Polytetrafluoroethylene microporous membrane as well as preparation method and application thereof

Abstract

The invention discloses a polytetrafluoroethylene microporous membrane as well as a preparation method and application thereof, and belongs to the technical field of polytetrafluoroethylene microporous membranes. According to the invention, the polytetrafluoroethylene microporous membrane added with different amounts and different types of lubricants is prepared by a thermomechanical two-way stretching method, and the pore structure is researched. According to the invention, the entanglement density, the average pore size, the porosity, the hydrophobic property and the mechanical property can be regulated and controlled by changing the quantity and the type of the lubricant. And a new idea is provided for preparing the high-performance PTFE biaxially oriented film, and the method has a good application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of polytetrafluoroethylene microporous membranes, and particularly relates to a polytetrafluoroethylene microporous membrane and a preparation method and application thereof. Background Art

[0002] Polytetrafluoroethylene (PTFE) is a unique material whose exceptional properties have garnered significant interest among researchers. PTFE, due to its high electronegativity and low polarizability, exhibits a small van der Waals radius, strong C—F bonds, and possesses superior toughness, chemical stability, and hydrophobicity. These excellent properties have led to its widespread application in a wide range of applications and industrial production. PTFE membranes are a novel membrane material with broad application prospects in gas-liquid separation, biomedicine, and new energy.

[0003] PTFE is infusible and insoluble, making it impractical for melt extrusion and stretching, as with polypropylene and polyethylene, or for phase inversion methods using polymer casting solutions, two common membrane-making methods. PTFE microporous films were initially prepared using porogen-based pore formation, which can be further divided into polymer decomposition and extraction. The former involves blending easily degradable polymers with polytetrafluoroethylene resin, heating the mixture above the decomposition temperature of the added polymer to cause cleavage and form pores within the membrane. The latter involves adding a porogen, such as NaCl, to polytetrafluoroethylene, pressing the mixture into a film, and then extracting the porogen to produce the microporous material. However, the PTFE porous materials produced by these two processes suffer from poor uniformity, low porosity, and low strength, as well as complex processing and high pollution levels. Compared to the porogen-based pore formation method, the high-temperature stretching pore formation method developed by Gore & Gore is simpler to prepare, pollution-free, and produces microporous products with uniform pore size distribution, relatively high porosity, and strength, leading to its widespread use. The production of polytetrafluoroethylene porous membranes by biaxial stretching using polytetrafluoroethylene fine powder is a commercial method with technical significance and suitable for mass production. In short, biaxially stretched PTFE porous membranes are produced by a dual operation consisting of a parallel and subsequent second stretch perpendicular to the first stretch. The typical preparation process for the production of polytetrafluoroethylene porous membranes by industrial mechanical stretching is a multi-step operation process in which relatively fine polytetrafluoroethylene powder is evenly dispersed in an oil lubricant (such as a hydrocarbon) as a processing aid and processed into a paste. This step is crucial because it is easy to manufacture and can reduce the extrusion force required to produce a uniform PTFE porous membrane. The resulting paste is then pressed into a mold to form a cylindrical billet without voids, which is then extruded into a sheet or rod shape using a punch press. The extrudate is then passed through two heated roller mills several times to achieve a precise thickness. The lubricant is then completely vaporized by heating, and the film is longitudinally stretched at high temperatures. After heat treatment, a polytetrafluoroethylene porous membrane is finally formed to fix the membrane morphology. The PTFE membrane formed under this process is composed of billions of tiny continuous fibers. These fibers are highly oriented along the direction of stretching operation and connected to each other by nodes, forming pores in the porous PTFE membrane between adjacent fibers.

[0004] To date, many efforts have been made to improve the PTFE membrane structure by optimizing the preparation process, but there are currently no reports in this field on the effects of lubricants on the hydrophobicity and mechanical properties of PTFE microporous membranes. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the purpose of the present invention is to provide a polytetrafluoroethylene microporous membrane and a preparation method and use thereof.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a polytetrafluoroethylene microporous membrane, which is prepared from polytetrafluoroethylene resin and lubricant as raw materials; the mass ratio of the lubricant to the polytetrafluoroethylene resin is 1: (0.5-5), the molecular weight of the polytetrafluoroethylene resin is 1 million to 2 million, the lubricant is an isoparaffin solvent, and the isoparaffin is C 10-20 Isoalkanes.

[0008] Furthermore, the mass ratio of the lubricant to the polytetrafluoroethylene resin is 1:(2.3-4), and the isoparaffin is C 11-16 Isoalkanes.

[0009] Furthermore, the mass ratio of the lubricant to the polytetrafluoroethylene resin is 1:2.3, 1:3 or 1:4, and the isoparaffin is C 11 、C 12 、C 13 or C 16 Isoalkanes.

