intensifies, resulting in a significant increase in the water-salt migration effect and salt expansion deformation effect. Under the action of low temperatures, the liquid water in the soil condenses into ice, and the salt crystallizes, causing the roadbed soil to suffer from varying degrees of frost heave and salt expansion diseases. These problems seriously affect the service performance of infrastructure, resulting in its inability to meet actual operational needs, shortening its service life, and increasing safety risks. Summary of the Invention

[0003] To address these issues, the present invention proposes a water- and salt-blocking composite geomembrane and its preparation method. Through its physical barrier properties, the geomembrane effectively inhibits water and salt migration and salt expansion deformation in saline soil roadbeds, thereby improving the stability and durability of the roadbed. Through the coordinated optimization design of a high-density polyethylene (HDPE) substrate, a nanoclay-modified layer, and a UV-resistant coating, this geomembrane achieves breakthrough improvements in mechanical properties, environmental barrier properties, and durability, demonstrating significant engineering application value.

[0004] The water-proof and salt-blocking composite geomembrane of the present invention comprises a high-density polyethylene substrate, a nano-clay modified layer and an anti-ultraviolet coating in order from the substrate to the surface.

[0005] The thickness of the high-density polyethylene substrate is 0.8 mm, the thickness of the nano-clay modified layer is 0.3 mm, and the thickness of the anti-ultraviolet coating is 0.1 mm.

[0006] The nanoclay modified layer is prepared by uniformly dispersing nanoclay in a high-density polyethylene substrate through a melt blending process, and the mass ratio of the main components of the nanoclay modified layer is sodium montmorillonite: hexadecyltrimethylammonium bromide = 2.8:1-3.2:1.

[0007] The anti-ultraviolet coating is made of high-performance polyurethane material.

[0008] The preparation method of the water-proof and salt-blocking composite geomembrane of the present invention comprises the following steps:

[0009] S1. Pretreatment of high-density polyethylene substrate;

[0010] S2. Functional modification of nanoclay;

[0011] S3. Premixing and masterbatch preparation;

[0012] S4. Melt blending process;

[0013] S5. Nano-dispersion strengthening;

[0014] S6. Molding;

[0015] S7. Composite structure integration.

[0016] The high-density polyethylene substrate pretreatment is to crystallize and dry the high-density polyethylene substrate under vacuum conditions at 80° C. for 4 hours, and then grind it at low temperature and sieve it into particles with a particle size of 0.5-1.0 mm.

[0017] The specific steps of the functional modification of nanoclay are: first, sodium montmorillonite is subjected to ion exchange reaction with hexadecyltrimethylammonium bromide at 80°C for 6 hours, then centrifuged, and the precipitate is washed with ethanol after centrifugation until there is no Br - Detection; the cleaned precipitate was placed in a 2wt% γ-methacryloxypropyltrimethoxysilane ethanol solution and ultrasonically treated at pH = 4.5 for 2 hours to obtain a modified clay suspension, and then the clay suspension was dried in a vacuum oven at 120°C for 24 hours. After drying, it was sieved through a 400-mesh sieve to finally obtain the modified clay.

[0018] The premixing refers to premixing 85wt%-91wt% of high-density polyethylene, 8wt%-12wt% of nanoclay and 1wt%-3wt% of maleic anhydride grafted polyethylene according to mass percentage.

[0019] The kinetic parameters in the melt blending process are as follows: screw speed: 250 rpm-350 rpm, vacuum degree: -0.095 MPa, melt pressure: 10 MPa-15 MPa, specific mechanical energy: 0.25 kWh / kg-0.35 kWh / kg, temperature range of the feeding section: 150°C-160°C, temperature range of the melting section: 180°C-185°C, temperature range of the high shear section: 190°C-195°C, temperature range of the mixing section: 185°C-190°C, and temperature range of the die head: 175°C-180°C.

[0020] The composite structure integration comprises performing corona treatment on the surface of high-density polyethylene in the composite film and coating the surface with a polyurethane coating.

