A polytetrafluoroethylene film and preparation method thereof

Abstract

The present invention provides a polytetrafluoroethylene (PTFE) film and a method for preparing the same. The film has a thickness of less than 0.01 mm, significantly reducing the thickness of existing PTFE films. The film is not only ultra-thin but also exhibits high surface smoothness and flatness, meeting the high-end demands of special-purpose films such as medical films and circuit board protective films. The method comprises compression molding, sintering, and lathing a modified PTFE resin to produce a skived film. The skived film is then subjected to a primary calendering at a temperature above the melting point of the PTFE, followed by a secondary calendering at a temperature below the melting point of the PTFE, to produce a PTFE film with a thickness of less than 0.01 mm. The method has a simple preparation process and achieves a PTFE film with a thickness of less than 0.01 mm and a high degree of surface flatness and finish.

Description

Technical Field

[0001] The invention belongs to the technical field of film processing and manufacturing, and particularly relates to a polytetrafluoroethylene film and a preparation method thereof. Background Art

[0002] PTFE film is made by molding, sintering, and cooling PTFE suspension resin into a blank, which is then turned. The turned film is a non-oriented film, which becomes an oriented film after calendering.

[0003] Oriented polytetrafluoroethylene (PTFE) films are generally used for insulation of wires and cables, capacitor dielectrics, and as a backing material for specialty tapes. However, more specialized applications, such as medical films and protective films for semiconductor circuit boards, require films with high cleanliness and flatness, while also requiring the thinnest thickness possible.

[0004] For example, artificial dura mater used in medical membranes generally needs to meet at least the following requirements: ① a smooth surface that does not adhere to the arachnoid mater and brain tissue; ② as a replacement for human meninges, it must be as thin as possible; ③ possess a certain level of strength, elasticity, and elongation; and ④ be easy to sterilize and store. Therefore, ideal dura mater repair materials must possess the following characteristics: sufficient material resources and a simple preparation process; stable chemical properties; a smooth and flat surface that is as thin as possible; and not only good mechanical properties, but also good density and excellent permeability resistance.

[0005] At present, the new medical polymer material expanded polytetrafluoroethylene (ePTFE) has been successfully used to make artificial dura mater as a substitute for the human meninges. It is used to repair defects in the dura mater or spinal meninges caused by craniocerebral or spinal cord injuries, tumors and other cranial diseases to restore their integrity, thereby effectively preventing the occurrence of serious complications such as cerebrospinal fluid leakage, intracranial infection, encephalocele, brain adhesions and scarring. Although expanded polytetrafluoroethylene (ePTFE) has a good effect in repairing the dura mater during surgery, because ePTFE is a microporous membrane, the material cannot meet high requirements in terms of permeability resistance, limiting its scope of application. In comparison, the permeability resistance of oriented polytetrafluoroethylene films is better. If the thickness and surface smoothness of the polytetrafluoroethylene film meet the requirements, then the polytetrafluoroethylene film has great application prospects in the field of medical membranes.

[0006] However, according to the existing processing technology of oriented films, if the film thickness is generally greater than 0.03 mm while ensuring good surface smoothness and flatness, this greatly limits the application of polytetrafluoroethylene films.

[0007] Chinese patent CN111016206A discloses a method for preparing a polytetrafluoroethylene (PTFE) film. The method involves sieving a polytetrafluoroethylene (PTFE) suspension resin powder through a 10-mesh screen, then molding, sintering, and lathing to produce a skived film. The skived film is then heat-stretched at a temperature equal to or above the glass transition temperature of the polytetrafluoroethylene resin and below its melting point. After the heat-stretching process, the temperature is lowered to below the glass transition temperature to produce the polytetrafluoroethylene (PTFE) film. However, the thickness of this polytetrafluoroethylene film is 0.1-1.0 mm, which does not meet the requirements for artificial dura mater materials or ultra-thin PTFE films.