[0010] The present invention also provides a method for preparing the above-mentioned polytetrafluoroethylene microporous membrane, the method comprising the following steps:

[0011] (1) Mix polytetrafluoroethylene resin and lubricant evenly;

[0012] (2) standing and ripening to obtain a paste;

[0013] (3) pressing the paste, rolling, and drying;

[0014] (4) Asynchronous biaxial stretching and sintering to obtain the product.

[0015] Furthermore, the mixing temperature in step (1) is ≤19°C.

[0016] Furthermore, the standing temperature in step (2) is 10 to 80° C., and the standing time is 2 to 20 hours.

[0017] Furthermore, the standing temperature is 40° C., and the standing time is 8 hours.

[0018] Furthermore, in step (3), the temperature for rolling is 30-100° C., the temperature for drying is 10-80° C., and the time for drying is 5-30 hours.

[0019] Furthermore, in step (3), the rolling temperature is 60° C., the drying temperature is 50° C., and the drying time is 12 h.

[0020] Furthermore, the asynchronous biaxial stretching in step (4) is first preheated for the first time, with a stretching ratio of 300% to 500% and a stretching rate of 80% / s to 120% / s for the first stretching; and then preheated for the second time, with a stretching ratio of 500% to 700% and a stretching rate of 80% / s to 120% / s for the second stretching; the temperature of the first preheating is 80°C to 120°C, and the temperature of the second preheating is 100°C to 150°C.

[0021] Furthermore, the asynchronous biaxial stretching in step (4) is to first perform a first stretching at a stretching ratio of 400% and a stretching rate of 100% / s, and then perform a second stretching at a stretching ratio of 600% and a stretching rate of 100% / s, the first preheating temperature is 100°C, and the second preheating temperature is 115°C.

[0022] Furthermore, the sintering temperature in step (4) is 100-600° C., and the sintering time is 1-60 min.

[0023] Furthermore, the sintering temperature is 340° C., and the sintering time is 10 min.

[0024] The present invention also provides uses of the polytetrafluoroethylene microporous membrane in the fields of gas-liquid separation, biomedicine, and new energy.

[0025] The present invention has achieved the following beneficial effects:

[0026] This paper uses a thermomechanical biaxial stretching method to prepare polytetrafluoroethylene (PTFE) microporous membranes containing varying amounts and types of lubricants. The pore structure is then studied, and the concept of entanglement density is introduced to clarify the role of the lubricant in the fibrillation process. By varying the amount and type of lubricant, the present invention can control the entanglement density, average pore size, porosity, and mechanical properties. This provides a new approach for the preparation of high-performance biaxially stretched PTFE membranes and has promising application prospects.

[0027] For the PTFE microporous membrane samples with different types of lubricants added, when the lubricant is Isopar G, the water contact angle of the obtained sample P2 (25) is the smallest and the hydrophilicity is the best; when the lubricant is iso-chain, the water contact angle of the obtained sample P1 (25) is the largest and the hydrophobicity is the best.

[0028] Among the PTFE microporous membranes prepared by the present invention, P1 (20) has the best elastic modulus; for the PTFE microporous membranes with different lubricant addition amounts, P1 (30) has the largest elongation at break and the best toughness; for the PTFE microporous membranes with different types of lubricants added, when the lubricant is Isopar M, the obtained P4 (25) has the largest elongation at break and the best toughness.

[0029] Definitions of terms used in the present invention: Unless otherwise stated, the initial definitions provided for groups or terms in this document apply to the groups or terms throughout the specification; for terms that are not specifically defined herein, they should be given the meaning that a person skilled in the art would give them based on the disclosure and context.

[0030] The minimum and maximum carbon atom content in a hydrocarbon group is indicated by a prefix, for example, the prefix C a-b Alkyl refers to any alkyl group containing from "a" to "b" carbon atoms. For example, C 10-15 Alkyl refers to straight or branched chain alkyl groups containing 10, 11, 12, 13, 14 or 15 carbon atoms, and so on.

[0031] Obviously, based on the above contents of the present invention, according to common technical knowledge and customary means in this field, without departing from the above basic technical ideas of the present invention, other various forms of modifications, replacements or changes can be made.

[0032] The following further describes the above content of the present invention in detail through specific embodiments in the form of examples. However, this should not be construed as limiting the scope of the above subject matter of the present invention to the following examples. All technologies implemented based on the above content of the present invention fall within the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The relationship between the loss modulus and frequency of PTFE membranes with different amounts (a) and different types of lubricants (b), and the entanglement density of PTFE membranes with different amounts (c) and different types of lubricants (d).

[0034] Figure 2 The first melting curve of the PTFE membrane with different amounts (a) and different types of lubricants (c), and the crystallization curve of the PTFE membrane with different amounts (b) and different types of lubricants (d).

[0035] Figure 3 Tensile curves of PTFE membranes with different amounts (a) and types of lubricants (b).

[0036] Figure 4 Water contact angles of PTFE membranes (pre-stretch) and PTFE microporous membranes (post-stretch) with different amounts (a) and types of lubricants (b).

