A waterproof and moisture-permeable bulked polyethylene film and a method for preparing the same

Abstract

The present application relates to a waterproof and moisture-permeable bulked polyethylene film and a preparation method thereof, and belongs to the technical field of high polymer microporous materials. The film has a three-dimensional through nodular fiber network structure, the nodular diameter is 0.8-2 mu m, the fiber diameter is 100-300 nm, the porosity is 78-90%, and the average pore size is 0.05-0.5 mu m. The present application successfully constructs the nodular fiber network structure similar to ePTFE on the polyethylene base material through the improvement of thermal phase separation and step-by-step bidirectional stretching process, so that the film has excellent waterproofness and moisture permeability, and the longitudinal and transverse tensile strengths are both greater than or equal to 22 MPa. The present application has low raw material cost and simple process, and can be widely applied in the fields of protective clothing, medical protection and electronic device protection.

Description

Technical Field

[0001] This invention relates to the field of polymer microporous materials technology, and more particularly to a waterproof and breathable expanded polyethylene film and its preparation method. Background Technology

[0002] Waterproof and breathable membranes are functional polymer materials that can simultaneously block liquid water penetration while allowing gaseous water vapor to pass through. Their performance directly determines the comfort and reliability of end products such as protective clothing, medical protective clothing, outdoor equipment, and precision electronic devices, making them an indispensable core functional layer for these products. With the increasing demands for wearing comfort and equipment reliability in fields such as outdoor sports, medical protection, and consumer electronics, the market demand for high-performance waterproof and breathable membranes continues to grow.

[0003] Currently, commercially available waterproof and breathable membranes mainly fall into the following three categories: Expanded polytetrafluoroethylene (ePTFE) membranes are formed through a mechanical stretching process, creating a unique "node-fiber" microporous structure that achieves a good balance between waterproofness and moisture permeability, while also exhibiting excellent weather resistance and chemical stability. However, the raw material for ePTFE membranes, polytetrafluoroethylene (PTFE), is expensive, the processing technology is demanding, and production costs are high. Furthermore, fluorine-containing materials face difficulties in downstream processing and waste recycling, making it challenging to meet increasingly stringent environmental protection requirements.

[0004] Polyurethane (PU) hydrophilic coating films transfer water molecules through hydrophilic groups on the molecular chain, resulting in excellent initial waterproof performance. However, this type of material has an inherent defect: the ester or ether groups in the polyurethane molecular chain are prone to hydrolysis in humid and hot environments, causing the moisture permeability to decrease significantly with prolonged use, thus limiting its service life.

[0005] Polyolefin microporous membranes, represented by polyethylene (PE), are inexpensive to produce, possess excellent chemical stability and high mechanical strength. Industrially, they are often prepared using thermally induced phase separation (TIPS) technology, which employs a highly thermally stable solvent as a pore-forming agent and involves screw extrusion, casting, extraction, and stretching. However, existing PE microporous membranes often exhibit a disordered, sponge-like microporous structure. This type of membrane has significant drawbacks: excessively large pore sizes result in insufficient waterproofing, while small pore sizes lead to low water vapor permeability, making it difficult to achieve both waterproofing and breathability. Furthermore, current stretching processes struggle to create a balanced molecular orientation in both the longitudinal and transverse directions, resulting in significant anisotropy in tear resistance and tensile strength. This makes them unsuitable for applications requiring high bidirectional mechanical properties, such as protective clothing and electronic device protection.

[0006] Therefore, developing a polyethylene-based waterproof and breathable membrane with controllable microstructure, synergistic waterproof and breathable properties, balanced mechanical properties, and a preparation process adapted to industrial production is of great technical significance and has broad market application prospects. Summary of the Invention

[0007] To address the aforementioned problems in the existing technology, this invention provides a waterproof and breathable expanded polyethylene membrane and its preparation method. Through a combination of thermally induced phase separation and stepwise biaxial stretching processes, an expanded polyethylene membrane with a nodular-fiber network structure is prepared, achieving a good balance between waterproofness, breathability, and mechanical properties. Furthermore, the process is simple, low-cost, and suitable for large-scale production.