[0008] Chinese patent CN101491945A discloses a method for preparing an ultra-thin wide-width polytetrafluoroethylene oriented film, which includes the steps of raw material screening, pre-pressing, high-temperature sintering, turning film formation, calendering and orientation. Although the thickness of the oriented film reaches 5-50 μm, it can be used to manufacture advanced insulating gaskets, sheaths, anti-friction materials and chemical anti-corrosion materials in the electromechanical and electronic industries, and can be used for a long time in the range of -200 to 250 ° C. However, fillers are added to the polytetrafluoroethylene raw materials of the film, so it is not suitable for fields with extremely high cleanliness requirements such as medical care and semiconductors, and can only be used in ordinary fields such as sealing and anti-corrosion. In addition, if the film thickness described in the patent is to be achieved, the turning process requirements are extremely high. It is necessary to control the thickness of the extremely thin turning film during turning film formation, and the general turning film thickness needs to be controlled at 2.8 times the thickness of the oriented film. It can be seen that the existing turning process is difficult to meet this demand. Furthermore, when a filler-added blank is turned into a film, the filler is exposed on the film surface, reducing smoothness and making it difficult to control the uniformity of the turned thickness, making it difficult to achieve a good flatness and finish, and thus affecting the subsequent calendering and shaping effect. Furthermore, if the thickness of the filled film is extremely thin, microscopic voids are easily formed, significantly reducing properties such as strength and permeability resistance, making it difficult for the film to achieve excellent application performance. Summary of the Invention

[0009] The present invention provides a polytetrafluoroethylene film having a thickness of less than 0.01 mm. Compared with existing polytetrafluoroethylene films, the thickness of the film is greatly reduced. The film is not only ultra-thin but also has high surface smoothness and flatness, which can meet the high-end requirements of special-purpose films such as medical films and circuit board protective films.

[0010] The specific technical solutions are as follows:

[0011] A polytetrafluoroethylene film is an oriented film with a thickness of less than 0.01 mm and a surface roughness Ra of less than 0.130 μm.

[0012] Furthermore, the tensile strength of the polytetrafluoroethylene film is greater than 80 MPa, the elongation at break is greater than 100%, and the chloride ion permeation concentration after 30 days is less than 100 ppm. Therefore, the polytetrafluoroethylene film has also been improved in terms of mechanical strength and permeation resistance.

[0013] Furthermore, the polytetrafluoroethylene film has a melt viscosity of less than 3.5×10 10 It is made from modified polytetrafluoroethylene resin containing Pa·S.

[0014] In addition, the present invention also provides a method for preparing the polytetrafluoroethylene film, which can produce a film that is not only thin but also has a surface with high smoothness and flatness.

[0015] The specific technical solutions are as follows:

[0016] A method for preparing the above-mentioned polytetrafluoroethylene film comprises the following steps:

[0017] (1) The melt viscosity is less than 3.5×10 10 The modified polytetrafluoroethylene resin powder of Pa·S is first sieved through a mesh, then molded, sintered, and turned to obtain a turning film;

[0018] (2) The skived film obtained in step (1) is first calendered at a roller temperature higher than the melting point of the modified polytetrafluoroethylene resin used, and then calendered for a second time at a roller temperature lower than the melting point of the modified polytetrafluoroethylene resin used, to obtain an oriented film with a thickness of less than 0.01 mm.

[0019] If the film is calendered only at a temperature above the melting point, the surface finish and flatness of the film will be poor. However, the problem of poor surface flatness can be effectively solved by performing a second calendering at a temperature below the melting point of PTFE after the first calendering.

[0020] The above-mentioned two processing steps of primary calendering and secondary calendering can be continuous processes or intermittent processes. Preferably, the process between the primary calendering and the secondary calendering is continuous.

[0021] Furthermore, the blank obtained after sintering in step (1) of the polytetrafluoroethylene film preparation method is first kept at 100-200° C. for 5-20 hours before turning.

[0022] After testing, it was found that keeping the blank within this temperature range for a sufficient time before turning can further ensure that the film has a lower surface roughness.

[0023] Furthermore, in step (1) of the polytetrafluoroethylene film preparation method, the particle size of the modified polytetrafluoroethylene resin powder is 10-50 μm, and the standard relative density (SSG) of the modified polytetrafluoroethylene resin is 2.130-2.200.

[0024] By using the modified polytetrafluoroethylene resin within the particle size and standard relative density range for compression molding, the density of the obtained pressed green body is more uniform and the arrangement is dense.