[0037] Figure 5SEM images of PTFE microporous membranes prepared by adding different amounts and types of lubricants (A: P1(20); B: P1(25); C: P1(30); D: P2(25); E: P3(25); F: P4(25)). a, b, c, d, e and f are magnifications of A, B, C, D, E and F, respectively).

[0038] Figure 6 Pore ​​size distribution of PTFE microporous membranes prepared by adding different amounts and types of lubricants. a: P1 (20); b: P1 (25); c: P1 (30); d: P2 (25); e: P3 (25); f: P4 (25)), average pore size (g) and porosity (h).

[0039] Figure 7 2D-WAXS spectra of PTFE microporous membranes prepared by adding different amounts and types of lubricants. a: P1 (20); b: P1 (25); c: P1 (30); d: P2 (25); e: P3 (25); f: P4 (25). Azimuthal integral of 2D-WAXS spectra of PTFE microporous membranes prepared by adding different amounts (g) and different types of lubricants (h).

[0040] Figure 8 2D-SAXS spectra of PTFE microporous membranes prepared by adding different amounts and types of lubricants. a: P1(20); b: P1(25); c: P1(30); d: P2(25); e: P3(25); f: P4(25).

[0041] Figure 9 Tensile curves of PTFE microporous membranes with different amounts (a) and types of lubricants (b).

[0042] Figure 10 is the contact angle between different types of lubricants and PTFE.

[0043] Figure 11 The storage modulus and loss tangent tanδ of PTFE membranes with different amounts of isoparaffin added change with temperature.

[0044] Figure 12 Gurley value of PTFE microporous membrane with different amounts of isoparaffin and different types of lubricants added. Figure 13 Azimuthal integral of 2D-SAXS spectra of PTFE microporous membranes prepared by adding different amounts (a) and different types of lubricants (b). DETAILED DESCRIPTION

[0045] The raw materials and equipment used in the present invention are all known products and are obtained by purchasing commercially available products.

[0046] The following experiments without temperature indication are for normal temperature reactions, where normal temperature is room temperature, which is 25±5℃.

[0047] The materials used in the embodiments of the present invention are as follows:

[0048] Polytetrafluoroethylene (PTFE) dispersion resin, brand F104, number average molecular weight 1.15×10 6 g / mol, provided by Daikin Fluorochemical (China) Co., Ltd., and its bulk density is 0.45 g / cm 3 , the average particle size is 550μm and the moisture content is ≤0.004%.

[0049] The four lubricants (isoparaffin solvents) are ISO-C12 (isododecane) from Dongfang Electric Corporation, Dongfang Electric Corporation; Total Isane 185 (isododecane) from TotalEnergies SE, HTS18 (isododecane) from TotalEnergies SE; 11-13 The physical properties of the four lubricants (including surface tension, viscosity, and density) are shown in Table 1.

[0050] Table 1 Physical properties of different types of lubricants

[0051]

[0052] Example 1: Preparation of PTFE microporous membrane

[0053] A 10-mesh sieve was used to remove any agglomerated PTFE dispersion resin. The sieved PTFE dispersion resin powder was then mixed with a lubricant (the lubricant mass fraction is the percentage of the lubricant mass to the total mass of the PTFE and lubricant) in a three-dimensional mixer at 30 rpm for 30 minutes according to the formulation in Table 2. The mixing process was performed below 19°C. The mixture was then aged in a 40°C oven for 8 hours to uniformly disperse the lubricant throughout the PTFE dispersion resin powder, forming a PTFE paste. The aged PTFE paste was pressed at 4 MPa for 10 minutes to form a uniform billet, which was then extruded at 20 mm / min. The resulting film was then rolled multiple times on a two-roll calender at 60°C to form a membrane sheet approximately 700 μm thick. The rolled membrane was then oven-dried at 50°C for 12 hours to completely remove the liquid lubricant from the rolled film, resulting in a PTFE membrane. The dried membrane was subjected to asynchronous biaxial stretching on a universal electronic material testing machine (KaroIV, Bruckner, Germany) with a temperature control device. First, the dried membrane was preheated at 100°C for 1 minute, and then the first stretching (MD direction) was performed at a stretching ratio of 400%. Then, it was moved to the second chamber and preheated at 115°C for 1 minute, and then the second stretching (TD direction) was performed at a stretching ratio of 600% in the direction perpendicular to the first stretching direction. The stretching rate was 100% / s. Finally, the stretched film was sintered at 340°C for 10 minutes to prevent shrinkage, and the PTFE microporous membranes were obtained, which were named P1(20), P1(25), P1(30), P2(25), P3(25), and P4(25) respectively.

[0054] The codes of samples prepared according to the above method with different amounts and types of lubricants are shown in Table 2.

[0055] Table 2 Codes of samples prepared with different amounts and types of lubricants

[0056]

[0057]

[0058] The beneficial effects of the present invention are demonstrated by experimental examples below.

