Water-resisting and salt-resisting composite geomembrane and preparation method thereof

Abstract

The invention discloses a water-resisting and salt-resisting composite geomembrane and a preparation method thereof, and the water-resisting and salt-resisting composite geomembrane sequentially comprises a high-density polyethylene base material, a nano clay modified layer and an anti-ultraviolet coating from the base material to the surface. The HDPE base material is used as a core structure layer, the high tensile strength and tear resistance of the HDPE base material are fully exerted, the geomembrane is endowed with excellent mechanical bearing capacity, and soil deformation, external loads and construction abrasion in engineering application can be resisted; nano clay is uniformly dispersed in an HDPE base material through a melt blending process to form a three-dimensional nano barrier structure, so that the water vapor permeability is greatly reduced, the barrier efficiency is improved by 80% or above compared with that of a traditional geomembrane, and permeation migration of moisture, salt ions and harmful substances is effectively inhibited; the surface polyurethane coating obviously reduces the photooxidative degradation rate by absorbing and reflecting ultraviolet rays; through the synergistic effect of the coating and the base material, the material keeps flexibility in the extreme temperature range of-50 DEG C to 80 DEG C, and failure caused by low-temperature brittle rupture or high-temperature softening is avoided.

Description

Technical Field

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

[0002] The widely distributed saline soil in the northwest region and its special engineering geological conditions and climate conditions lead to significant coupling effects among multiple physical fields (such as temperature field, moisture field, salt field, and mechanical field) inside the soil mass, thereby triggering various engineering diseases. During the large-scale cooling process in the cold season, the temperature is transmitted from the ground surface downward, and the interaction among the temperature field, moisture field, salt field, and mechanical field inside the soil mass intensifies, resulting in a significant enhancement of the water-salt migration effect and salt expansion deformation effect. Under the action of low temperature, the liquid water inside the soil mass freezes into ice, and at the same time, the salt crystallizes, causing different degrees of frost heave and salt expansion diseases in the subgrade soil. These problems seriously affect the service performance of the infrastructure, making it unable to meet the actual operation requirements, shortening the service life, and increasing potential safety hazards. Summary of the Invention

[0003] In order to solve the above problems, the invention proposes a water-proof and salt-blocking composite geomembrane and a preparation method thereof. Through the physical barrier effect of the water-proof and salt-blocking composite geomembrane, the water-salt migration and salt expansion deformation of the saline soil subgrade are effectively inhibited, thereby improving the stability and durability of the subgrade. Through the synergistic optimization design of the high-density polyethylene (HDPE) substrate, nano-clay modified layer, and anti-ultraviolet coating, a breakthrough improvement has been achieved in mechanical properties, environmental barrier properties, and durability, with significant engineering application value.

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

[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 nano-clay modified layer is obtained by uniformly dispersing nano-clay in the high-density polyethylene substrate through a melt blending process. The mass ratio of the main components of the nano-clay modified layer is sodium-based montmorillonite: cetyltrimethylammonium bromide = 2.8:1 to 3.2:1.

[0007] The anti-ultraviolet coating uses a high-performance polyurethane material.

[0008] The preparation method of the water-proof and salt-blocking composite geomembrane of the invention comprises the following steps: S1. Pretreatment of the high-density polyethylene substrate; S2. Functional modification of nano-clay; S3. Premixing and masterbatch preparation; S4. Melt blending process; S5. Nano-dispersion strengthening; S6. Molding; S7. Composite structure integration.

[0009] The pretreatment of the high-density polyethylene substrate refers to subjecting the high-density polyethylene to substrate crystallization drying under vacuum conditions at 80°C for 4 hours, followed by low-temperature pulverization and screening to particles with a particle size of 0.5 - 1.0 mm.

[0010] The specific steps for the functional modification of the nano-clay are as follows: First, the sodium-based montmorillonite is subjected to an ion exchange reaction with cetyltrimethylammonium bromide at 80°C for 6 hours, followed by centrifugation. After centrifugation, the precipitate is washed with ethanol until no Br - is detected; the washed precipitate is placed in a 2 wt% ethanol solution of γ-methacryloxypropyltrimethoxysilane, and ultrasonically treated for 2 hours under the condition of pH = 4.5 to obtain a modified clay suspension. Then, the clay suspension is dried in a vacuum oven at 120°C for 24 hours, and after drying, it is sieved through a 400-mesh sieve to finally obtain the modified clay.