[0025] Furthermore, the specific operation of the compression molding in step (1) of the polytetrafluoroethylene film preparation method is as follows: at room temperature, increase the pressure to 3-8 MPa at a pressure rate of 10-100 mm / min, and retract the press; after 3-6 seconds, re-pressurize the film in both directions at a pressure rate of 10-100 mm / min to 20-50 MPa, and maintain the pressure for more than 10 minutes;

[0026] Preferably, the pressurization rate is 10-30 mm / min.

[0027] Pressing should be done at room temperature, and it is especially important to avoid molding at temperatures below 20°C, otherwise it will easily lead to obvious or microscopic cracking of the molded body after sintering.

[0028] The compression molding process involves pre-pressing and main pressing, with the main pressing being bidirectional. Bidirectional pressing helps avoid the pressure drop that occurs during unidirectional pressing, which can affect the density distribution of the blank, particularly with larger blanks. This compression molding process improves the film's flatness and density uniformity.

[0029] Furthermore, in step (1) of the polytetrafluoroethylene film preparation method, the sintering is performed in a step-by-step heating manner, specifically: heating from room temperature to 250-300°C at a heating rate of 1-3°C / min, and keeping the temperature for 10-80 minutes; then heating from 250-300°C to 360-390°C at a heating rate of 1-3°C / min, and keeping the temperature for 6-20 hours to completely melt the green body obtained by compression molding; finally, cooling to 280-290°C at a rate of 1-3°C / min, keeping the temperature for at least 30 minutes, and then naturally cooling from 280-290°C to room temperature;

[0030] The thickness of the turning film obtained by turning in the step (1) is 0.03-0.05 mm.

[0031] Furthermore, in step (2) of the polytetrafluoroethylene film preparation method, the temperature of the rollers during the primary calendering is controlled to be above 330°C; the temperature of the rollers during the secondary calendering is controlled to be between 200°C and 320°C;

[0032] Preferably, the roller temperature of the primary calendering is 330-400°C;

[0033] The speed of the first calendering and the second calendering is 0.5-5m / min;

[0034] The rolling ratios of the primary rolling and the secondary rolling are 1.1-5.0 times.

[0035] The polytetrafluoroethylene film or the polytetrafluoroethylene film prepared by the preparation method is used for medical films and circuit board protective films.

[0036] The beneficial effects of the present invention are:

[0037] 1. The thickness of the polytetrafluoroethylene film of the present invention reaches less than 0.01 mm, which greatly reduces the thickness of the film compared with the existing polytetrafluoroethylene film. Not only is the film ultra-thin but also has high surface smoothness and flatness, it can fully meet the market requirements for ultra-thin polytetrafluoroethylene films, especially the high-end demand for special-purpose films such as medical films and circuit board protective films.

[0038] 2. The method for preparing the polytetrafluoroethylene film provided by the present invention first selects a polytetrafluoroethylene film having a melt viscosity of less than 3.5×10 10 After in-depth research on modified polytetrafluoroethylene resin with a low melt viscosity of Pa·S, it was found that only by turning and double calendering can the pores between polymer molecules be minimized, the arrangement is dense, and defects are reduced to the greatest extent. Under the premise of ensuring that the performance and appearance of the film meet the application requirements, the thickness can be reduced to below 0.01mm.

[0039] 3. The method for preparing the polytetrafluoroethylene film provided by the present invention involves first performing a primary calendering and orientation process at a temperature above the melting point of the modified polytetrafluoroethylene resin, significantly reducing the film thickness. This primary calendering process, performed while the polytetrafluoroethylene is molten, minimizes voids within the film, reduces defects, and improves its mechanical properties. A secondary calendering process, performed at a temperature below the melting point of the modified polytetrafluoroethylene resin, enhances the film's smoothness and flatness. The resulting polytetrafluoroethylene film not only achieves an extremely low thickness but also exhibits superior strength, permeability resistance, and flexural resistance, achieving a smooth, flat appearance and excellent performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 The present invention is a flow chart of the method for preparing the polytetrafluoroethylene film.

[0041] Figure 2 This is a photo of the film prepared by the preparation method described in Example 1.

[0042] Figure 3 This is a photo of the film prepared by the preparation method described in Comparative Example 1.

