A polytetrafluoroethylene film and a method for producing the same

Abstract

The application provides a polytetrafluoroethylene film and a preparation method thereof, and specifically comprises the following steps: step one, preparing a spinning liquid; step two, spinning into a film; step three, gradient temperature sintering; step four, preheating and stretching the film material; step five, jet flow treatment of the film material; step six, high-voltage electrostatic spraying of a composite solution, drying, and solidification to obtain the polytetrafluoroethylene film; the application breaks through the technical bottleneck of the existing PTFE film through material modification, process innovation and function integration, and improves the performance in all aspects of high porosity and conductivity, and the nanofiber network is generated by multi-needle electrospinning, and the uniform bead structure is formed by forced melting of the fiber in combination with pulse pressure sintering, so that the porosity is significantly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high polymer materials, and particularly relates to a polytetrafluoroethylene film and a preparation method thereof. BACKGROUND

[0002] The polytetrafluoroethylene (PTFE) film is widely used in new energy (such as fuel cell separators), filtration and separation (such as air / water filtration) and medical materials due to its excellent chemical stability, high temperature resistance and hydrophobicity. However, the traditional polytetrafluoroethylene film and the preparation method thereof have the following key defects:

[0003] 1) The traditional stretching method leads to uneven pore size distribution and low porosity. The traditional PTFE film is formed by a one-way or two-way stretching process, but the molecular chain orientation is difficult to control accurately in the stretching process, leading to a wide pore size distribution, such as a mixture of micron-sized large pores and nanometer-sized small pores, and low porosity. In the field of new energy (such as lithium ion battery separators), uneven pore size may cause dendrite penetration risk; in the medical field, low porosity will reduce the air permeability. 2) The traditional PTFE film lacks reinforcing phase, and has low tear resistance, especially in a wet environment, and is prone to brittle fracture. Pure PTFE film is prone to cracking under dynamic load, which limits its application in high-pressure filtration or flexible electronics. For example, in seawater desalination, the membrane material is prone to breakage, and medical dressings need to withstand repeated bending, and insufficient mechanical properties may lead to failure. 3) The traditional PTFE film relies on single hydrophobicity, but the strong hydrophobicity leads to poor liquid permeability, such as low blood filtration efficiency, and lacks additional functions such as antibacterial and conductive. 4) The traditional process uses organic solvents (such as NMP, DMF) to disperse PTFE, which has the risk of solvent residue, and the high-temperature sintering has high energy consumption. The cost of solvent recovery accounts for more than 30% of the total production cost. Medical materials are sensitive to solvent residue, and green processes are needed in the field of new energy to reduce carbon emissions.

[0004] Therefore, it is an urgent problem to provide a polytetrafluoroethylene film with strong mechanical properties, uniform pore size distribution, strong hydrophobicity and an environmentally friendly preparation process, and a preparation method thereof. SUMMARY

[0005] In order to solve the above problems, the application provides a polytetrafluoroethylene film and a preparation method thereof, which comprises the following steps:

[0006] Step one, vacuum drying PTFE resin particles and modifiers at 60-70 DEG C for 4-5 h, and crushing thermoplastic elastomer (TPEE) and functional polymer to a particle size of less than or equal to 100 microns.

[0007] Preferably, the particle size of the PTFE resin particles is 200-500 nm.

[0008] Step two, heat the solvent to 50-60℃, add the adhesive, 500-600rpm stirring until dissolved, then add PTFE, TPEE, PVDF and modifier in turn, 3000-4000rpm, shear time 40-60min, stand for 24-26h, get the textile liquid.

[0009] Preferably, the mass ratio of PTFE resin, modifier, TPEE, functional polymer, adhesive and solvent is (60-80):(4-11):(5-10):(5-15):(5-10):(150-200).

[0010] Preferably, the modifier is SiO2 nanoparticles and graphene, the mass ratio is (3-8):(1-3). Most preferably, the SiO2 nanoparticles have a particle size of 10-50nm, and the graphene has a sheet thickness of ≤5nm.

[0011] Preferably, the TPEE is purchased from Dongguan Hongyi Plastic Technology Co., Ltd. and has a melt index of 10-30g / 10min.

[0012] Preferably, the functional polymer is polyvinylidene fluoride (PVDF). Most preferably, the PVDF has a molecular weight of 500-600 thousand.

[0013] Preferably, the adhesive is polyethylene oxide (PEO), and most preferably, the PEO has a molecular weight of 10-300 thousand.

[0014] Preferably, the solvent is deionized water.