[0011] The premixing refers to premixing 85 wt% - 91 wt% of high-density polyethylene, 8 wt% - 12 wt% of nano-clay, and 1 wt% - 3 wt% of maleic anhydride-grafted polyethylene by mass percentage.

[0012] The kinetic parameters in the melt blending process are respectively: 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, temperature range of the die head: 175°C - 180°C.

[0013] The composite structure integration includes corona treatment of the surface of the high-density polyethylene in the composite film and coating with a polyurethane coating.

[0014] The beneficial effects of the present invention are as follows: 1. Taking the HDPE substrate as the core structural layer, giving full play to its high tensile strength and tear resistance, endowing the geomembrane with excellent mechanical load-bearing capacity, and being able to 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.

[0015] 2. The nano-clay is uniformly dispersed in the HDPE substrate through a melt blending process to form a three-dimensional nano-barrier structure, which significantly reduces the water vapor transmission rate (≤0.1 g / m²·24 h), and the barrier efficiency is increased by more than 80% compared with traditional geomembranes, effectively inhibiting the permeation and migration of moisture, salt ions and harmful substances. In cold region projects, this structure can block the frost heaving damage caused by capillary water rise and salt crystallization; in saline soil areas, it can prevent material corrosion and the exacerbation of soil salinization caused by salt penetration.

[0016] 3. The surface polyurethane coating absorbs and reflects ultraviolet rays (the shielding rate of the UV-A / UV-B band ≥ 95%, UV-A (long-wave ultraviolet rays): the wavelength range is 315–400 nm, with strong penetration, which can cause photo-oxidative aging of materials. UV-B (medium-wave ultraviolet rays): the wavelength range is 280–315 nm, with higher energy, which is easy to cause surface degradation of materials.), significantly reducing the photo-oxidative degradation rate, and extending the service life of the material under outdoor exposure conditions to more than 20 years, which is 2-3 times longer than that of conventional geomembranes. The synergistic effect between the coating and the substrate enables the material to maintain flexibility within the extreme temperature range of -50°C to 80°C, avoiding failures caused by low-temperature embrittlement or high-temperature softening. The dense surface coating can prevent microbial attachment and root penetration, and is suitable for scenarios vulnerable to biological erosion such as landfills and ecological slopes. The three-layer composite design avoids the complexity of layered laying of materials in traditional multi-layer impermeable systems, reduces the risk of joint leakage, and improves the construction efficiency by more than 30%. The long-life characteristics reduce the replacement frequency. Combined with the comprehensive functions of anti-seepage, anti-salt and anti-frost heaving, the engineering maintenance cost can be reduced by more than 50%, especially suitable for harsh environments such as alpine regions, coastal saline areas, and arid areas with strong ultraviolet rays.

[0017] 4. The nano-clay modified layer uses natural mineral materials, reducing the dosage of petroleum-based additives and reducing carbon emissions during production; the polyurethane coating can be degradably modified, reducing the risk of environmental residues. By controlling the permeability balance, the local ecological water replenishment needs (such as moisture regulation of vegetation roots) can be taken into account in anti-seepage projects, promoting the coordinated development of the project and the environment. Through the integrated innovation of material-structure-function, it provides an efficient, economical and sustainable technical solution to solve the performance short board of geomembranes in complex environments, with significant social, economic and ecological benefits. Description of the Drawings

[0018] Figure 1 It is a schematic structural diagram of the water-proof and salt-blocking composite geomembrane of the present invention.

[0019] Figure 2 It is a diagram of the unidirectional freezing process of the soil column under the condition of non-pressure water replenishment of the present invention.

[0020] Figure 3It is the moisture content distribution map after the unidirectional freezing test of the soil column under the condition of unpressurized water supply for the composite geomembrane of the present invention.

[0021] Figure 4 It is the sulfate ion content distribution map after the unidirectional freezing test of the soil column under the condition of unpressurized water supply with different mass ratios of sodium-based montmorillonite: CTAB.

[0022] Figure 5 It is the temperature and salt expansion amount change map after the unidirectional freezing test of the undisturbed soil sample under the condition of unpressurized water supply.

[0023] Figure 6 It is the temperature and salt expansion amount change map after the unidirectional freezing test of the soil column under the condition of unpressurized water supply for the composite geomembrane of the present invention.