[0021] The beneficial effects of the present invention are:

[0022] 1. With HDPE substrate as the core structural layer, its high tensile strength and tear resistance are fully utilized, giving the geomembrane excellent mechanical bearing capacity, which can resist soil deformation, external loads and construction wear in engineering applications; the chemical stability of the HDPE substrate can withstand the erosion of acids, alkalis, salts and organic solvents, ensuring the long-term performance stability of the material in complex chemical environments such as landfills and chemical pollution sites.

[0023] 2. Nanoclay is evenly dispersed within the HDPE substrate through a melt blending process, forming a three-dimensional nano-barrier structure. This significantly reduces water vapor transmission rate (≤0.1g / m² / 24h), increasing barrier efficiency by over 80% compared to traditional geomembranes. This effectively inhibits the permeation and migration of water, salt ions, and harmful substances. In cold-region projects, this structure can block frost heave damage caused by capillary water rise and salt crystallization. In saline soils, it can prevent material corrosion and increased soil salinization caused by salt penetration.

[0024] 3. The surface polyurethane coating absorbs and reflects UV rays (UV-A / UV-B shielding rate ≥ 95%. UV-A (long-wave ultraviolet): wavelength range 315–400 nm, with strong penetrating power, can cause photooxidative degradation of the material. UV-B (medium-wave ultraviolet): wavelength range 280–315 nm, with higher energy, can easily cause surface degradation). This significantly reduces the rate of photooxidative degradation, extending the material's service life under outdoor exposure to over 20 years, 2-3 times longer than conventional geomembranes. The synergistic effect of the coating and the substrate ensures the material maintains flexibility in extreme temperatures ranging from -50°C to 80°C, preventing failure caused by low-temperature brittle cracking or high-temperature softening. The dense surface coating prevents microbial attachment and root penetration, making it suitable for use in locations susceptible to bioerosion, such as landfills and ecological slope protection. The three-layer composite design avoids the complexity of layered material installation in traditional multi-layer anti-seepage systems, reduces the risk of joint leakage, and improves construction efficiency by over 30%. The long lifespan reduces the frequency of replacement, and combined with the comprehensive functions of anti-seepage, salt resistance, and frost heave resistance, it can reduce project maintenance costs by more than 50%. It is especially suitable for harsh environments such as high altitude, coastal salinization, drought, and strong ultraviolet rays.

[0025] 4. The nanoclay modified layer utilizes natural mineral materials, reducing the use of petroleum-based additives and lowering production carbon emissions. The polyurethane coating is biodegradable and modified, minimizing environmental residual risks. By controlling the permeability balance, the anti-seepage project can address local ecological water replenishment needs (such as water regulation for plant roots), promoting the coordinated development of the project and the environment. Through integrated innovation in materials, structure, and function, this provides an efficient, economical, and sustainable technical solution to address the performance shortcomings of geomembranes in complex environments, with significant social, economic, and ecological benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1It is a structural schematic diagram of the water-proof and salt-blocking composite geomembrane of the present invention.

[0027] Figure 2 This is a diagram of the one-way freezing process of a soil column under the condition of pressureless water replenishment of the present invention.

[0028] Figure 3 It is a moisture content distribution diagram of the composite geomembrane of the present invention after a one-way freezing test of a soil column under the condition of no pressure water replenishment.

[0029] Figure 4 This is the distribution diagram of sulfate ion content after the unidirectional freezing test of soil columns with different mass ratios of sodium montmorillonite:CTAB and under pressureless water replenishment conditions.

[0030] Figure 5 This is a diagram showing the changes in temperature and salt expansion after a one-way freezing test of a soil column under the condition of no-pressure water replenishment on an original soil sample.

[0031] Figure 6 This is a graph showing changes in temperature and salt expansion after a one-way freezing test of a soil column under no-pressure water replenishment conditions for the composite geomembrane of the present invention.

[0032] Figure 7 This is a graph showing the changes in temperature and salt expansion after a unidirectional freezing test of a soil column under pressureless water replenishment conditions when the mass ratio of sodium montmorillonite to CTAB is 3.2:1.