[0043] Figure 4 This is a photo of the film prepared by the preparation method described in Comparative Example 4. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to specific examples, but is not limited thereto. The resins used in the following examples and comparative examples are commercially available products and can be obtained from commercial channels.

[0045] Terminology Notes:

[0046] Room temperature: has the meaning commonly known in the art, i.e., 25±5°C.

[0047] 1. Surface roughness Ra: The arithmetic mean of the absolute values ​​of the profile deviation within the sampling length (lr). See GB / T1031-2009.

[0048] 2. Test of tensile strength and elongation at break: refer to GB / T13022-1991.

[0049] 3. Test method for permeability resistance (30-day chloride ion permeability concentration): Place the polytetrafluoroethylene film between two diffusion cells. Fill the two diffusion cells with pure water and concentrated hydrochloric acid respectively. After 30 days, use a chloride ion concentration tester to detect the chloride ion concentration in the pure water.

[0050] Example 1

[0051] The thickness of the polytetrafluoroethylene film is 0.0095 mm, and the surface roughness Ra is 0.118 μm.

[0052] The tensile strength of the polytetrafluoroethylene film is 91 MPa, the elongation at break is 105%, and the chloride ion permeation concentration after 30 days is 85 ppm.

[0053] The polytetrafluoroethylene film has a melt viscosity of 9.3×10 9 It was prepared by modified polytetrafluoroethylene suspension resin with Pa·S, particle size of 22μm and SSG of 2.163.

[0054] The specific preparation steps of the polytetraethylene film are as follows:

[0055] (1) Raw material screening: sieve the modified polytetrafluoroethylene suspension resin through a 10-mesh sieve and set aside;

[0056] (2) Compression molding: The sieved modified polytetrafluoroethylene suspension resin is placed in a mold. At room temperature, the pressure is increased to 3 MPa at a rate of 15 mm / min. The press is retracted. After 4-5 seconds, the pressure is increased again in both directions at a rate of 15 mm / min to 25 MPa. After maintaining the pressure for 15 minutes, the molded body is removed and set aside.

[0057] (3) Sintering: Place the green body obtained in step (2) into a sintering furnace, heat the temperature from room temperature to 290°C at a rate of 3°C / min, and keep it at 290°C for 10 minutes; then heat it from 290°C to 380°C at a rate of 1°C / min, and keep it at 380°C for 7 hours; finally, cool it from 380°C to 290°C at a rate of 1°C / min, and keep it at 290°C for 30 minutes. After cooling naturally from 290°C to room temperature, take out the sintered green body and set it aside;

[0058] (4) Turning: The blank obtained in step (3) was kept at 100° C. for 10 h and then turned on a lathe, and the thickness of the turning film was 0.045 mm;

[0059] (5) Primary calendering and secondary calendering: First, the skived film obtained in step (4) was subjected to primary calendering on a double-roll calender, with the roller temperature of the primary calendering being 350° C. and the primary calendering rate being 1.2 m / min, to obtain a film with a thickness of 0.010 mm;

[0060] Then it enters the calendering roller with a roller temperature of 300℃ for secondary calendering treatment;

[0061] The primary calendering and the secondary calendering were carried out continuously, and the magnifications of the primary calendering and the secondary calendering were both 4.7 times, to obtain a film with a thickness of 0.0095 mm.

[0062] Depend on Figure 2 It can be seen that the surface of the film prepared by the preparation method is flat and smooth.

[0063] Example 2

[0064] The thickness of the polytetrafluoroethylene film is 0.0055 mm, and the surface roughness Ra is 0.120 μm.

[0065] The tensile strength of the polytetrafluoroethylene film is 96 MPa, the elongation at break is 110%, and the chloride ion permeation concentration after 30 days is 91 ppm.

[0066] The polytetrafluoroethylene film has a melt viscosity of 2.2×10 10 It was prepared from a modified polytetrafluoroethylene suspension resin with a particle size of 32 μm and an SSG of 2.170.