[0024] Figure 7 It is the temperature and salt expansion amount change map after the unidirectional freezing test of the soil column under the condition of unpressurized water supply when the mass ratio of sodium-based montmorillonite: CTAB is 3.2:1.

[0025] Figure 8 It is the temperature and salt expansion amount change map after the unidirectional freezing test of the soil column under the condition of unpressurized water supply when the mass ratio of sodium-based montmorillonite: CTAB is 2.8:1. Detailed implementation manners

[0026] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the drawings. The embodiments described below by referring to the drawings are exemplary and are intended to explain the present invention, and should not be construed as a limitation to the present invention.

[0027] For the water-proof and salt-blocking composite geomembrane of the present invention, from the base material to the surface, there are a high-density polyethylene base material (HDPE), a nano-clay modified layer, and an anti-ultraviolet coating in sequence. Among them, the HDPE base material, as the main structural layer, has excellent mechanical strength and chemical stability, providing basic support for the composite geomembrane.

[0028] As Figure 1 shown, in the water-proof and salt-blocking composite geomembrane of the present invention, the thickness of the high-density polyethylene base material 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.

[0029] The nano-clay modified layer is formed by uniformly dispersing nano-clay in the high-density polyethylene base material through a melt blending process to form a dense nano-scale barrier network structure. This unique structural design not only significantly improves the barrier performance of the material, but also effectively prevents the penetration of moisture and salt, and its water vapor transmission rate can be reduced to less than 0.1 g / m²·24h.

[0030] The mass ratio of the main components of the nano-clay modified layer is sodium-based montmorillonite : cetyltrimethylammonium bromide = 2.8:1 to 3.2:1.

[0031] Preferably, the mass ratio of the main components of the nano-clay modified layer is sodium-based montmorillonite : cetyltrimethylammonium bromide = 3:1. To verify the salt-blocking effects of different mass ratios of sodium-based montmorillonite and cetyltrimethylammonium bromide, a control experiment was further conducted. According to the experimental results, when the mass ratio is 3:1, the best salt-blocking effect is achieved; when the mass ratio is within the range of 2.8:1 to 3.2:1, 98% of the best salt-blocking effect can be achieved; if it exceeds this range, it will deviate from the original design intention of the "three-dimensional nano-barrier structure", resulting in a performance attenuation of ≥30%, significantly affecting the salt-blocking performance. Therefore, in the actual process, the mass ratio of sodium-based montmorillonite and cetyltrimethylammonium bromide can be controlled within the range of 2.8:1 to 3.2:1.

[0032] The anti-ultraviolet coating uses a high-performance polyurethane material and forms a uniform protective layer through a precision coating process. This coating has excellent anti-aging performance and can effectively resist the influence of environmental factors such as ultraviolet radiation and temperature changes, extending the service life of the composite geomembrane in outdoor use environments to more than 20 years.

[0033] Through the synergistic effect, this three-layer composite structure not only ensures the mechanical properties of the material but also realizes excellent barrier functions and durability, enabling it to meet the engineering application requirements under various complex environmental conditions.

[0034] The preparation method of the water-blocking and salt-blocking composite geomembrane of the present invention includes the following steps: S1. Pretreatment of the high-density polyethylene substrate; Select high-flowability HDPE as the substrate, crystallize and dry the HDPE under vacuum conditions at 80°C for 4 hours, and after low-temperature crushing, screen it to particles with a particle size of 0.5 - 1.0 mm.

[0035] S2. Functional modification of nano-clay; At 80°C, ion exchange is carried out between sodium-based montmorillonite and cetyltrimethylammonium bromide (CTAB), the mass ratio of sodium-based montmorillonite : CTAB is 3:1, the reaction time is 6 hours, and then centrifugation is carried out. After centrifugation, the precipitate is washed with ethanol until no Br - is detected; the washed precipitate is put into a 2wt% ethanol solution of γ-methacryloxypropyltrimethoxysilane (KH570), and ultrasonic treatment is carried out for 2 hours at pH = 4.5, with a power of 300 W and a frequency of 40 kHz to obtain modified clay. The modified clay is dried in a vacuum oven at 120°C for 24 hours, and after drying, it is sieved through a 400-mesh sieve to finally obtain modified clay.

[0036] KH570 is used as a functional modifier for nano-clay (sodium-based montmorillonite), and the organic modification of the clay is achieved through a chemical grafting reaction.