[0033] Figure 8 This is a graph showing the changes in temperature and salt expansion after a unidirectional freezing test of a soil column under pressureless water replenishment conditions when the mass ratio of sodium montmorillonite to CTAB is 2.8:1. DETAILED DESCRIPTION

[0034] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0035] The water- and salt-blocking composite geomembrane of this invention consists of a high-density polyethylene (HDPE) substrate, a nanoclay-modified layer, and a UV-resistant coating, from the substrate to the surface. The HDPE substrate, as the primary structural layer, possesses excellent mechanical strength and chemical stability, providing fundamental support for the composite geomembrane.

[0036] like Figure 1 As shown, in the water-barrier and salt-blocking composite geomembrane of the present invention, the thickness of the high-density polyethylene substrate is 0.8 mm, the thickness of the nano-clay modified layer is 0.3 mm, and the thickness of the anti-ultraviolet coating is 0.1 mm.

[0037] The nanoclay-modified layer is made by evenly dispersing nanoclay in a high-density polyethylene substrate through a melt blending process, forming a dense nanoscale barrier network structure. This unique structural design not only significantly improves the barrier properties of the material, but also effectively prevents the penetration of moisture and salt, reducing its water vapor transmission rate to below 0.1g / m²·24h.

[0038] The mass ratio of the main components of the nanoclay modified layer is sodium montmorillonite: hexadecyltrimethylammonium bromide = 2.8:1~3.2:1.

[0039] Preferably, the mass ratio of the main components of the nanoclay-modified layer is sodium montmorillonite: hexadecyltrimethylammonium bromide = 3:1. To verify the salt-blocking effects of different mass ratios of sodium montmorillonite to hexadecyltrimethylammonium bromide, further control experiments were conducted. The experimental results show that the optimal salt-blocking effect is achieved when the mass ratio is 3:1; when the mass ratio is within the range of 2.8:1 to 3.2:1, 98% of the optimal salt-blocking effect is achieved. Exceeding this range deviates from the original design intent of the "three-dimensional nano-barrier structure," resulting in a performance degradation of ≥30%, significantly affecting salt-blocking performance. Therefore, in actual processes, the mass ratio of sodium montmorillonite to hexadecyltrimethylammonium bromide can be controlled within the range of 2.8:1 to 3.2:1.

[0040] The anti-UV coating is made of high-performance polyurethane material and forms a uniform protective layer through a precision coating process. The coating has excellent anti-aging properties and can effectively resist the influence of environmental factors such as ultraviolet radiation and temperature changes, thereby extending the service life of the composite geomembrane in outdoor use environments to more than 20 years.

[0041] Through synergistic effects, this three-layer composite structure not only ensures the mechanical properties of the material, but also achieves excellent barrier function and durability, enabling it to adapt to engineering application requirements under various complex environmental conditions.

[0042] The preparation method of the water-proof and salt-blocking composite geomembrane of the present invention comprises the following steps:

[0043] S1. Pretreatment of high-density polyethylene substrate;

[0044] High-fluidity HDPE was selected as the substrate, and the HDPE was crystallized and dried under vacuum conditions at 80°C for 4 hours. After low-temperature crushing, the particles were sieved to a particle size of 0.5-1.0 mm.

[0045] S2. Functional modification of nanoclay;

[0046] At 80 ° C, sodium montmorillonite was ion exchanged with hexadecyltrimethylammonium bromide (CTAB) at a mass ratio of 3:1. The reaction time was 6 hours, and then centrifuged. After centrifugation, the precipitate was washed with ethanol until there was no Br.- Detection; the cleaned precipitate was placed in a 2wt% γ-methacryloxypropyltrimethoxysilane (KH570) ethanol solution, and ultrasonically treated for 2 hours at pH = 4.5, with a power of 300W and a frequency of 40kHz to obtain modified clay. The modified clay was dried in a vacuum oven at 120°C for 24 hours, and after drying, passed through a 400-mesh sieve to finally obtain the modified clay.