[0059] Experimental Example 1: PTFE Microporous Membrane Performance

[0060] 1. Experimental Methods

[0061] 1.DMA test

[0062] The entanglement density of PTFE membrane is tested by DMA test. In all tube models, the basic parameter describing the topological network is the molecular weight Me between entanglements, which is the most basic material parameter. Me is usually calculated from the platform modulus G. 0 NInferred (see Equation 1), the plateau modulus G 0 N It can be determined by measuring the dynamic moduli G' and G" in oscillatory shear experiments. Raju et al. (Raju VR, Menezes EV, Marin G, Graessley WW, Fetters LJ. Macromolecules 1981; 14: 1668) found an empirical "universal" end spectrum based on observations of different monodisperse polymer species. For long chains with sufficiently narrow dispersion, the area under the loss modulus peak divided by the maximum value of the G" end peak should give a universal proportionality constant K (see Equation 2):

[0063]

[0064] Raju et al. found that K = 3.56, where ω max It refers to the frequency corresponding to the maximum loss modulus. It is much easier to analyze the maximum value of G' (Z ~ 20) than to analyze the entire G' end peak (Z> 50) (Z = M / Me, where Z is the "entanglement number" of each molecule, equal to the number of tube segments per molecule), because the maximum value of G' is not affected by fluctuations and high-frequency Rouse modes, which are affected by frequencies ω> ω. max The modulus is dominated by the loss modulus. Due to its simplicity, this so-called "maximum" method (MAX) has been widely used. To obtain the relationship between the loss modulus and frequency, a dynamic thermomechanical analyzer (TAQ800) was used to perform an isothermal frequency sweep (in tension mode) on the membrane before double tensioning. Equations 1 and 2 were then used to calculate the entanglement density (Ve = ρ / Me) of different membranes.

[0065] 2.PTFE microporous membrane pore structure test

[0066] The morphology of the PTFE membrane and PTFE microporous membrane was studied by field emission scanning electron microscopy (SEM, NovaNanoSEM450). The sample was coated with Au, the spraying time was 120s, and the acceleration voltage was 5kV. The test was carried out according to the Gurley value in GB / T3636-2018, that is, a certain volume of gas passes through 6.45cm 2The time required for the microporous membrane to be pore-permeable is typically linearly negatively correlated with the Gurley value. The average pore size and pore size distribution of each PTFE membrane were obtained by a bubble pressure pore size analyzer (BSD-PB). Before testing, the dry PTFE membrane sample was cut into discs with a diameter of 11.2 mm and then completely wetted in a standard wetting solution with a surface tension of 15.9 dyn / cm. The open porosity of the membrane was determined by the dry-wet film weight method. Due to the hydrophobicity of PTFE, n-butanol was used as the wetting solution. The porosity (ε) of the membrane was determined by formula (3):

[0067]

[0068] Where w1 is the weight of the wet film (g), w2 is the weight of the dry film (g), D1 is the density of the infiltration liquid n-butanol (0.809 g / ml), and D P is the density of PTFE (2.20g / cm 3 ).

[0069] 3.DSC test

[0070] Thermal analysis was performed using a differential scanning calorimeter (DSC, DSC3+, Mettler, Toledo, Switzerland). Approximately 5 mg of PTFE membrane was weighed and heated from 25°C to 380°C at a rate of 10°C / min under nitrogen (50 ml / min). The crystallinity was calculated according to Equation 4:

[0071]

[0072] Where ΔH m is the melting enthalpy of the sample, ΔH 0 m It is the melting enthalpy of polytetrafluoroethylene when it is 100% crystallized (82 J / g).

[0073] 4. Water contact angle test

[0074] The contact angle (WCA) of distilled water was measured at room temperature using a contact angle tester (DSA30, KRUSS, Germany) with an automatic droplet dispenser. The volume of a single water droplet used for measurement was 4 μl. For each sample, three WCA values ​​at different locations were measured and the average was used to obtain the final contact angle.

[0075] 5.WAXS and SAXS testing

[0076] The 2D wide-angle X-ray diffractometer (Rigaku Home Lab diffactometer) used a CuKα target (λ = 0.154 nm) as the X-ray source, an operating voltage of 40 kV, an operating current of 40 mA, and a 5-min exposure time. A 2D small-angle X-ray scattering (2D-SAXS) instrument (Xenocs, France) was used, equipped with a Pilatus 300K detector with a resolution of 680 × 600 and a pixel size of 172 μm. The X-ray source was a CuKα target (λ = 0.154 nm). The sample-to-detector distance was 2500 mm, with the sample surface positioned perpendicular to the X-ray direction. The exposure time was 5 min.

[0077] 6. Mechanical properties test

[0078] The mechanical properties of PTFE in the MD direction before and after double stretching were measured using a universal material testing machine (Instron 4320). The stretching rate was 50 mm / min, the stretching temperature was room temperature, and each sample was tested 5 times.