[0037] The precipitate after centrifugation is the clay modified by CTAB. After the modified clay is ultrasonically treated in a KH570 ethanol solution, a KH570-modified clay suspension is obtained. Then, the clay suspension is dried and sieved to obtain the final modified clay.

[0038] S3. Premixing and masterbatch preparation; S301. Quick mechanical premixing Using a double-cone mixer, to cope with different production conditions, 85wt%-91wt% of high-density polyethylene, 8wt%-12wt% of nano-clay, and 1wt%-3wt% of maleic anhydride-grafted polyethylene are premixed. When the price of the clay is high, the lower limit (8%) can be adopted to reduce costs while still maintaining the basic performance. For the coastal saline environment, the upper limit (12% clay + 2.5% MAH-PE) can be adopted to enhance the salt resistance. When the grafting rate of MAH-PE fluctuates, the interfacial bonding force can be ensured by adjusting the dosage (±0.5%).

[0039] Preferably, 88wt% of HDPE + 10wt% of nano-clay + 2wt% of maleic anhydride-grafted polyethylene (MAH-PE) are premixed at 56°C and a rotation speed of 200 rpm for 15 minutes to obtain a mixture.

[0040] S302. Extrusion granulation The obtained mixture is used in a single-screw pre-extruder (L / D = 25), and a premixed masterbatch is prepared through a temperature gradient: the feeding section at 160°C, the compression section at 175°C, and the die head at 170°C.

[0041] S4. Melt blending process; A molten HDPE / clay mixture is produced, and at this time, the nano-clay is dispersed in the HDPE matrix in an intercalated or partially exfoliated form.

[0042] S401. Equipment configuration Co-rotating twin-screw extruder (screw diameter D = 65 mm, L / D = 48); 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 preliminary transportation and compression of the materials. The melting section completely melts the matrix before the shearing section. The shearing section uses kneading blocks and reverse-thread elements, and the mixing section uses helical disk elements. The shearing section, the mixing section, and the vacuum devolatilization section respectively correspond to the functions of high-shear dispersion, melt homogenization, and removal of volatile components.

[0043] S402. Temperature field control Table 1 Section Temperature range (°C) Functional objective 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 Promote clay platelet exfoliation Mixing section 185-190 Maintain melt stability Die head 175-180 Avoid melt fracture S403. Kinetic Parameter Control Screw speed: 250 rpm - 350 rpm. The screw speed of 250 rpm - 350 rpm is converted into the actual shear rate range experienced by the melt through screw design, which is 800 s -1 -1500 s- 1 , and this shear rate corresponds to the local shear rate experienced by the melt in the screw shear section.

[0044] Vacuum degree: -0.095 MPa. The purpose of setting the vacuum degree to -0.095 MPa is to remove volatile components (such as residual solvents, low-molecular-weight substances) and bubbles generated during the melt blending process. Because when HDPE is blended with nano-clay, volatile components and bubbles will cause pores or interfacial defects in the finished product, affecting the barrier properties and mechanical strength.

[0045] Melt pressure: 10 MPa - 15 MPa. Since the HDPE melt has a high viscosity, a higher pressure is required to drive the melt through the die head and ensure uniform extrusion. By using a closed-loop control of the melt pressure through a die head pressure sensor, the screw speed or feeding rate can be adjusted in real time to avoid thickness unevenness or melt fracture caused by pressure fluctuations.

[0046] Specific Mechanical Energy (SME): 0.25 kWh / kg - 0.35 kWh / kg. Specific Mechanical Energy refers to the mechanical energy consumed per unit mass of the material during the processing. This parameter is the core index for evaluating the energy consumption and dispersion efficiency of the extrusion process.

[0047] During the melt blending process, the anhydride group (-CO-O-CO-) of MAH-g-HDPE reacts with the hydroxyl group (-OH) on the surface of nano-clay in an esterification reaction (as shown in Equation 1): Clay-OH + MAH-g-HDPE → Clay-O-CO-R + H2O (1) MAH-g-HDPE is short for maleic anhydride grafted high density polyethylene, which is a pre-grafted and modified HDPE material added in the premixing step. Its function is realized through chemical reactions during the melt blending process, ultimately improving the interfacial properties and barrier properties of the composite material.

[0048] This reaction forms a covalent bond connection, significantly enhancing the interfacial binding force between clay and HDPE. The function of MAH-g-HDPE is realized through chemical reactions during the melt blending process, ultimately improving the interfacial properties and barrier properties of the composite material.