[0047] KH570 is used as a functional modifier for nanoclay (sodium montmorillonite), achieving organic modification of the clay through chemical grafting reaction.

[0048] The precipitate after centrifugation is the CTAB-modified clay. The modified clay is ultrasonically treated in a KH570 ethanol solution to obtain a KH570-modified clay suspension, which is then dried and sieved to obtain the final modified clay.

[0049] S3. Premixing and masterbatch preparation;

[0050] S301. Speedy mechanical premixing

[0051] To accommodate varying production conditions, a double-cone mixer premixes 85-91wt% high-density polyethylene (HDPE), 8-12wt% nanoclay, and 1-3wt% maleic anhydride-grafted polyethylene (MAH-PE) in a premix. When clay prices are high, a lower limit (8%) can be used to reduce costs while maintaining basic performance. For coastal saline environments, an upper limit (12% clay + 2.5% MAH-PE) can be used to enhance salt resistance. When the MAH-PE grafting ratio fluctuates, adjusting the dosage (±0.5%) ensures interfacial adhesion.

[0052] Preferably, 88 wt% of HDPE, 10 wt% of nanoclay, and 2 wt% of maleic anhydride grafted polyethylene (MAH-PE) are premixed at 56° C. and 200 rpm for 15 minutes to obtain a mixture.

[0053] S302. Extrusion granulation

[0054] The obtained mixture was extruded through a single screw pre-extruder (L / D=25) with a temperature gradient of: 160°C in the feeding section, 175°C in the compression section, and 170°C in the die head to prepare a premixed masterbatch.

[0055] S4. Melt blending process;

[0056] The resulting molten HDPE / clay mixture contains nanoclay dispersed in the HDPE matrix in the form of intercalated or partially exfoliated layers.

[0057] S401. Equipment Configuration

[0058] Co-rotating twin-screw extruder (screw diameter D=65mm, L / D=48);

[0059] The combined screw includes a feeding section, a melting section, a shearing section, a mixing section and a vacuum devolatilization section. The feeding section is mainly responsible for the initial conveying and compression of the material. The melting section realizes complete melting of the matrix before the shearing section. The shearing section adopts kneading blocks and reverse screw elements, and the mixing section adopts helical toothed disc elements. The shearing section, mixing section and vacuum devolatilization section correspond to the functions of high shear dispersion, melt homogenization and removal of volatiles respectively.

[0060] S402. Temperature field control

[0061] Table 1

[0062] Segment Temperature range (℃) Functional goals Feeding section 150-160 Prevent thermal degradation of raw materials Melting section 180-185 Achieve complete melting of the matrix High shear section 190-195 Promotes clay exfoliation Mixing section 185-190 Maintain melt stability die head 175-180 Avoid melt fracture

[0063] S403. Kinetic parameter control

[0064] Screw speed: 250rpm-350rpm. The screw speed of 250rpm-350rpm is converted into the shear rate range of 800s actually experienced by the melt through the screw design. -1 -1500s- 1 , which corresponds to the local shear rate experienced by the melt in the shear section of the screw.

[0065] Vacuum level: -0.095 MPa. The purpose of setting the vacuum level to -0.095 MPa is to remove volatiles (such as residual solvents and low-molecular-weight substances) and bubbles generated during the melt blending process. When HDPE and nanoclay are blended, volatiles and bubbles can cause porosity or interfacial defects in the finished product, affecting barrier properties and mechanical strength.

[0066] Melt Pressure: 10MPa-15MPa. HDPE melt has high viscosity, requiring higher pressure to drive the melt through the die and ensure uniform extrusion. Closed-loop control of melt pressure via a die pressure sensor allows real-time adjustment of screw speed or feed rate to avoid uneven thickness or melt fracture caused by pressure fluctuations.

[0067] Specific Mechanical Energy (SME): 0.25kWh / kg-0.35kWh / kg. Specific mechanical energy refers to the mechanical energy consumed per unit mass of material during the processing process. This parameter is the core indicator for evaluating the energy consumption and dispersion efficiency of the extrusion process.