[0079] 2. Experimental Results

[0080] (1) PTFE diaphragm performance

[0081] 1. Calculation of DMA and entanglement density

[0082] The results are as follows Figure 1 As shown, Figure 1 a and b respectively represent the relationship between the loss modulus and frequency of PTFE membranes (i.e., calendered membranes) with different amounts and types of lubricants added. The entanglement density of different PTFE membranes is obtained by calculation using formulas 1 and 2 (see Figure 1 c and d). Figure 1 c shows that as the amount of lubricant added increases, the entanglement density of the PTFE diaphragm decreases. Figure 1 d indicates that the entanglement density of the PTFE diaphragm gradually decreases from P1 (25) to P4 (25). It can be seen from Table 1 that the entanglement density of the PTFE diaphragm is directly related to the surface tension of the lubricant added thereto. The smaller the surface tension of the lubricant added, the lower the entanglement density of the PTFE diaphragm.

[0083] 2.DSC test

[0084] DSC was used to investigate the effect of lubricants on PTFE crystallization during processing. Figure 2 , as shown in Table 3.

[0085] Table 3 Crystallization and melting parameters of PTFE membrane

[0086]

[0087] From the first melting curve ( Figure 2 a, Table 3) shows that as the amount of lubricant added increases, the crystallinity of the PTFE diaphragm gradually increases. Figure 1 It can be clearly seen from Figure c that as the amount of lubricant added increases, the crystallinity of the PTFE membrane increases. In addition, the entanglement density of the PTFE membrane decreases. Samples P1(20), P1(25), and P1(30) were subjected to DMA tests with constant frequency temperature sweeps (see Figure 11 ), it was found that the Tg of samples P1(20), P1(25), and P1(30) were 61.26℃, 59.90℃, and 57.88℃, respectively. This further verified that the increase in the amount of lubricant added made the PTFE membrane molecular chain movement ability stronger and its crystallinity increased. Similarly, the surface tension of the lubricant added to samples P1(25) to P4(25) gradually decreased, resulting in a gradual decrease in their entanglement density. The less their ribbon crystal structure was destroyed, the greater their crystallinity. In addition, it was found that the crystallization temperature of samples P1(25) to P4(25) gradually decreased (see Figure 2 d).

[0088] 3. Mechanical properties

[0089] Figure 3 The stress-strain curves of calendered films prepared by adding different amounts and types of lubricants at a tensile speed of 50 mm / min at room temperature are shown. The trend of the stress-strain curves of each sample is similar. At low strain, the stress increases rapidly, then reaches yield, and after yielding, the stress gradually increases and finally remains stable. Figure 3 It can be seen from a that as the amount of lubricant added increases, the strain of the calendered membrane at the yield point decreases slightly. The yield strain represents the minimum stretching multiple required for the PTFE calendered membrane to form micropores. In addition, the tensile strengths of the three PTFE membranes, samples P1 (20), P1 (25) and P1 (30), are also quite different, 5.20 MPa, 3.97 MPa and 2.79 MPa respectively (see Table 5). The tensile strength decreases with the amount of lubricant added, proving that the activation energy required for the P1 (30) calendered membrane to be stretched into pores is lower, and it is easier to stretch into pores. It is worth noting that during the stretching process of the three PTFE membranes, the molecular chains are more likely to be oriented in the same direction at the entanglement points, forming a stable tensile orientation structure, thereby improving the tensile strength. Therefore, the more entanglement points and the greater the entanglement density, the higher the tensile strength. Similarly, samples P1 (25) to P4 (25) (see Figure 3 b) The tensile strength gradually decreases. In addition, their elastic modulus also decreases with the decrease of entanglement density. This is because the more entanglement points produced by microfiber entanglement, the greater the obstruction to the movement of molecular chains, which will lead to an increase in the elastic modulus of the material.

[0090] Table 5 Mechanical properties of PTFE diaphragm

[0091]

[0092] 4. Wetting properties

[0093] Polytetrafluoroethylene materials are widely used in waterproof and antifouling coatings, membrane distillation, and waterproof and breathable functional fabrics due to their excellent hydrophobicity. Therefore, the present invention establishes the relationship between the action mechanism of the lubricant and the membrane performance by testing the wettability of the water contact angle of the PTFE diaphragm and the PTFE microporous membrane. There are many factors that affect the wettability of the solid surface, mainly including the properties of the surface and environmental conditions. When the environmental conditions are constant, the chemical structure and geometric structure of the solid surface are the main factors affecting the wettability of the solid surface, among which the geometric structure is mainly roughness, structural size, crystallization, etc. For partially crystalline polymers, during the crystal-melt phase transition, the surface energy of the crystalline phase is different from that of the amorphous phase γ c The surface energy γ a The relationship is shown in Formula 5:

[0094]