[0067] The specific preparation steps of the polytetraethylene film are as follows:

[0068] (1) Raw material screening: sieve the modified polytetrafluoroethylene suspension resin through a 10-mesh sieve and set aside;

[0069] (2) Compression molding: The sieved modified polytetrafluoroethylene suspension resin is placed in a mold. At room temperature, the pressure is increased to 4 MPa at a rate of 80 mm / min. The press is retracted. After 4-5 seconds, the pressure is increased again in both directions at a rate of 80 mm / min to 35 MPa. After maintaining the pressure for 30 minutes, the molded body is removed and set aside.

[0070] (3) Sintering: Place the green body obtained in step (2) into a sintering furnace, heat the temperature from room temperature to 260°C at a rate of 1°C / min, and keep it at 260°C for 60 minutes; then heat it from 260°C to 360°C at a rate of 2°C / min, and keep it at 360°C for 18 hours; finally, cool it from 360°C to 290°C at a rate of 2°C / min, and keep it at 290°C for 60 minutes. After cooling naturally from 290°C to room temperature, take out the sintered green body and set it aside;

[0071] (4) Turning: The blank obtained in step (3) was kept at 200°C for 5 hours and then turned on a lathe, and the thickness of the turning film was 0.03 mm;

[0072] (5) Primary calendering and secondary calendering: First, the skived film obtained in step (4) was subjected to primary calendering on a double-roll calender, with the roller temperature of the primary calendering being 380° C. and the primary calendering rate being 4.5 m / min, to obtain a film with a thickness of 0.0060 mm;

[0073] Then it enters the calendering roller with a roller temperature of 200℃ for secondary calendering treatment;

[0074] The primary calendering and the secondary calendering were carried out continuously, and the magnification of the primary calendering and the secondary calendering was 5 times, to obtain a film with a thickness of 0.0055 mm.

[0075] Comparative Example 1

[0076] The method for preparing the polytetrafluoroethylene film in this comparative example is different from that in Example 1 in that:

[0077] Step (4) turning: the sintered blank is not subjected to heat preservation treatment, but is directly turned on a lathe, and the turning film thickness is 0.03 mm;

[0078] Step (5) calendering: the skived film obtained in step (4) is calendered on a double-roll calender, the calendering roller temperature is 250°C, the calendering rate is 2m / min, and the calendering ratio is 2.5 times.

[0079] The other steps of the preparation method and the modified polytetrafluoroethylene suspension resin used are the same as those in Example 1.

[0080] The film prepared by the method of this comparative example has a thickness of 0.012 mm, a surface roughness Ra of 0.150 μm, a tensile strength of 65 MPa, and an elongation at break of 88%. The chloride ion permeation concentration after 30 days is 217 ppm.

[0081] pass Figure 3 It can be seen that the film prepared by the preparation method described in this comparative example has slight turning marks on its surface.

[0082] Comparative Example 2

[0083] The specific steps of the method for preparing the polytetrafluoroethylene film in this comparative example are as follows:

[0084] (1) Raw material screening: The same modified polytetrafluoroethylene suspension resin as in Example 1 was used, and sieved through a 10-mesh sieve and set aside;

[0085] (2) Compression molding: The sieved modified polytetrafluoroethylene suspension resin is placed in a mold. At room temperature, the pressure is increased to 3 MPa at a rate of 15 mm / min. The press is retracted. After 4-5 seconds, the pressure is increased again in both directions at a rate of 15 mm / min to 30 MPa. After maintaining the pressure for 10 minutes, the molded body is removed and set aside.

[0086] (3) Sintering: The green body obtained in step (2) is placed in a sintering furnace, and the temperature is raised from room temperature to 290°C at a rate of 3°C / min, and kept at 290°C for 10 minutes. The temperature is then raised from 290°C to 380°C at a rate of 1°C / min, and kept at 380°C for 7 hours. The temperature is then lowered from 380°C to 290°C at a rate of 1°C / min, and kept at 290°C for 30 minutes. After the green body is naturally cooled from 290°C to room temperature, it is taken out for use.

[0087] (4) Turning: The sintered blank obtained in (3) is turned on a lathe, and the turning film thickness is 0.03 mm;

[0088] (5) Calendering: The skived film obtained in (4) was calendered on a double-roll calender with a calendering roll temperature of 360°C, a calendering rate of 2 m / min, and a calendering ratio of 3.3 times.