[0068] During the melt blending process, the anhydride groups (-CO-O-CO-) of MAH-g-HDPE react with the hydroxyl groups (-OH) on the surface of nanoclay to form an esterification reaction (as shown in Equation 1):

[0069] Clay-OH+MAH-g-HDPE→Clay-O-CO-R+H2O (1)

[0070] MAH-g-HDPE, short for maleic anhydride grafted high-density polyethylene, is a pre-grafted modified HDPE material added in the premixing step. Its function is achieved through chemical reactions during the melt blending process, ultimately improving the interfacial and barrier properties of the composite material.

[0071] This reaction forms a covalent bond, significantly enhancing the interfacial bonding between the clay and HDPE. The functionality of MAH-g-HDPE is achieved through chemical reactions during the melt blending process, ultimately improving the composite's interfacial and barrier properties.

[0072] After the nanoclay is modified with KH570 silane, its methoxy group (-OCH3) hydrolyzes to form silanol (-Si-OH), which condenses with the hydroxyl groups on the clay surface to form Si-O-Si bonds. At the same time, the retained methacryloyloxy group (CH2=C(CH3)COO-) is entangled with the HDPE chain segment through van der Waals forces, achieving molecular-level interface strengthening.

[0073] Organic modification expands the interlayer spacing of clay sheets from 1.2nm to 3.5-4.0nm. HDPE molecular chains are inserted into the interlayers through melt shear force, forming an intercalated structure. When the shear stress exceeds the critical value, the clay sheets are peeled off into a single layer or a few layers with a specific surface area of ​​750m² / g, producing the following effects: the tortuosity of the water molecule permeation path increases by 200 times; the negative charge on the clay surface adsorbs Na through ion-dipole interaction. + 、Cl - Plasma.

[0074] S5. Nano-dispersion enhancement, optimized dispersion state, HDPE / clay mixture is still in molten state;

[0075] S501. Multi-stage shear field design

[0076] The primary dispersion generates an extensional flow field through reverse threaded elements, the secondary dispersion uses 45° staggered kneading blocks to form local pressure oscillations, and the tertiary dispersion introduces chaotic convection in the mixing section.

[0077] S502 in situ intercalation control.

[0078] By dynamically adjusting the screw assembly, the interlayer spacing of the clay flakes was expanded from the original 1.2 nm to 3.8 nm-4.2 nm (verified by XRD); the complex viscosity was controlled within the range of 800 Pa·s-1200 Pa·s through online rheometer monitoring.

[0079] S6. Molding and structural locking;

[0080] S601. Multi-layer coextrusion die design

[0081] A coat-hanger-type wide-width die head is used, a static mixer is set in the flow channel, and the die lip gap is 0.8mm-1.2mm.

[0082] S602. Rapid cooling and shaping process

[0083] The final HDPE / clay composite film is formed by cooling and solidification. A three-roll calendering system is used with a roll temperature gradient of 60°C for the top roll, 40°C for the middle roll, and 25°C for the bottom roll. The cooling rate is >50°C / s and the pulling speed is 5-8m / min.

[0084] S603. Online detection and process control

[0085] Near infrared spectroscopy (NIR) was used to detect clay dispersion online, and ultrasonic attenuation was used to monitor aggregate size in real time.

[0086] S7. Composite structure integration.

[0087] The integration of the three-layer composite structure involves the following steps: corona treatment of the HDPE surface in the composite membrane and application of a polyurethane coating to the HDPE / clay composite membrane. The polyurethane coating is first cured by UV light to form a dense protective layer.

[0088] S701. Interlayer interface strengthening

[0089] The HDPE surface is corona treated to increase surface activity with a power density of 3.5W / cm² and a treatment speed of 10m / min, increasing the surface tension to ≥38mN / m.