[0015] Step three, inject the textile liquid into a multi-needle electrospinning machine to spin a film, the needle inner diameter is 0.3-0.5mm, the needle spacing is 5cm, the receiving device is a stainless steel drum with a diameter of 30cm, the surface is coated with aluminum foil, the rotating speed is 100-500rpm adjustable, the temperature is 25±2℃, the humidity is ≤30%, the textile liquid flow rate is 0.5-1.5mL / h, the voltage is 20-30kV, the receiving distance is 18-22cm, and the film thickness is 400-500μm.

[0016] Preferably, the initial voltage is 20kV, and it is increased to 30kV at a rate of 2kV / h, the initial flow rate is 0.5mL / h, and it is increased to 1.5mL / h at a rate of 0.2mL / h.

[0017] Step four, sinter the film material of step three in an oxygen-deficient environment, increase the temperature from room temperature to 120-130℃ at a rate of 5℃ / min, keep the temperature for 2-3h, then increase the temperature to 360-370℃ at a rate of 2℃ / min, keep the temperature for 3-3.5h, then increase the temperature to 400-410℃ at a rate of 1℃ / min, and keep the temperature for 1.5-2h.

[0018] Preferably, the oxygen content of the oxygen-free environment is less than or equal to 50 ppm.

[0019] Preferably, the pulse pressure is introduced during the 400-410℃ holding stage, with a pressure of 0.1-0.5 MPa and a frequency of 1-2 Hz, to force the molten fibers to form uniform beads with a diameter of 50-200 nm.

[0020] Step five, the film material of step four is preheated at 260-280℃ for 5-10 min, and then is placed in a biaxial stretching machine for stretching, with a longitudinal stretching ratio of 4:1, a stretching speed of 40-50 mm / s, a stretching time of 50-70 s, a transverse stretching ratio of 5:1, a stretching speed of 20-30 mm / s, and a stretching time of 40-50 s, and the film material is immediately quenched to room temperature by using cold air at 15-20℃.

[0021] Preferably, the longitudinal stretching is completed in two stages, in the first stage, the stretching ratio is 2:1, the stretching speed is 40-50 mm / s, and the stretching time is 20-30 s, and the film material is stopped for 10 s, and in the second stage, the stretching ratio is 2:1, the stretching speed is 40-50 mm / s, and the stretching time is 20-30 s.

[0022] Step six, the film material of step five is placed in an atmospheric pressure plasma jet device, and the film material is jetted by using a composite gas, with a gas flow of 10-12 L / min, a power of 80-100 W, a nozzle distance of 5-6 mm from the film surface, and a scanning speed of 10-12 mm / s, and the process is repeated for 3-4 times.

[0023] Preferably, the composite gas is Ar and O2, with a volume ratio of 9:1.

[0024] Step seven, a composite solution is coated on the surface of the film material of step five by using high-voltage electrostatic spraying, with a voltage of 15-20 kV, a nozzle diameter of 0.3-0.4 mm, and a spraying amount of 0.5-1.0 mg / cm 2 , and after the spraying is completed, the film material is dried at 80-90℃ for 10-15 min, and then is cured at 120-130℃ for 30-40 min, to obtain a polytetrafluoroethylene film.

[0025] Preferably, the composite solution includes heptadecafluorodecyltrimethoxysilane (fluorosilane), nano-SiO2 particles, and anhydrous ethanol, with a mass ratio of 1:4:45. Most preferably, the particle size of the nano-SiO2 particles is 20-30 nm.

[0026] The present application has the following advantages:

[0027] (1) The application adopts multiple needle electrospinning to generate nanofiber network, combines with pulse pressure sintering, and forcibly melts the fiber to form a uniform bead structure, significantly improves the porosity, solves the problem of uneven pore size and low porosity in traditional stretching method, and in the field of new energy, the uniform pore size can inhibit dendrite growth, and in filtration and separation, high porosity improves flux.

[0028] (2) The application adds SiO2 nanoparticles and graphene, the SiO2 nanoparticles and the graphene with a sheet structure form physical crosslinking points, two-way stretching is completed in stages, the stretching ratio reaches 4:1 (longitudinal) + 5:1 (transverse), and the tear resistance is significantly improved, solving the problem of insufficient mechanical strength of traditional polytetrafluoroethylene film.

[0029] (3) The plasma jet processing of the application introduces oxygen-containing groups, regulates the contact angle, and improves the liquid permeability, the fluorosilane coating and nano-SiO2 of the application can endow antibacterial property, solving the problem of single function of traditional membranes.

[0030] (4) The application uses deionized water instead of organic solvents, the solvent residue is less than 0.01%, and low-temperature sintering is carried out through anoxic environment and gradient heating, solving the problems of high energy consumption and solvent residue in traditional processes. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the application will be described below in a clear and complete manner. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0032] Embodiment 1

[0033] Raw materials: the mass ratio of the PTFE resin, the modifier, TPEE, PVDF, PEO and deionized water is 70:8:6:10:7:200, the modifier is SiO2 nanoparticles and graphene, and the mass ratio is 5:2. The particle size of the PTFE resin particles is 300 nm, the particle size of the SiO2 nanoparticles is 30 nm, the sheet thickness of the graphene is ≤5 nm, the TPEE is purchased from Dongguan Hongyi Plastic Co., Ltd., and the melt index is 20 g / 10 min, the molecular weight of the PVDF is 550,000, and the molecular weight of the PEO is 200,000.