[0008] The objective of this invention can be achieved through the following technical solutions: In a first aspect, a waterproof and breathable expanded polyethylene membrane is provided, wherein the expanded polyethylene membrane has a three-dimensional interconnected nodule-fiber network structure; the nodules are polyethylene crystalline aggregates with a nodule diameter of 0.8 μm to 2 μm; the fibers are nanoscale oriented fibrils formed by biaxial stretching and have a fiber diameter of 100 nm to 300 nm, and the fibers overlap in multiple directions within the membrane surface to form a three-dimensional network; the porosity of the expanded polyethylene membrane is 78% to 90%, and the average pore size is 0.05 μm to 0.5 μm.

[0009] Scanning electron microscopy revealed that the microstructure consisted of discretely distributed polyethylene nodules and oriented fibers connecting adjacent nodules.

[0010] Furthermore, the expanded polyethylene film has a water contact angle of 130°–150°, a hydrostatic pressure of 150–185 kPa, and a moisture permeability of 8200–9500 g / (m³). 2 (24h), longitudinal tensile strength and transverse tensile strength are both ≥22 MPa.

[0011] Furthermore, the substrate of the expanded polyethylene film is one or a blend of two of high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE).

[0012] Furthermore, the weight-average molecular weight of the ultra-high molecular weight polyethylene is 1.6 × 10⁻⁶. 6 g / mol~5×10 6 The weight-average molecular weight of the high-density polyethylene is 3.5 × 10 g / mol. 5 g / mol~5×10 5 g / mol.

[0013] Secondly, a method for preparing a waterproof and breathable expanded polyethylene film as described in the first aspect is provided, comprising the following steps: (1) Raw material blending and screw extrusion casting: Polyethylene resin and solvent are mixed evenly at a mass ratio of 1:(1~4) and fed into a twin-screw extruder for melt blending; the extrusion temperature is controlled at 160℃~220℃, and the melt is extruded through the die and then cast to the cooling roller for cooling and shaping to obtain polyethylene castings with a thickness of 0.5 mm~2 mm. (2) Extraction and deoiling: Immerse the cast sheet obtained in step (1) in the extractant to remove the solvent pore-forming agent inside the cast sheet. After extraction, dry the cast sheet to completely remove the residual extractant and obtain a dry cast sheet. (3) Biaxial stretching: The dry casting obtained in step (2) is preheated to 100℃~130℃ for 10~20min. A step-by-step biaxial stretching process is adopted, first longitudinal stretching and then transverse stretching. (4) Heat setting and winding: The film after biaxial stretching in step (3) is placed at 110℃~135℃ for heat setting for 1~10 min, cooled and wound up to obtain a waterproof and breathable expanded polyethylene film.

[0014] This invention uses polyethylene as a matrix and a solvent as a pore-forming agent. A homogeneous system is formed through twin-screw melt blending. After cooling and casting, the solvent is uniformly dispersed in the polyethylene matrix as a microphase region. After extraction, the solvent is removed, and an initial microporous structure is formed inside the matrix. During stepwise biaxial stretching, the polyethylene flakes break and oriented. Some of the thicker flake stacked regions remain as micron-sized nodules. The broken molecular chains recrystallize under stress to form nano-sized oriented fibers. The fibers and nodules overlap to construct a three-dimensional network structure. This structure has both suitable pore size and through-pores, preventing liquid water from penetrating while allowing water vapor to pass through rapidly. At the same time, the biaxial orientation structure significantly improves the biaxial mechanical properties of the membrane.

[0015] Furthermore, in step (1), when the polyethylene resin is a blend of ultra-high molecular weight polyethylene and high-density polyethylene, the proportion of ultra-high molecular weight polyethylene is 20% to 80% by mass.