[0095] Where ρ c is the crystal density, ρ a is the density of amorphous solid. During the crystal-melt phase transition, due to the discontinuous change in density, the change in surface energy is also discontinuous. c Greater than ρ a , so the surface energy of the crystalline phase is generally greater than that of the amorphous phase. Generally, the surface energy of a polymer is proportional to the fourth power of its density. Surface tension is the main physical quantity that describes the surface state and is a physical quantity that can be measured. The degree of wetting of a solid by a liquid depends primarily on its surface tension. When a liquid droplet reaches thermodynamic equilibrium on a solid surface, it should satisfy Young's equation (Formula 6):

[0096] γ S =γ L *cosθ+γ SL (6)

[0097] For solid surfaces, the relationship between the degree of wetting and the contact angle between the liquid and the solid surface is as follows (Formula 7):

[0098] A=γ S -γ SL =γ L *cosθ (7)

[0099] Where A represents the degree of wetting (mN·m -1 ), γS Represents the surface tension of the solid (mN·m -1 ), γ L Represents the surface tension of the liquid (mN·m -1 ), γ SL Represents the surface tension between solid and liquid (mN·m -1 ), θ represents the contact angle. As can be seen from the above formula, for a specific liquid, the size of the solid surface energy is the key to determining the degree of wetting. The carbon chains in the PTFE molecular structure are filled with fluorine atoms, which are much larger than hydrogen atoms. The polarizability of fluorine atoms is small, and the cohesive energy between PTFE molecules is low, which leads to low surface free energy of PTFE film, making it extremely difficult to be wetted. Figure 4 It can be clearly seen that the water contact angle of both the PTFE membrane before stretching and the PTFE microporous membrane after stretching is greater than 90°. Figure 4 It can be seen from a that the water contact angle of the PTFE membrane before stretching decreases with the increase of lubricant addition. Figure 2 It can be concluded that the role of lubricant in the processing process is to protect the primary particles, reduce their direct contact, and prevent the crystal structure of PTFE primary particles from being destroyed due to shear friction. Therefore, the crystallinity of samples P1 (20), P1 (25), and P1 (30) gradually increases (see Figure 2 and Table 3), and according to formula (5), it is obtained that the surface energy of the crystalline phase is greater than the surface energy of the amorphous phase, so the surface energy of the sample gradually increases from P1 (20) to P1 (30). Combined with formula (7), it is obtained that the greater the surface energy of the sample, the better its wetting performance, and the smaller its water contact angle.

[0100] (2) PTFE microporous membrane performance

[0101] 1. Membrane morphology

[0102] Figure 5The SEM images of PTFE microporous membranes obtained under the same double-drawing conditions with different amounts and types of lubricants are shown. All PTFE membranes contain similar structural features and are composed of three different regions: the first is a region composed of undrawn PTFE resin particle aggregates, called "nodes"; the second is a fiber region drawn out from the PTFE "ribbon" crystals, with the fibers oriented along the drawing direction, and the nodes and fibers forming a spatially periodic distribution structure; the third part is the blank area between the fibers. These blank areas constitute the microporous part of the ePTFE membrane, providing it with a high porosity. The most prominent structural feature of the PTFE membrane is its spatial periodicity. However, there are obvious differences in the morphology of the PTFE membranes prepared by adding different amounts of lubricants. The fibers of the microporous membrane P1 (20) are relatively thick and short, the nodes are long strips, and the fiber orientation direction tends to the TD direction; the fibers of the microporous membrane P1 (30) are relatively slender, the nodes are more broken, and are spherical. The fiber orientation direction is not as obvious as that of P1 (20), and the surface pore size is larger. The morphological structures of the PTFE membranes prepared by adding different types of lubricants also have great differences. The degree of node fragmentation of the microporous membranes P2 (25), P3 (25) and P4 (25) increases successively, but they are all lower than that of the microporous membrane P1 (25); the fibers pulled out of the microporous membranes P1 (25), P2 (25) and P3 (25) have relatively obvious orientation in the TD direction, while the orientation of the microporous membrane P4 (25) is not obvious.

[0103] Figure 6 ac shows that with the increase of lubricant addition, the pore size distribution gradually becomes wider and the average pore size of the PTFE microporous membrane gradually becomes larger (see Figure 6 g), and has the same variation pattern as the entanglement density of the PTFE membrane before biaxial stretching (see Figure 1 ), that is, the greater the entanglement density of the PTFE membrane, the smaller the average pore size of the PTFE microporous membrane. For the system with different types of lubricants added, the entanglement density of sample P1 (25) is the highest among the samples with four different lubricants added, but its average pore size is the largest. According to Figure 10 It shows that the contact angle between the lubricant iso-chain and PTFE is the largest, that is, its wettability with PTFE is poor, resulting in a large number of entanglement points in the diaphragm during the processing, and the entanglement points are unevenly distributed. The areas with more entanglement points will gather together to form large entanglement points during stretching, resulting in a large pull-out pore size, while the areas with fewer entanglement points will not agglomerate during stretching, resulting in a smaller pull-out pore size. This is also the reason why the use of lubricants such as iso-chain leads to a wider range of pull-out pore size distribution.