[0090] S702. Co-curing with UV-resistant layer

[0091] Applying a polyurethane coating to the surface of the HDPE / clay composite film and then curing it with UV light strengthens the bond with the activated HDPE surface. The wet film thickness is 200-300 μm. UV curing conditions are: wavelength 365 nm, irradiation intensity 1200 mW / cm², and curing time 90 seconds under nitrogen protection.

[0092] The polyurethane coating is a two-component polyurethane, including two raw materials: -NCO prepolymer + polyol curing agent. The two raw materials are mixed before coating, and then a chain extension reaction occurs to form a polyurethane coating.

[0093] -NCO-terminated prepolymer is a polymer intermediate with an isocyanate group (-NCO) as the end group. The polyol curing agent is a compound containing multiple hydroxyl groups (-OH). The polyol curing agent and the -NCO group form a carbamate bond (-NHCOO-) through a chain extension reaction, ultimately forming a polyurethane network structure.

[0094] After the two-component polyurethane is coated, a chain extension reaction occurs (as shown in Formula 2):

[0095] R-NCO+HO-R'→R-NH-CO-O-R'R-NCO+HO-R'→R-NH-CO-O-R' (2)

[0096] The urethane bond energy formed by curing is as high as 326kJ / mol, which is resistant to UV degradation.

[0097] There are narrow channels of about 0.3 nm between clay sheets (smaller than the diameter of hydrated ions: Na + Hydrated diameter 0.72nm, Cl − 0.66 nm), preventing ion migration by size exclusion.

[0098] The glass transition temperature (Tg) of the polyurethane coating is designed to be -40°C. During freeze-thaw cycles, the polyurethane coating absorbs volume deformation energy through the movement of molecular chain segments, thereby inhibiting the propagation of microcracks.

[0099] The freeze-thaw cycle temperature range is -30℃~50℃, which is the anti-freeze test of the later geomembrane. The freeze-thaw cycle test verifies the low-temperature crack resistance and salt expansion resistance of the composite geomembrane by simulating extreme temperature changes.

[0100] Through the synergistic effects of a chemically bonded interface design, nanoscale topological barriers, and energy dissipation mechanisms, the composite geomembrane of this invention achieves, at multiple molecular, micro, and macroscale levels: a water molecule permeation path tortuosity greater than 150 times (seepage prevention); an ion migration activation energy increased to 85 kJ / mol (salt resistance); and a tensile strength retention rate greater than 95% after 500 freeze-thaw cycles (frost heave resistance). This dual chemical and physical protection mechanism ensures over 20 years of durability in harsh environments.

[0101] Example 1

[0102] The one-way freezing test of the soil column with the composite geomembrane of the present invention added under the condition of no pressure water replenishment is as follows: Figure 2 shown.

[0103] ① Sample preparation: prepare a soil sample with a mass ratio of sodium montmorillonite: CTAB of 3:1, weigh the test soil sample, fill it into the mold in 5 layers and compact it layer by layer. After compaction, the height of each layer is 3 cm, 3.5 cm, 4 cm, 3.5 cm, 4 cm from bottom to top, and after the second layer is compacted, add the composite geomembrane of this application to each layer; install the top plate of the sample tube and the displacement sensor, insert the temperature sensor into the temperature measuring holes at 3 cm, 6.5 cm, 10.5 cm, 14 cm, and 18 cm from the bottom, and wrap the sample tube with insulation material to reduce temperature loss.

[0104] Figure 2In the figure, the blue temperature above the specimen represents the temperature applied to the top plate during the test, which is the cold end temperature and simulates the actual surface temperature. The yellow temperature below the specimen represents the temperature applied to the bottom plate during the test, which is the warm end temperature and simulates the actual constant temperature underground.

[0105] ② Initial condition setting: Adjust the freeze-thaw cycle test chamber, precision constant temperature liquid bath and secondary temperature control system so that the temperature of the bottom plate, top plate and ambient temperature of the sample tube reaches 3.8°C. Let it stand for 24 hours to ensure uniform temperature and moisture distribution inside the sample.