[0089] The film prepared by the method of this comparative example has a thickness of 0.009 mm, a surface roughness Ra of 0.203 μm, a tensile strength of 58 MPa, and an elongation at break of 78%. The chloride ion permeation concentration after 30 days is 175 ppm.

[0090] Comparative Example 3

[0091] The polytetrafluoroethylene film of the comparative example is made of a polytetrafluoroethylene film having a melt viscosity of 1.68×10 11It is made from unmodified polytetrafluoroethylene resin with a Pa·S, a particle size of 18μm, and an SSG of 2.153.

[0092] The specific steps of the method for preparing the polytetrafluoroethylene film in this comparative example are as follows:

[0093] (1) Raw material screening: the unmodified polytetrafluoroethylene suspension resin with high melt viscosity is screened through a 10-mesh screen and set aside;

[0094] (2) Compression molding: The sieved unmodified polytetrafluoroethylene resin is placed in a mold. At room temperature, the pressure is increased to 4 MPa at a rate of 20 mm / min. The press is retracted. After 4-5 seconds, the pressure is increased again in both directions at a rate of 20 mm / min to 33 MPa. After maintaining the pressure for 12 minutes, the molded body is removed and set aside.

[0095] (3) Sintering: The molded product obtained in step (2) was placed in a sintering furnace, and the temperature was raised from room temperature to 290°C at a rate of 3°C / min, and kept at 290°C for 10 minutes. The temperature was then raised from 290°C to 380°C at a rate of 1°C / min, and kept at 380°C for 8 hours. The temperature was then lowered from 380°C to 290°C at a rate of 1°C / min, and kept at 290°C for 30 minutes. After the temperature was naturally lowered from 290°C to room temperature, the product was taken out and set aside.

[0096] (4) Turning: The sintered blank obtained in step (3) is turned on a lathe, and the turning film thickness is 0.03 mm;

[0097] (5) Primary calendering and secondary calendering: The skived film obtained in step (4) was calendered on a double-roll calendering machine with a calendering roller temperature of 345°C and a calendering rate of 1.2 m / min to obtain a film with a thickness of 0.011 mm. The film was then calendered on a calendering roller at a temperature of 200°C with a calendering ratio of 2.8 times.

[0098] The film prepared by the method of this comparative example has a thickness of 0.0105 mm, a surface roughness Ra of 0.128 μm, a tensile strength of 70 MPa, and an elongation at break of 95%. The chloride ion permeation concentration after 30 days is 155 ppm.

[0099] Comparative Example 4

[0100] The specific steps of the method for preparing the polytetrafluoroethylene film in this comparative example are as follows:

[0101] (1) Raw material screening: The same modified polytetrafluoroethylene suspension resin as in Example 1 was used, and sieved through a 10-mesh sieve and set aside;

[0102] (2) Compression molding: The sieved modified polytetrafluoroethylene resin is placed in a mold. At room temperature, the pressure is increased to 4 MPa at a rate of 20 mm / min. The press is retracted. After 4-5 seconds, the pressure is increased again in both directions at a rate of 20 mm / min to 33 MPa. After maintaining the pressure for 12 minutes, the molded body is removed and set aside.

[0103] (3) Sintering: Place the green body obtained in step (2) into a sintering furnace, raise the temperature from room temperature to 290°C at a rate of 3°C / min, keep it at 290°C for 10 min, raise the temperature from 290°C to 380°C at a rate of 1°C / min, keep it at 380°C for 8 h, then cool it from 380°C to 290°C at a rate of 1°C / min, keep it at 290°C for 30 min, cool it naturally from 290°C to room temperature, and then take it out for use;

[0104] (4) Turning: The sintered blank obtained in (3) was kept at 150°C for 10 h and then turned on a lathe. The thickness of the turning film was 0.03 mm.

[0105] (5) Primary calendering and secondary calendering: The skived film obtained in step (4) was calendered on a double-roll calender, with the primary calendering roll temperature at 150° C. and the primary calendering rate at 2 m / min, to obtain a film with a thickness of 0.015 mm.

[0106] Then it enters the calendering roller with a roller temperature of 370℃ for secondary calendering treatment;

[0107] Calendering and secondary calendering are carried out continuously, and the calendering ratio is 4 times.