[0034] Step one, vacuum drying the PTFE resin particles and the modifier at 65℃ for 4.5h, and crushing TPEE and PVDF to a particle size ≤100μm.

[0035] Step two, heat the solvent to 60℃, add PEO, stir at 600 rpm until dissolved, then add PTFE, TPEE, PVDF and modifier in turn, 4000 rpm, shear time 50 min, stand for 24 h to obtain the spinning solution.

[0036] Step three, spin the spinning solution into a film in a multi-needle electrospinning machine, with an inner diameter of 0.4 mm, a needle spacing of 5 cm, a receiving device of a stainless steel drum with a diameter of 30 cm and a surface coated with aluminum foil, a rotating speed of 100-500 rpm, a temperature of 25±2℃, a humidity of ≤30%, an initial voltage of 20 kV, an increase of 2 kV / h to 30 kV, an initial flow rate of 0.5 mL / h, and an increase of 0.2 mL / h to 1.5 mL, and a film thickness of 400-500 μm.

[0037] Step four, sinter the film material of step three in a nitrogen atmosphere, with an oxygen content of ≤50 ppm, an increase of 5℃ / min from room temperature to 120℃, a holding time of 2 h, an increase of 2℃ / min to 360-℃, a holding time of 3 h, an increase of 1℃ / min to 400℃, a holding time of 1.5 h, and the introduction of a pulsed pressure of 0.35 MPa at 400℃ with a frequency of 1 Hz.

[0038] Step five, preheat the film material of step four at 270℃ for 10 min, then put it into a biaxial stretching machine for stretching, with the longitudinal stretching completed in two stages, the first stage with a stretching ratio of 2:1, a stretching speed of 50 mm / s, a stretching time of 20 s, and a stop time of 10 s, the second stage with a stretching ratio of 2:1, a stretching speed of 50 mm / s, a stretching time of 20 s, and a transverse stretching ratio of 5:1, a stretching speed of 30 mm / s, and a stretching time of 40 s, and immediately after stretching, quenching to room temperature with 15℃ cold air.

[0039] Step six, put the film material of step five into an atmospheric pressure plasma jet device, and treat the film material with Ar:O2=9:1, a gas flow rate of 10 L / min, a power of 100 W, a nozzle distance from the film surface of 5 mm, and a scanning speed of 10 mm / s, and repeat this process three times.

[0040] Step seven, coat a composite solution on the surface of the film material of step five by high-voltage electrostatic spraying, with a voltage of 20 kV, a nozzle diameter of 0.3 mm, and a spraying amount of 1.0 mg / cm 2 , dry at 90℃ for 10 min after spraying, and then cure at 120℃ for 30 min to obtain a polytetrafluoroethylene film. The composite solution comprises fluorosilane, nano-SiO2 particles, and anhydrous ethanol, with a mass ratio of 1:4:45. The nano-SiO2 particles have a particle size of 25 nm.

[0041] Test Example 1

[0042] Experimental equipment and method: The porosity was calculated by measuring the difference between the apparent density and the true density of the material using a mercury porosimeter (Micromeritics AutoPore IV 9500) combined with the density method.

[0043] Samples: The PTFE membrane sample prepared in Example 1 (experimental group) and the traditional polytetrafluoroethylene membrane purchased from Dongguan Shengli New Material Co., Ltd. (control group) were recorded for the film thickness h (pm) respectively. The samples were dried in a vacuum drying oven at 60°C for 24 hours to remove moisture.

[0044] Apparent density measurement: The mass m (g) of the dried sample was weighed using a precision balance (accuracy 0.1 mg).

[0045] Apparent density p apparent = m / (A x h), where A is the sample area (cm 2 ).

[0046] The true density p true of the PTFE membrane and the traditional polytetrafluoroethylene membrane was measured by helium specific gravity method (AccuPyc II 1340).

[0047] Porosity = (1 - p apparent / p true ) x 100%, five replicates per group, the results are shown in Table 1.

[0048] Test Example 2

[0049] Equipment: Universal material testing machine (Instron 5967) in accordance with ASTM D882 standard.

[0050] Test conditions: Room temperature 25°C, humidity 50%, tensile speed 50 mm / min.

[0051] Steps: Cut the sample into dumbbell-shaped samples, total length 75 mm, gauge length 25 mm x 5 mm.