[0104] Figure 6 h shows the porosity of PTFE microporous membranes prepared by adding different amounts and types of lubricants. It was found that the porosity of the samples in the two systems was consistent with their average pore size. Figure 12 The Gurley values ​​of these six PTFE microporous membranes were characterized. The Gurley value is a parameter used to characterize the gas permeability of a material, indicating the tortuosity of its pores. Specifically, larger pore sizes and higher porosity indicate better gas permeability and smaller Gurley values. The Gurley values ​​of the samples in both systems were consistent with their average pore size and porosity, further verifying the accuracy of the experiment.

[0105] 2. Wetting properties

[0106] Figure 4 It can be seen that compared with the PTFE diaphragm before stretching, the water contact angle of the PTFE microporous membrane obtained after stretching increases significantly. After the PTFE diaphragm is stretched, the special ribbon crystals of the PTFE dispersed resin will be destroyed to form fibers, so the crystallinity of the PTFE microporous membrane is very low, and the crystallization factor has little effect on the wetting properties of PTFE. However, the PTFE microporous membrane obtained after the PTFE diaphragm is stretched will form a pore structure, and this pore structure will increase the roughness of the surface of the PTFE microporous membrane. According to the Wenzel equation (see formula 8), roughness will increase the surface hydrophobicity / oleophobicity of the intrinsically hydrophobic / oleophobic surface (i.e., according to the Wenzel equation, θ>90° of the surface), so the water contact angle of the PTFE microporous membrane obtained after stretching increases significantly.

[0107]

[0108] Where A r is the actual contact area, A is the nominal contact area, r is the surface roughness, θ r is the corrected rough surface contact angle, θ s is the contact angle of a smooth surface. Figure 4 a shows that the water contact angle of the stretched PTFE microporous membrane increases with the increase of lubricant addition. Figure 6 It can be seen that the increase in the amount of lubricant added reduces the entanglement density of the PTFE membrane, resulting in an increase in the pore size of the stretched PTFE microporous membrane, and the surface roughness of the PTFE microporous membrane increases. According to formula 8, for hydrophobic materials, the greater the surface roughness of the material, the greater its water contact angle. Figure 4 b, For PTFE microporous membrane samples with different types of lubricants added, their water contact angles are consistent with their pore sizes.

[0109] The above results show that for PTFE microporous membrane samples with different types of lubricants added, when the lubricant is Isopar G, the water contact angle of the obtained sample P2 (25) is the smallest and the hydrophilicity is the best; when the lubricant is iso-chain, the water contact angle of the obtained sample P1 (25) is the largest and the hydrophobicity is the best.

[0110] 3.WAXS and SAXS

[0111] Figure 7 The 2D-WAXS spectra of PTFE microporous membranes prepared by adding different amounts and types of lubricants are shown. Since these PTFE microporous membranes are prepared by biaxial stretching, the diffraction rings tend to be circular. In order to better explore the effect of lubricants on the crystal orientation of PTFE microporous membranes, the (100) crystal plane is selected for an azimuthal angle integration (see Figure 7 g and h). Figure 7 In g and h, the horizontal coordinate angles of 0° and 180° represent the MD direction, and 90° and 270° represent the TD direction. It is found that there are sharp peaks at 0° and 180°. This is because the stretching degree in the TD direction is greater than that in the MD direction, which makes the diffraction signal intensity in the equatorial direction higher than that in the meridian direction, and the overall orientation of the crystal is toward the TD direction. Here, the half-peak width of the peak is used to measure the orientation degree of the PTFE membrane crystal (see Table 4). The smaller the half-peak width, the sharper the peak, and the greater the orientation degree of the crystal. It can be seen from Table 4 that the orientation degree in both systems (different amounts and different types) decreases with the decrease of the entanglement density (see Figure 1 ) and reduced.

[0112] Table 4 Half-peak width of azimuth-integrated peak of 2D-WAXS pattern of PTFE microporous membrane

[0113]

[0114] Figure 8 The 2D-SAXS spectra of PTFE microporous membranes prepared by adding different amounts and types of lubricants are shown. Like WAXS, since these PTFE microporous membranes are prepared by biaxial stretching, the scattering patterns tend to be isotropic. However, since the stretching ratios in the TD and MD directions are different, the lamellae stacking structure will be tilted. For this reason, the azimuthal angle integration of the 2D-SAXS spectra is performed (see Figure 13 ), it was found that the orientation law of the platelet stacking structure was consistent with that of WAXS (see Table 6), which further verified that the lubricant had an effect on the degree of crystal orientation of the PTFE microporous membrane.