[0049] After the nano-clay is modified by 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 remaining methacryloyloxy groups (CH2=C(CH3)COO-) are wound around the HDPE chain segments through van der Waals forces to achieve molecular-level interfacial strengthening.

[0050] The organophilic modification expands the clay interlayer spacing from 1.2 nm to 3.5 - 4.0 nm. The HDPE molecular chains are inserted into the interlayer through melt shear force to form an intercalated structure. When the shear stress exceeds the critical value, the clay sheets are exfoliated into single-layer or few-layer structures, with a specific surface area of 750 m² / g, resulting in the following effects: the tortuosity of the water molecule penetration path increases by 200 times; the negative charges on the clay surface adsorb Na + 、Cl - and other plasma ions through ion-dipole interactions.

[0051] S5. Nano-dispersion strengthening to optimize the dispersion state, and the HDPE / clay mixture remains in a molten state; S501. Multi-stage shear field design The primary dispersion generates a tensile flow field through a reverse-thread element, the secondary dispersion uses a 45° staggered kneading block to form local pressure oscillations, and the tertiary dispersion introduces chaotic convection in the mixing section.

[0052] S502 In-situ intercalation control. Dynamically adjust the screw combination to expand the clay interlayer spacing from the original 1.2 nm to 3.8 nm - 4.2 nm (verified by XRD); monitor through an on-line rheometer and control the complex viscosity in the range of 800 Pa·s - 1200 Pa·s.

[0053] S6. Molding and structure locking; S601. Multi-layer coextrusion die head design Adopt a hanger-type wide-width die head, set a static mixer in the runner, and the die lip gap is 0.8 mm - 1.2 mm.

[0054] S602. Quenching and setting process Form the final HDPE / clay composite film through cooling and solidification. Adopt a three-roll calendering system, with the roll temperature gradient: upper roll 60 °C / middle roll 40 °C / lower roll 25 °C, cooling rate: > 50 °C / s, and traction speed: 5 m / min - 8 m / min.

[0055] S603. On-line detection and process control On-line detect the clay dispersion degree by near-infrared spectroscopy (NIR), and monitor the aggregate size in real time by ultrasonic attenuation method.

[0056] S7. Composite structure integration.

[0057] Complete the integration of the three-layer composite structure, mainly including the following steps: corona treat the surface of HDPE in the composite film and coat a polyurethane coating on the surface of the HDPE / clay composite film. The polyurethane coating is first cured by ultraviolet light to form a dense protective layer.

[0058] S701. Interlayer interface strengthening Corona treat the surface of HDPE to improve surface activity, with a power density of 3.5 W / cm², a treatment speed of 10 m / min, and the surface tension is increased to ≥38 mN / m.

[0059] S702. Co-cure with the ultraviolet-resistant layer Coat a polyurethane coating on the surface of the HDPE / clay composite film. After coating, it is cured by ultraviolet light, which can bind more firmly to the activated HDPE surface. The wet film thickness is 200 μm - 300 μm, and the ultraviolet light curing conditions are: wavelength 365 nm, irradiation intensity 1200 mW / cm², and the curing time is 90 seconds under nitrogen protection.

[0060] 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 will occur to form a polyurethane coating.

[0061] The -NCO-terminated prepolymer is a polymer intermediate with isocyanate groups (-NCO) as the end groups, and the polyol curing agent is a compound containing multiple hydroxyl groups (-OH). The polyol curing agent reacts with the -NCO groups through a chain extension reaction to generate urethane bonds (-NHCOO-), and finally forms a polyurethane network structure.

[0062] After the two-component polyurethane is coated, a chain extension reaction occurs (as shown in Equation 2): R-NCO + HO-R’ → R-NH-CO-O-R’ R-NCO + HO-R’ → R-NH-CO-O-R’ (2) The urethane bonds formed by curing have an energy as high as 326 kJ / mol, which can resist ultraviolet light degradation.

[0063] There are slit pores of about 0.3 nm between the clay platelets (less than the hydrated ion diameter: Na + hydrated diameter 0.72 nm, Cl − 0.66 nm), and ion migration is blocked by size exclusion.

[0064] The glass transition temperature Tg of the polyurethane coating is designed to be -40 °C, and the volume deformation energy is absorbed through the movement of molecular segments during freeze-thaw cycles to inhibit the propagation of microcracks.