[0016] Further, in step (1), the solvent is No. 20 white oil, with a kinematic viscosity of 20 mm at 40°C. 2 / s.

[0017] Further, in step (2), the extractant is one or more of n-hexane, cyclohexane, and methyl ethyl ketone, the extraction temperature is 20℃~60℃, and the extraction time is 10 min~60 min.

[0018] Further, in step (3), the longitudinal stretching temperature is 100℃~125℃, the stretching ratio is 3~8 times, and the stretching rate is 50 mm / s~200 mm / s; the transverse stretching temperature is 110℃~130℃, the stretching ratio is 3~10 times, and the stretching rate is 50 mm / s~200 mm / s.

[0019] Furthermore, in step (4), the membrane tension during the heat setting process is 5 N / m to 20 N / m, and the heat setting temperature is 5°C to 10°C higher than the transverse stretching temperature in step (3), in order to eliminate residual stress in the membrane and stabilize the micropore and fiber structure.

[0020] The beneficial effects of this invention are as follows: (1) Controllable microstructure. This invention combines improvements to thermally induced phase separation and stepwise biaxial stretching processes. Initial micropores are formed by extraction with low-viscosity white oil, followed by stepwise biaxial stretching to cause polyethylene flakes to break and oriented. Some of the incompletely broken flake stacking areas are retained as micron-sized nodules. The broken molecular chains recrystallize under stress to form nano-sized oriented fibers, which overlap to construct a three-dimensional interconnected nodule-fiber network structure. This structure has uniform fiber and nodule distribution and good micropore connectivity, solving the problem of the sponge-like disordered structure of traditional polyethylene microporous membranes.

[0021] (2) Excellent waterproof and breathable properties. The expanded polyethylene membrane prepared by this invention has a three-dimensional interconnected nodular-fiber network structure with an average pore size of 0.05 μm to 0.5 μm. On the one hand, water vapor molecules can freely pass through the interconnected micropores, achieving efficient moisture permeability. In Example 1, the moisture permeability was 8200 g / (m³). 2 • 24h); On the other hand, liquid water droplets, due to their size being much larger than the pore size and being hindered by surface tension, cannot penetrate the membrane. Example 1: hydrostatic pressure 185 kPa. The two work together to achieve a good balance between waterproofing and moisture permeability in the polyethylene system.

[0022] (3) Excellent mechanical properties. This invention employs a stepwise biaxial stretching process, which orients the polyethylene molecular chains in both directions. The stepwise biaxial stretching causes the molecular chains to orient sequentially in the longitudinal and transverse directions, ultimately resulting in the fibers overlapping in multiple directions within the membrane to form a three-dimensional network. The longitudinal and transverse biaxial tensile strengths of the membrane are simultaneously improved, resulting in strong tear resistance. In Example 1, the longitudinal strength is 32 MPa and the transverse strength is 28 MPa, which is significantly better than that of the uniaxially stretched membrane. In Comparative Example 1, the transverse strength is only 8 MPa. At the same time, the subsequent heat setting process, through heat treatment at a temperature 5℃~10℃ higher than the stretching temperature, effectively eliminates residual stress in the membrane, fixes the microporous structure, and ensures good dimensional stability of the membrane, making it less prone to shrinkage and deformation during long-term use.