[0115] Table 6 Half-peak width of azimuth-integrated peak of 2D-WAXS pattern of PTFE microporous membrane

[0116]

[0117] 4. Mechanical properties

[0118] Figure 9 The stress-strain curves of PTFE microporous membranes prepared by adding different amounts and types of lubricants are shown. Figure 3 ) compared with the PTFE microporous membrane after biaxial stretching, the elastic modulus of the PTFE microporous membrane after biaxial stretching is greatly reduced. For example, the elastic modulus of sample P1 (20) before biaxial stretching is 106.35 MPa, and the elastic modulus after biaxial stretching is 22.63 MPa (see Table 7), which is a decrease of 79%. However, the maximum tensile strength of the PTFE microporous membrane after biaxial stretching is lower than that of the PTFE membrane before biaxial stretching (see Figure 3 ) should be high. Figure 9 a shows that as the amount of lubricant added increases, the maximum tensile strength of the PTFE microporous membrane decreases. Similarly, for systems with different types of lubricants added, the maximum tensile strength of the PTFE microporous membrane is consistent with the entanglement density of the PTFE membrane before biaxial stretching ( Figure 9 b).

[0119] Table 7 Mechanical properties parameters of PTFE microporous membrane

[0120]

[0121] The above experimental results show that the elastic modulus of the PTFE microporous membrane P1 (20) of the present invention is the best; for the PTFE microporous membranes with different lubricant addition amounts, P1 (30) has the largest elongation at break and the best toughness; for the PTFE microporous membranes with different types of lubricants added, when the lubricant is Isopar M, the obtained P4 (25) has the largest elongation at break and the best toughness.

[0122] In summary, this invention uses a thermomechanical biaxial stretching method to prepare polytetrafluoroethylene (PTFE) microporous membranes containing varying amounts and types of lubricants. The pore structure is then studied, and the concept of entanglement density is introduced to clarify the role of lubricants in the fibrillation process. By varying the amount and type of lubricant, the invention allows for the regulation of entanglement density, average pore size, porosity, and mechanical properties. This provides a new approach for the preparation of high-performance biaxially stretched PTFE membranes and holds great promise for future applications.

Claims

1. A polytetrafluoroethylene microporous membrane, characterized in that The polytetrafluoroethylene microporous membrane is prepared from polytetrafluoroethylene resin and lubricant as raw materials; the mass ratio of the lubricant to the polytetrafluoroethylene resin is 1: (0.5-5), the molecular weight of the polytetrafluoroethylene resin is 1 million to 2 million, the lubricant is an isoparaffin solvent, and the isoparaffin is C 10-20 Isoalkanes.

2. The polytetrafluoroethylene microporous membrane according to claim 1, characterized in that: The mass ratio of the lubricant to the polytetrafluoroethylene resin is 1:(2.3-4), and the isoparaffin is C 11-16 Isoparaffin; preferably, the mass ratio of the lubricant to the polytetrafluoroethylene resin is 1:2.3, 1:3 or 1:4, and the isoparaffin is C 11 、C 12 、C 13 or C 16 Isoalkanes.

3. A method for preparing the polytetrafluoroethylene microporous membrane according to claim 1 or 2, characterized in that: The method comprises the following steps: (1) Mix polytetrafluoroethylene resin and lubricant evenly; (2) standing and ripening to obtain a paste; (3) pressing the paste, rolling, and drying; (4) Asynchronous biaxial stretching and sintering to obtain the product.

4. The method according to claim 3, characterized in that The mixing temperature in step (1) is ≤19°C.

5. The method according to claim 3, characterized in that: The standing temperature in step (2) is 10-80° C., and the standing time is 2-20 h; preferably, the standing temperature is 40° C., and the standing time is 8 h.

6. The method according to claim 3, characterized in that: The temperature for rolling in step (3) is 30-100°C, the temperature for drying is 10-80°C, and the drying time is 5-30h; preferably, the temperature for rolling in step (3) is 60°C, the temperature for drying is 50°C, and the drying time is 12h.

7. The method according to claim 3, characterized in that The asynchronous biaxial stretching in step (4) is performed by preheating for the first time, and performing the first stretching at a stretching ratio of 300% to 500% and a stretching rate of 80% / s to 120% / s; Preheating for the second time, and performing the second stretching at a stretching ratio of 500% to 700% and a stretching rate of 80% / s to 120% / s; The temperature of the first preheating is 80°C to 120°C, and the temperature of the second preheating is 100°C to 150°C.

8. The method according to claim 7, characterized in that: The asynchronous biaxial stretching in step (4) is to first perform a first stretching at a stretching ratio of 400% and a stretching rate of 100% / s, and then perform a second stretching at a stretching ratio of 600% and a stretching rate of 100% / s. The first preheating temperature is 100°C, and the second preheating temperature is 115°C.

9. The method according to claim 3, characterized in that: The sintering temperature in step (4) is 100-600° C., and the sintering time is 1-60 min; preferably, the sintering temperature is 340° C., and the sintering time is 10 min.

10. Use of the polytetrafluoroethylene microporous membrane according to claim 1 or 2 in the fields of gas-liquid separation, biomedicine, and new energy.

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