[0065] The freeze-thaw cycle temperature range is -30°C to 50°C, which is the anti-freeze test of the geomembrane in the later stage. The freeze-thaw cycle test verifies the low-temperature crack resistance and anti-salt expansion performance of the composite geomembrane by simulating extreme temperature changes.

[0066] Through the synergistic effect of interface design dominated by chemical bonding, nano-scale topological barrier, and energy dissipation mechanism, the composite geomembrane of the present invention achieves, at the molecular-micro-macro multi-scale: the tortuosity of the water molecule penetration path > 150 times (seepage prevention); the activation energy of ion migration is increased to 85 kJ / mol (salt resistance); the tensile strength retention rate > 95% after 500 freeze-thaw cycles (anti-freeze swelling). This chemical-physical dual protection mechanism enables it to achieve durability guarantee of more than 20 years in harsh environments.

[0067] Example 1

[0068] Unidirectional freezing test of soil columns with the composite geomembrane of the present invention added under the condition of unpressurized water supply, as Figure 2 shown.

[0069] ① Specimen preparation: Prepare soil samples with a mass ratio of sodium-based montmorillonite to CTAB of 3:1. Weigh the test soil samples, fill them into the mold in 5 layers and compact them layer by layer. After compaction, the height of each layer from bottom to top is 3 cm, 3.5 cm, 4 cm, 3.5 cm, 4 cm, and start adding the composite geomembrane of this application from the second layer after compaction; install the top plate of the specimen cylinder and displacement sensors, insert temperature sensors into the temperature measurement holes at 3 cm, 6.5 cm, 10.5 cm, 14 cm, and 18 cm from the bottom end, and wrap the specimen cylinder with heat insulation materials to reduce temperature loss.

[0070] Figure 2 In, the blue temperature above the specimen represents the temperature applied by the top plate during the test, that is, the cold-end temperature, simulating the surface temperature in the actual situation. The yellow temperature below the specimen represents the temperature applied by the bottom plate during the test, that is, the warm-end temperature, simulating the underground constant temperature in the actual situation.

[0071] ② Initial condition setting: Adjust the freeze-thaw cycle test chamber, precision constant temperature liquid bath, and secondary temperature control system to make the bottom plate, top plate, and ambient temperature of the specimen cylinder all reach 3.8°C, and let it stand for 24 hours to make the internal temperature and moisture of the specimen evenly distributed.

[0072] ③ Cooling test: Adjust the equipment for unidirectional gradient cooling, that is, adjust the freeze-thaw cycle test chamber and precision constant temperature liquid bath to keep the temperature at 3.8°C unchanged. The secondary temperature control system controls the temperature at -5°C, -10°C, and -15°C respectively in the first stage, second stage, and third stage, and each stage is maintained for 15 hours, and record the frost heave deformation of the soil body and the internal temperature change.

[0073] After the internal temperature of the soil sample stabilizes and the frost heave deformation amount stabilizes, the test is completed. Take out the soil column and take samples at the bottom and at 3 cm, 6.5 cm, 10.5 cm, 14 cm, and 18 cm from the bottom plate to measure the water content and sulfate ion content at different positions.

[0074] Refer to Figure 2 - 6 , by comparing and analyzing the test data of the composite geomembrane of the present invention in the unidirectional freezing test of the soil column under the condition of unpressurized water replenishment with the test data of the undisturbed soil sample, the results show that: the composite geomembrane of the present invention shows significant beneficial effects in preventing salt heaving of saline soil subgrade. Specifically, the composite geomembrane of the present invention has excellent water and salt barrier performance, can effectively prevent the migration of underground water and salt, and significantly reduce the salt heaving deformation of the saline soil subgrade. The test data show that the salt heaving rate is greatly reduced from 6.3% of the undisturbed soil sample to 0.017%, fully proving its water and salt barrier effect. At the same time, the water content above and below the composite geomembrane of the present invention drops suddenly from 22.75% to 10.07%, and the sulfate ion content also drops significantly, indicating its outstanding salt barrier and salt adsorption effects. Through physical barrier action, the composite geomembrane can inhibit the condensation of liquid water and the crystallization of salt inside the soil body, thereby reducing the occurrence of frost heave and salt heave, and significantly improving the stability and durability of the subgrade. In addition, the composite geomembrane of the present invention has good heat insulation performance, can reduce the influence of temperature change on the subgrade, and the cooling rate of the soil body below it slows down, further reducing the damage of freeze-thaw cycle to the soil structure. In terms of mechanical properties, the composite geomembrane has excellent tensile and flexural resistance properties, can adapt to complex geological conditions and construction environments, and ensure its stability and reliability in long-term use.