[0023] (4) Strong industrial adaptability. The polyethylene and white oil used in this invention are far cheaper than fluorinated polytetrafluoroethylene raw materials. The entire process can be realized using existing mature equipment. The process route is simple and can be directly applied to large-scale industrial production. It has a wide range of applications. Attached Figure Description

[0024] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0025] Figure 1 Scanning electron microscope (SEM) image of the expanded polyethylene film prepared for an embodiment of the present invention; Figure 2 The pore size distribution diagram of the expanded polyethylene film prepared for the embodiments of the present invention shows that pores with an average pore size of 0.190 micrometers (190 nanometers) account for 64.6%; Figure 3 The image shows the scanning electron microscope (SEM) morphology of the expanded polyethylene film of Comparative Example 9 of the present invention, which was preheated for 25 min by dry casting. Figure 4 The image shows the scanning electron microscope (SEM) morphology of the expanded polyethylene film with a stretching ratio of 5×5, which is Comparative Example 5 of the present invention. Figure 5 The image shows the scanning electron microscope (SEM) morphology of the expanded polyethylene film, which was stretched twice at different stretching ratios (first 2×2, then 5×5) as a comparative example of the present invention. Detailed Implementation

[0026] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided.

[0027] Example 1 (1) Raw material ratio: Ultra-high molecular weight polyethylene (weight average molecular weight 3×10 6 g / mol) and high-density polyethylene (weight-average molecular weight 4×10⁻⁶ g / mol) 5 The mixed polyethylene resin was prepared by mixing the poly(ethylene) resin (g / mol) at a mass ratio of 1:1; the mixed polyethylene resin and solvent were then mixed evenly at a mass ratio of 1:3 to obtain the mixture.

[0028] (2) Extrusion casting: The mixture is fed into a twin-screw extruder for melt blending. The extrusion temperature is 180℃~210℃, the die temperature is 200℃, and the melt is cast to a 25℃ cooling roller to obtain a polyethylene casting with a thickness of 1 mm.

[0029] (3) Extraction and deoiling: Immerse the polyethylene casting in n-hexane to remove the solvent pore-forming agent inside the casting. The extraction temperature is 30℃ and the extraction time is 30 min. After extraction, place it in a 60℃ oven to dry for 30 min to remove residual solvent and obtain a dried casting.

[0030] (4) Biaxial stretching: The dried casting is preheated to 115°C for 17 minutes. A step-by-step biaxial stretching process is adopted. First, longitudinal stretching is carried out at 115°C, with a stretching ratio of 5 times and a stretching rate of 100 mm / s. Then, transverse stretching is carried out at 120°C, with a stretching ratio of 6 times and a stretching rate of 100 mm / s.

[0031] (5) Heat setting and winding: The biaxially stretched film is placed at 125℃ for 5 minutes for heat setting. During the heat setting process, the film tension is controlled at 10 N / m. After cooling and winding, the finished waterproof and breathable expanded polyethylene film is obtained.

[0032] Example 2 (1) Raw material ratio: single ultra-high molecular weight polyethylene (weight average molecular weight 4×10 6 The mixture (g / mol) and solvent are mixed evenly at a mass ratio of 1:4 to obtain the mixture.

[0033] (2) Extrusion casting: The mixture is fed into a twin-screw extruder for melt blending. The extrusion temperature is 190℃~220℃, the die temperature is 200℃, and the melt is cast to a 25℃ cooling roller to obtain a polyethylene casting with a thickness of 1.5mm.

[0034] (3) Extraction and deoiling: Immerse the polyethylene casting in cyclohexane at an extraction temperature of 40°C for 40 min; after extraction, dry the casting in a 60°C oven for 30 min to obtain a dried casting.

[0035] (4) Biaxial stretching: The dried casting is preheated to 120°C for 15 minutes. A step-by-step biaxial stretching process is adopted. First, longitudinal stretching is carried out at 115°C, with a stretching ratio of 6 times and a stretching rate of 100 mm / s. Then, transverse stretching is carried out at 125°C, with a stretching ratio of 8 times and a stretching rate of 100 mm / s.

[0036] (5) Heat setting and winding: The biaxially stretched membrane is placed at 130℃ for 8 minutes for heat setting. During the heat setting process, the membrane tension is controlled at 10 N / m. After cooling and winding, the finished waterproof and breathable expanded polyethylene membrane is obtained.