[0106] ③ Cooling test: The equipment was adjusted to perform unidirectional gradient cooling, i.e. the freeze-thaw cycle test chamber and the precision constant temperature liquid bath were adjusted to maintain a constant temperature of 3.8°C. The secondary temperature control system adjusted the temperature to -5°C, -10°C, and -15°C in the first, second, and third stages, respectively, and maintained for 15 hours each. The frost heave deformation of the soil and the internal temperature changes were recorded.

[0107] The test was completed after the internal temperature of the soil sample stabilized and the frost heave deformation stabilized. The soil column was removed and samples were taken at the bottom and at locations 3 cm, 6.5 cm, 10.5 cm, 14 cm, and 18 cm from the bottom plate to measure the moisture content and sulfate ion content at different locations.

[0108] Reference Figure 2-6 , by comparing and analyzing the test data of the unidirectional freezing of soil columns under the condition of no-pressure water replenishment of the composite geomembrane of the present invention with the test data of the original soil sample, the results show that the composite geomembrane of the present invention exhibits significant beneficial effects in the prevention and control of salt swelling of saline soil roadbed. Specifically, the composite geomembrane of the present invention has excellent water-proof and salt-blocking properties, can effectively prevent the migration of underground water and salt, and significantly reduce the salt swelling deformation of saline soil roadbed. The test data show that the salt swelling rate is greatly reduced from 6.3% of the original soil sample to 0.017%, which fully demonstrates its water-proof and salt-blocking effect. At the same time, the moisture content above and below the composite geomembrane of the present invention dropped sharply from 22.75% to 10.07%, and the sulfate ion content was also greatly reduced, indicating that it has outstanding salt blocking and salt adsorption effects. Through the physical barrier effect, the composite geomembrane can inhibit the condensation of liquid water and salt crystallization inside the soil, thereby reducing the occurrence of frost heave and salt swelling, and significantly improving the stability and durability of the roadbed. Furthermore, the composite geomembrane of the present invention exhibits excellent thermal insulation properties, reducing the impact of temperature fluctuations on the roadbed. This slows the cooling rate of the soil beneath it, further minimizing damage to the soil structure caused by freeze-thaw cycles. In terms of mechanical properties, the composite geomembrane exhibits excellent tensile and flexural strength, adapting to complex geological conditions and construction environments, ensuring its stability and reliability over long-term use.

[0109] The comparison results of the tensile and flexural properties of the water-proof and salt-blocking composite geomembrane of this application and the traditional geomembrane are shown in Table 2:

[0110] Table 2

[0111]

[0112] Example 2

[0113] In order to verify the salt blocking effect of different mass ratios of sodium montmorillonite and hexadecyltrimethylammonium bromide, further control experiments were conducted, such as Figure 4 、 Figure 7 and Figure 8 As shown in the experimental results, the optimal salt rejection effect is achieved when the mass ratio is 3:1. When the mass ratio is within the range of 2.8:1 to 3.2:1, 98% of the optimal salt rejection effect is achieved. If it exceeds this range, the original design intention of the "three-dimensional nano-barrier structure" will be deviated, resulting in a performance degradation of ≥30%, significantly affecting the salt rejection performance. Therefore, in actual processes, the mass ratio of sodium montmorillonite to hexadecyltrimethylammonium bromide can be controlled within the range of 2.8:1 to 3.2:1.

[0114] In the present invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification, as well as features of different embodiments or examples, unless they are mutually inconsistent.

[0115] Although the above embodiments have been shown and described, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those skilled in the art are all within the scope of protection of the present invention.