[0075] The comparison results of the tensile and flexural resistance properties between the water and salt barrier composite geomembrane of this application and the traditional geomembrane are shown in Table 2: Table 2

[0076] Example 2

[0077] To verify the salt barrier effect of different mass ratios of sodium montmorillonite and cetyltrimethylammonium bromide, a control experiment was further carried out, as shown in Figure 4 , Figure 7 and Figure 8 . According to the experimental results, when the mass ratio is 3:1, the best salt barrier effect is achieved; when the mass ratio is in the range of 2.8:1 to 3.2:1, 98% of the best salt barrier effect can be achieved; if it exceeds this range, it will deviate from the original design intention of the "three-dimensional nano-barrier structure", resulting in a performance attenuation of ≥30%, significantly affecting the salt barrier performance. Therefore, in the actual process, the mass ratio of sodium montmorillonite to cetyltrimethylammonium bromide can be controlled in the range of 2.8:1 to 3.2:1.

[0078] In the present invention, terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in connection 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 one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0079] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Variations, modifications, substitutions and alterations made by those of ordinary skill in the art to the above embodiments are all within the scope of protection of the present invention.

Claims

1. A water and salt barrier composite geomembrane, characterized in that, From the substrate to the surface, there are a high-density polyethylene substrate, a nano-clay modified layer, and an anti-ultraviolet coating in sequence. The nano-clay modified layer is obtained by uniformly dispersing nano-clay in the high-density polyethylene substrate through a melt blending process. The mass ratio of the main components of the nano-clay modified layer is sodium montmorillonite: cetyltrimethylammonium bromide = 2.8:1 to 3.2:

1.

2. The water-blocking and salt-barrier composite geomembrane according to claim 1, wherein 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.

3. The water-blocking and salt-barrier composite geomembrane according to claim 1, wherein, The anti-ultraviolet coating uses a high-performance polyurethane material.

4. A method for preparing a water-blocking and salt-barrier composite geomembrane according to any one of claims 1-3, characterized in that, It includes the following steps: S1. Pretreatment of the high-density polyethylene substrate; S2. Functional modification of nano-clay; S3. Premixing and masterbatch preparation; S4. Melt blending process; S5. Nano-dispersion strengthening; S6. Molding; S7. Composite structure integration.

5. The preparation method of the water-proof and salt-proof composite geomembrane according to claim 4, wherein, The pretreatment of the high-density polyethylene substrate means that the high-density polyethylene is crystallized and dried for 4 hours under vacuum conditions at 80°C, and then screened to particles with a particle size of 0.5 mm - 1.0 mm after low-temperature pulverization.

6. The preparation method of the water-blocking and salt-barrier composite geomembrane according to claim 4, characterized in that, The specific steps of the functional modification of nano-clay are as follows: First, an ion exchange reaction is carried out between sodium montmorillonite and cetyltrimethylammonium bromide, and then centrifugation is performed. After centrifugation, the precipitate is washed with ethanol until no Br⁻ is detected; the washed precipitate is put into a 2 wt% ethanol solution of γ-methacryloxypropyltrimethoxysilane, and ultrasonic treatment is carried out for 2 hours under the condition of pH = 4.5 to obtain a modified clay suspension. Then, the clay suspension is dried in a vacuum oven at 120°C for 24 hours, and after drying, it is sieved through a 400-mesh sieve to finally obtain modified clay.

7. The preparation method of the water-proof and salt-blocking composite geomembrane according to claim 4, characterized in that, The premixing refers to premixing 85 wt% - 91 wt% of high-density polyethylene, 8 wt% - 12 wt% of nano-clay, and 1 wt% - 3 wt% of maleic anhydride-grafted polyethylene by mass percentage.

8. The preparation method of the water-blocking and salt-barrier composite geomembrane according to claim 4, characterized in that, The kinetic parameters in the melt blending process are respectively: 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, temperature range of the die head: 175°C - 180°C.

9. The preparation method of the water-proof and salt-resistant composite geomembrane according to claim 4, characterized in that, The composite structure integration includes: corona treatment of the surface of the high-density polyethylene in the composite film and coating a polyurethane coating on the surface of the high-density polyethylene / clay composite film.

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