[0037] Example 3 (1) Raw material ratio: Ultra-high molecular weight polyethylene (weight average molecular weight 2×10 6g / mol) and high-density polyethylene (weight-average molecular weight 5×10 5 The mixed polyethylene resin was prepared by compounding the poly(ethylene) resin (g / mol) at a mass ratio of 3:7; the mixed polyethylene resin and solvent were then mixed evenly at a mass ratio of 1:2 to obtain the mixture.

[0038] (2) Extrusion casting: The mixture is fed into a twin-screw extruder for melt blending. The extrusion temperature is 180℃~210℃, the die temperature is 200℃, and the melt is cast to a 25℃ cooling roller to obtain a polyethylene casting with a thickness of 1 mm.

[0039] (3) Extraction and deoiling: Immerse the polyethylene casting in n-hexane at an extraction temperature of 35°C for 25 min; after extraction, place it in a 60°C oven to dry for 30 min to remove residual solvent and obtain a dried casting.

[0040] (4) Biaxial stretching: The dried casting is preheated to 130℃ for 12 minutes. A step-by-step biaxial stretching process is adopted. First, longitudinal stretching is carried out at 120℃, stretching ratio is 7 times, and stretching rate is 150 mm / s. Then, transverse stretching is carried out at 120℃, stretching ratio is 7 times, and stretching rate is 80 mm / s.

[0041] (5) Heat setting and winding: The biaxially stretched film is placed at 125℃ for 7 minutes for heat setting. During the heat setting process, the film tension is controlled at 8 N / m. After cooling and winding, the finished waterproof and breathable expanded polyethylene film is obtained.

[0042] Comparative Example 1 Based on Example 1, the film is directly wound up after unidirectional longitudinal stretching without transverse stretching or heat setting. The rest is the same as in Example 1.

[0043] Comparative Example 2 Based on Example 1, replace No. 20 white oil with No. 26 white oil (whose kinematic viscosity at 40°C is 24-28 mm). 2 / s), the rest is the same as in Example 1.

[0044] Comparative Example 3 Based on Example 1, No. 20 white oil was replaced with No. 15 white oil (kinematic viscosity 13.5-16.5 mm at 40°C). 2 / s), the rest is the same as in Example 1.

[0045] Comparative Example 4 Based on Example 1, a process flow of stretching followed by extraction was adopted, with the rest being the same as in Example 1.

[0046] Comparative Example 5 Based on Example 1, in the step-by-step biaxial stretching process, the longitudinal stretching ratio is 5 times; the transverse stretching ratio is 5 times, and the rest is the same as in Example 1.

[0047] Comparative Example 6 Based on Example 1, in the step-by-step biaxial stretching process, the longitudinal stretching ratio is 10 times; the transverse stretching ratio is 12 times, and the rest is the same as in Example 1.

[0048] Comparative Example 7 Based on Example 1, in the stepwise biaxial stretching process, the longitudinal stretching rate is 30 mm / s; the transverse stretching rate is 30 mm / s, and the rest is the same as in Example 1.

[0049] Comparative Example 8 Based on Example 1, in the step-by-step biaxial stretching process, the longitudinal stretching rate is 250 mm / s; the transverse stretching rate is 250 mm / s, and the rest is the same as in Example 1.

[0050] Comparative Example 9 Based on Example 1, the preheating time for the dry casting sheet is 25 minutes, and the rest is the same as in Example 1.

[0051] Comparative Example 10 Based on Example 1, the preheating time for the dry casting sheet is 7 minutes, and the rest is the same as in Example 1.

[0052] Comparative Example 11 Based on Example 1, the heat setting temperature is 15°C higher than the transverse stretching temperature, i.e., the heat setting temperature is 135°C, and the rest is the same as in Example 1.

[0053] Comparative Example 12 Based on Example 1, the heat setting temperature is 5°C lower than the transverse stretching temperature, i.e., the heat setting temperature is 115°C, and the rest is the same as in Example 1.