Claims

1. A method for preparing a water-proof and salt-blocking composite geomembrane, characterized in that: The water-proof and salt-blocking composite geomembrane comprises a high-density polyethylene substrate, a nanoclay modified layer and an anti-ultraviolet coating from the substrate to the surface. The nanoclay modified layer is obtained by uniformly dispersing nanoclay in the high-density polyethylene substrate through a melt blending process. The mass ratio of the main components of the nanoclay modified layer is sodium montmorillonite: hexadecyltrimethylammonium bromide = 3:

1. The anti-ultraviolet coating is made of high-performance polyurethane material. The preparation method comprises the following steps: S1. High-density polyethylene substrate pretreatment: High-flow HDPE was selected as the substrate. The HDPE substrate was crystallized and dried under vacuum at 80°C for 4 hours. The substrate was then cryogenically crushed and sieved to a particle size of 0.5 mm to 1.0 mm. S2. Functional modification of nanoclay: First, sodium montmorillonite was subjected to ion exchange reaction with hexadecyltrimethylammonium bromide, and then centrifuged. After centrifugation, the precipitate was washed with ethanol until there was no Br. - Detection; the washed precipitate was placed in a 2 wt% γ-methacryloxypropyltrimethoxysilane ethanol solution and ultrasonically treated at pH = 4.5 for 2 hours to obtain a modified clay suspension, and then the clay suspension was dried in a vacuum oven at 120 ° C for 24 hours. After drying, it was passed through a 400 mesh sieve to finally obtain the modified clay; S3 premix and masterbatch preparation: 85wt%-91wt% of high-density polyethylene, 8wt%-12wt% nanoclay and 1wt%-3wt% of maleic anhydride grafted polyethylene were premixed and premixed masterbatch was prepared using a single-screw pre-extruder; S4. Melt blending process: produces a molten HDPE / clay mixture, where the nanoclay is dispersed in the HDPE matrix in the form of intercalated or partially exfoliated layers. The equipment used is a co-rotating twin-screw extruder, which adopts a combined screw, which includes a feeding section, a melting section, a shearing section, a mixing section and a vacuum devolatilization section. The shearing section adopts a kneading block and a reverse screw element, and the mixing section adopts a helical toothed disc element. The shearing section has a high shear dispersion function. S5. Nanodispersion Enhancement: This includes multi-stage shear field design and in-situ intercalation control. The multi-stage shear field design involves primary dispersion using reverse-threaded elements to generate an extensional flow field, secondary dispersion using 45° staggered kneading blocks to create localized pressure oscillations, and tertiary dispersion by introducing chaotic convection in the mixing section. In-situ intercalation control utilizes a dynamically adjustable screw assembly to expand the clay interlayer spacing from its original 1.2 nm to 4.2 nm. S6. Molding: including multi-layer co-extrusion die design, rapid cooling and shaping process, online testing and process control; S7. Composite structure integration: Corona treatment of the HDPE surface in the composite membrane and application of a polyurethane coating on the surface of the HDPE / clay composite membrane.

2. The method for preparing a water-proof and salt-blocking composite geomembrane according to claim 1, wherein: The kinetic parameters in the melt blending process are as follows: screw speed: 250 rpm-350 rpm, vacuum degree: -0.095 MPa, melt pressure: 10 MPa-15 MPa, specific mechanical energy: 0.25 kWh / kg-0.35 kWh / kg, temperature range of the feeding section: 150°C-160°C, temperature range of the melting section: 180°C-185°C, temperature range of the high shear section: 190°C-195°C, temperature range of the mixing section: 185°C-190°C, and temperature range of the die head: 175°C-180°C.

3. A water-barrier and salt-blocking composite geomembrane prepared by the preparation method of the water-barrier and salt-blocking composite geomembrane according to claim 1 or 2, characterized in that: From the substrate to the surface, there are high-density polyethylene substrate, nanoclay modified layer and anti-ultraviolet coating. The nanoclay modified layer is made by uniformly dispersing nanoclay in the high-density polyethylene substrate through a melt blending process. The mass ratio of the main components of the nanoclay modified layer is sodium montmorillonite: hexadecyltrimethylammonium bromide = 3:1; The thickness of the high-density polyethylene substrate is 0.8 mm, the thickness of the nanoclay modified layer is 0.3 mm, and the thickness of the anti-ultraviolet coating is 0.1 mm; The anti-ultraviolet coating is made of high-performance polyurethane material.

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