[0052] 2. Test operation:

[0053] Clamp both ends of the sample to ensure that the gauge length is vertically aligned.

[0054] Start the testing machine and stretch to break at a speed of 50 mm / min, record the maximum load F max (N).

[0055] Calculate the tensile strength = F max / A cross-section .

[0056] Cross-sectional area A cross-section = width x thickness, five replicates per group, the results are shown in Table 1.

[0057] Test Example 3

[0058] Equipment: Contact angle measurement instrument (Kruss DSA25), using static drop method.

[0059] Test liquid: ultrapure water (surface tension 72.8 mN / m), n-hexadecane (surface tension 27.5 mN / m).

[0060] Steps:

[0061] The sample surface was blown with nitrogen to remove dust and avoid touching the test area.

[0062] A microsyringe was used to deposit a 5 μL droplet on the film surface, ensuring that the droplet did not rebound or spread.

[0063] A high-speed camera recorded the droplet morphology, and the software automatically fitted the droplet profile to calculate the contact angle (average of left / right angles).

[0064] Five different positions were tested for each sample group, and the average value was taken. The results are shown in Table 1.

[0065] Test Example 4

[0066] Simulation conditions: The film material was placed in a circulating water flow system (flow rate 10 L / min, pressure 0.1 MPa, temperature 25℃) and run continuously for 1000 hours.

[0067] Steps:

[0068] Initial performance recording, testing initial porosity, tensile strength, contact angle.

[0069] Periodic testing:

[0070] After 1000 hours, the sample was removed and the performance parameters were retested according to the above method. Each group was repeated five times, and the results are shown in Table 1.

[0071] Table 1

[0072]

[0073]

[0074] As can be seen from Table 1, through systematic testing and comparison, the polytetrafluoroethylene film prepared by the present application comprehensively surpasses the traditional polytetrafluoroethylene film in porosity, mechanical strength, omniphobicity and stability, solving the core problems of traditional film such as uneven pore size, single function, and easy decay.

[0075] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a polytetrafluoroethylene (PTFE) membrane, characterized in that, Includes the following steps: Step 1: Mix PTFE resin, modifier, TPEE, functional polymer, adhesive and solvent in a mass ratio of (60-80):(4-11):(5-10):(5-15):(5-10):(150-200), stir and let stand to obtain spinning solution; Step 2: Inject the spinning solution into a multi-needle electrospinning machine to spin a film. The ambient temperature is 25±2℃, the ambient humidity is ≤30%, the spinning solution flow rate is 0.5-1.5mL / h, the voltage is 20-30kV, the receiving distance is 18-22cm, and the film thickness is 400-500μm. Step 3: The membrane material is sintered in an oxygen-deficient environment at 120-130℃ for 2-3 hours, then heated to 360-370℃ and held for 3-3.5 hours, and then heated to 400-410℃ and held for 1.5-2 hours. Step 4: After preheating the membrane material from Step 3, stretch it biaxially and then rapidly cool it to room temperature. Step 5: Perform plasma jet treatment on the membrane material from Step 4 using a composite gas at a flow rate of 10-12 L / min. Step six: Apply the composite solution to the surface of the membrane material from step five using high-voltage electrostatic spraying. The voltage is 15-20 kV, and the spraying amount is 0.5-1.0 mg / cm³. 2 After spraying, the coating is dried and cured to obtain a polytetrafluoroethylene film. The modifier mentioned in step one is SiO2 nanoparticles and graphene in a mass ratio of (3-8):(1-3), the functional polymer is PVDF, the adhesive is PEO, and the solvent is deionized water; In step three, pulsed pressure is introduced during the heat preservation stage at 400-410℃, with a pressure of 0.1-0.5 MPa and a frequency of 1-2 Hz; The composite gas mentioned in step five is Ar and O2, with a volume ratio of 9:1; The composite solution described in step six comprises fluorosilane, nano-SiO2 particles, and anhydrous ethanol in a mass ratio of 1:4:

45.

2. The method for preparing a polytetrafluoroethylene membrane according to claim 1, characterized in that, In step two, the initial voltage is 20kV, which is increased to 30kV at a rate of 2kV / h, and the initial flow rate is 0.5mL / h, which is increased to 1.5mL / h at a rate of 0.2mL / h.

3. The method for preparing a polytetrafluoroethylene membrane according to claim 1, characterized in that, Step four involves longitudinal stretching in two stages. In the first stage, the stretching ratio is 2:1, the stretching speed is 40-50 mm / s, the stretching time is 20-30 s, and the hold time is 10 s. In the second stage, the stretching ratio is 2:1, the stretching speed is 40-50 mm / s, and the stretching time is 20-30 s.

4. The polytetrafluoroethylene membrane prepared by the method according to any one of claims 1-3.

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