[0054] Comparative Example 13 Based on Example 1, no heat setting treatment is performed, and the rest is the same as in Example 1.

[0055] Comparative Example 14 Based on Example 1, the step-by-step biaxial stretching was replaced with biaxial synchronous stretching, the stretching temperature was 120°C, and the stretching ratio was 5 times in the longitudinal direction and 6 times in the transverse direction. The rest was the same as in Example 1.

[0056] Comparative Example 15 A waterproof and breathable membrane was prepared using the method described in Example 1 of Chinese Patent CN111619083A.

[0057] Comparative Example 16 Based on Example 1, the molecular weight of UHMWPE was adjusted to 800,000 and the molecular weight of HDPE was adjusted to 200,000 (the two were compounded in a mass ratio of 1:1), and the rest was the same as in Example 1.

[0058] Comparative Example 17 Based on Example 1, UHMWPE and HDPE were compounded at a mass ratio of 15:85 (i.e., UHMWPE accounted for 15%), and the rest was the same as in Example 1.

[0059] Comparative Example 18 Based on Example 1, UHMWPE and HDPE were compounded at a mass ratio of 90:10 (i.e., UHMWPE accounted for 90%), and the rest was the same as in Example 1.

[0060] Comparative Example 19 Based on Example 1, the stretching ratio is increased in two stages: first stretching to 2×2, then stretching to 5×5, with the rest being the same as in Example 1.

[0061] Performance testing I. Testing Methods (1) Nodule diameter and fiber diameter The microstructure of the film was observed using a JEM JSM-IT510 scanning electron microscope, with images acquired at magnifications of 5,000× and 10,000×. Measurements were performed using image analysis software as follows: Nodule diameter: Select a nodule with a complete shape and clear boundaries in the SEM image, outline the nodule edge, and calculate the equivalent circle diameter with the same projected area as the nodule using software.

[0062] Fiber diameter: Referring to GB / T 36422-2018 standard, select continuous and complete oriented fibrils in the SEM image, and measure the diameter perpendicular to the fiber axis at the middle section of the fiber.

[0063] (2) Air permeability The test was conducted according to the Gurley method in GB / T 458-2008, defined as 100 mL of gas passing through 25.4 mm of air at a pressure of 275.79 kPa. 2 The time required for the membrane to expand to the desired area.

[0064] (3) Porosity According to the national standard GB / T 36363-2018, the porosity is calculated using the following two formulas:

[0065]

[0066] Wherein, ρ1 (g / cm 3) is the membrane surface density, m (g) is the membrane mass, L (cm) is the membrane length, b (cm) is the membrane width, ε (%) is the membrane porosity, d (μm) is the membrane thickness, and ρ0 (g / cm²) is the membrane thickness. 3 ( ) is the density of the raw material; (4) Aperture According to the national standard GB / T 32361-2015, the pore size is calculated using the bubble point method and the following formula:

[0067] Where D (μm) is the pore diameter, γ (mN / m) is the liquid surface tension, θ is the liquid contact angle, and p (Psi) is the gas pressure.

[0068] (5) Tensile strength The test shall be conducted in accordance with GB / T 1040.3-2006, using a type 2 specimen with a width of (15±0.1) mm, an initial distance of (100±5) mm between the fixtures, and a test speed of (250±10) mm / min.

[0069] (6) Hydrostatic pressure refers to GB / T 4744-2013 standard; water contact angle refers to GB / T 30693-2014 standard; tear strength refers to QB / T 1130-1991 standard.

[0070] II. Test Results Table 1 Performance test results of Examples 1-3

[0071] Table 2-1 Performance test results for comparative examples 1-7

[0072] Table 2-2 Performance test results for Comparative Example 8-13

[0073] Table 2-3 Performance test results for comparative examples 14-19

[0074] Table 1 Figure 1 and Figure 2 The results showed that the waterproof and breathable expanded polyethylene membranes prepared in Examples 1-3 had a nodular-fiber three-dimensional network structure, with a porosity between 78% and 85%, an average pore size between 0.05 μm and 0.5 μm, a water contact angle of 130° to 150°, a hydrostatic pressure of 150 to 185 kPa, and a moisture permeability of 8200 to 9500 g / (m³). 2The tensile strength in both the longitudinal and transverse directions was ≥22 MPa after 24 hours. These results demonstrate that the expanded polyethylene film prepared by this invention achieves a synergistic balance of excellent waterproof performance, moisture permeability, and mechanical properties, based on controllable microstructure.

[0075] Tables 2-1, 2-2, and 2-3 and Figures 3-5 The results showed that Comparative Example 1, which did not undergo transverse stretching and heat setting treatment, had a transverse tensile strength of 8 MPa, a hydrostatic pressure of 90 kPa, and a moisture permeability of 4500 g / (m³). 2 • 24h). Comparative Example 2 used No. 68 high-viscosity white oil with a porosity of 55%, a hydrostatic pressure of 60 kPa, and a moisture permeability of 3200 g / (m³). 2 • 24h). Comparative Example 3 used No. 15 low-viscosity white oil, and although its performance was better than Comparative Example 2, it was still inferior to Example 1. Comparative Example 4 was subjected to stretching followed by extraction, and its transverse strength was 9 MPa, hydrostatic pressure was 85 kPa, and moisture permeability was 4200 g / m³. 2 • 24h. Comparative Example 5, with a stretch ratio of 5×5, failed to form a nodular-fiber network, had an average pore size of 0.25μm, and a moisture permeability of 5800g / (m²). 2 • 24h). Comparative Example 6: stretch ratio 10×12, hydrostatic pressure 120kPa, moisture permeability 4800g / m³. 2 • 24h. Comparative Example 7: Tensile rate 30mm / s, transverse strength 18MPa. Comparative Example 8: Tensile rate 250mm / s, hydrostatic pressure 100kPa. Comparative Example 9: Preheated for 25min, unable to form a uniform nodule-fiber network, hydrostatic pressure 110kPa, moisture permeability 5600g / (m²). 2 • 24h); Comparative Example 10 was preheated for 7min, which was insufficient, resulting in fine fibers with small pores. Comparative Example 11 had a heat-setting temperature difference of 15℃, causing fiber embrittlement and a moisture permeability of 4800g / (m²). 2 (24h). Comparative Example 12: Heat-setting temperature difference less than 5℃, transverse strength 18MPa. Comparative Example 13: No heat-setting, microporous structure shrinkage, transverse strength 10MPa. Comparative Example 14: After bidirectional synchronous stretching instead of step stretching, average pore size 1.20μm, hydrostatic pressure 100kPa. Comparative Example 15: Its product is a non-nodular-fiber network, average pore size 1.35μm. Comparative Example 16: UHMWPE molecular weight reduced to 800,000. Comparative Examples 17 and 18: Too low or too high UHMWPE ratio prevents the formation of an ideal nodular-fiber network.

[0076] In summary, this invention uses low-viscosity white oil as a solvent to ensure that the initial micropores are small and uniformly distributed after extraction. The process sequence of extraction followed by stretching avoids interference from the solvent on the molecular chain orientation. Stepwise bidirectional stretching with strict control of the stretching ratio and rate ensures that the molecular chains are fully oriented in both the longitudinal and transverse directions, forming a three-dimensional network of interlocking fibers and nodules. Precise control of preheating time and heat-setting temperature difference eliminates residual stress within the membrane and fixes the micropore morphology. Through the synergistic effect of the above techniques, the expanded polyethylene membrane prepared by this invention possesses a nodule-fiber three-dimensional network structure, achieving excellent waterproof and breathable properties and balanced mechanical properties. Furthermore, it features low raw material costs and process compatibility with existing equipment, enabling large-scale applications in fields such as protective clothing, medical protective equipment, and electronic device protection.

[0077] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A waterproof and breathable expanded polyethylene film, characterized in that, The expanded polyethylene film has a three-dimensional interconnected nodule-fiber network structure; the nodules are polyethylene crystalline aggregates with a diameter of 0.8 μm to 2 μm; the fibers are oriented fibrils with a diameter of 100 nm to 300 nm; the expanded polyethylene film has a porosity of 78% to 90% and an average pore size of 0.05 μm to 0.5 μm.

2. The waterproof and breathable expanded polyethylene membrane according to claim 1, characterized in that, The expanded polyethylene film has a water contact angle of 130°–150°, a hydrostatic pressure of 150–185 kPa, and a moisture permeability of 8200–9500 g / (m²). (24h), longitudinal tensile strength and transverse tensile strength are both ≥22 MPa.

3. The waterproof and breathable expanded polyethylene membrane according to claim 1, characterized in that, The substrate of the expanded polyethylene film is one or a blend of high-density polyethylene and ultra-high molecular weight polyethylene.

4. The waterproof and breathable expanded polyethylene membrane according to claim 3, characterized in that, The ultra-high molecular weight polyethylene has a weight-average molecular weight of 1.6 × 10⁻⁶. 6 g / mol~5×10 6 The weight-average molecular weight of the high-density polyethylene is 3.5 × 10 g / mol. 5 g / mol~5×10 5 g / mol.

5. A method for preparing a waterproof and breathable expanded polyethylene film as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Raw material blending and extrusion casting: Polyethylene resin and solvent are mixed at a mass ratio of 1:(1~4), melt-blended and cast in a twin-screw extruder, and cooled to obtain castings with a thickness of 0.5 mm to 2 mm; (2) Extraction and deoiling: Immerse the cast sheet obtained in step (1) in the extractant, and after extraction, dry it to obtain a dry cast sheet; (3) Biaxial stretching: The dry casting obtained in step (2) is preheated to 100℃~130℃ for 10min~20min. A step-by-step biaxial stretching process is adopted, first longitudinal stretching and then transverse stretching. (4) Heat setting and winding: The biaxially stretched film in step (3) is heat set at 110℃~135℃ for 1 min~10 min, cooled and wound up to obtain a waterproof and breathable expanded polyethylene film.

6. The method for preparing the waterproof and breathable expanded polyethylene film according to claim 5, characterized in that, In step (1), when the polyethylene resin is a blend of ultra-high molecular weight polyethylene and high-density polyethylene, the proportion of ultra-high molecular weight polyethylene is 20% to 80% by mass.

7. The method for preparing the waterproof and breathable expanded polyethylene film according to claim 5, characterized in that, In step (1), the solvent is No. 20 white oil, which has a kinematic viscosity of 20 mm² / s at 40°C.

8. The method for preparing the waterproof and breathable expanded polyethylene film according to claim 5, characterized in that, In step (2), the extractant is one or more of n-hexane, cyclohexane, and methyl ethyl ketone, the extraction temperature is 20℃~60℃, and the extraction time is 10 min~60 min.

9. The method for preparing the waterproof and breathable expanded polyethylene film according to claim 5, characterized in that, In step (3), the longitudinal stretching temperature is 100℃~125℃, the stretching ratio is 3~8 times, and the stretching rate is 50 mm / s~200 mm / s; the transverse stretching temperature is 110℃~130℃, the stretching ratio is 3~10 times, and the stretching rate is 50 mm / s~200 mm / s.

10. The method for preparing the waterproof and breathable expanded polyethylene film according to claim 5, characterized in that, In step (4), the film tension during the heat setting process is 5 N / m to 20 N / m, and the heat setting temperature is 5°C to 10°C higher than the transverse stretching temperature in step (3).

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