[0108] The film prepared by the method of this comparative example has a thickness of 0.0075 mm, a surface roughness Ra of 0.227 μm, a tensile strength of 61 MPa, and an elongation at break of 71%. The chloride ion permeation concentration after 30 days is 160 ppm.

[0109] Depend on Figure 4 It can be seen that the surface of the film prepared by the preparation method described in this comparative example is uneven, that is, the flatness is poor.

[0110] The performance measurement data of the polytetrafluoroethylene films prepared in Examples 1-2 and Comparative Examples 1-4 are summarized in Table 1.

[0111] Table 1 Performance measurement data of polytetrafluoroethylene films obtained in various embodiments and comparative examples

[0112]

[0113]

[0114] By comparing and analyzing the data in Table 1, we can see that:

[0115] (1) The thickness of the polytetrafluoroethylene films obtained in Examples 1 and 2 was significantly reduced, all below 0.01 mm, and the tensile strength and elongation were significantly improved. Furthermore, because the flexibility of the polytetrafluoroethylene film increases with decreasing thickness, and the calendering process improves the film's performance, the number of bending resistances was significantly increased. Furthermore, the resulting film had a smooth and flat appearance, with a surface roughness of ≤0.120 μm. Furthermore, the film had excellent permeation resistance, with a chloride ion permeation concentration of less than 100 ppm after 30 days.

[0116] (2) Although the modified polytetrafluoroethylene suspension resin was used in Comparative Example 1, only the conventional oriented film processing technology of calendering below the melting point was used. Therefore, the maximum calendering ratio did not exceed 3 times. Due to the high viscosity, the thickness could not be pressed to a thinner thickness. In addition, the mechanical properties, permeation resistance and surface smoothness of the obtained film were poor.

[0117] (3) Although the modified polytetrafluoroethylene suspension resin in Comparative Example 2 was rolled at a temperature above the melting point and the thickness could reach less than 0.01 mm, the film obtained by this process alone had poor flatness and smoothness, and poor mechanical strength and permeability resistance, which could not meet the application requirements.

[0118] (4) Comparative Example 3 uses unmodified polytetrafluoroethylene suspension resin. Although the resulting film has good surface flatness and smoothness, and its mechanical properties are not too low, due to its high molecular weight and high melt viscosity, the thickness cannot be pressed to below 0.01 mm, and its permeability resistance is also poor.

[0119] (5) Comparative Example 4 was calendered at a temperature lower than the melting point of PTFE and then calendered twice at a temperature higher than the melting point of PTFE. Although the thickness could reach below 0.01 mm, the surface flatness, smoothness, permeability resistance and mechanical strength of the film obtained under these conditions were poor and could not be used.

Claims

1. A polytetrafluoroethylene film, characterized in that: The polytetrafluoroethylene film is an oriented film with a thickness of less than 0.01 mm, and the surface roughness Ra of the oriented film is less than 0.130 μm; The polytetrafluoroethylene film is prepared by the following steps: (1) The melt viscosity is less than 3.5×10 10 The modified polytetrafluoroethylene resin powder of Pa·S is first sieved through a mesh, then molded, sintered, and turned to obtain a turning film; (2) firstly rolling the skived film obtained in step (1) at a roller temperature higher than the melting point of the modified polytetrafluoroethylene resin used; Then, secondary calendering is performed under the condition that the roller temperature is lower than the melting point of the modified polytetrafluoroethylene resin used to obtain an oriented film with a thickness of less than 0.01 mm.

2. The polytetrafluoroethylene film according to claim 1, wherein The tensile strength of the polytetrafluoroethylene film is greater than 80 MPa, the elongation at break is greater than 100%, and the chloride ion permeation concentration after 30 days is less than 100 ppm.

3. The polytetrafluoroethylene film according to claim 1, wherein The blank obtained after sintering in step (1) is first kept at 100-200° C. for 5-20 hours before turning.

4. The polytetrafluoroethylene film according to claim 1, wherein The particle size of the modified polytetrafluoroethylene resin powder in step (1) is 10-50 μm, and the standard relative density of the modified polytetrafluoroethylene resin is 2.130-2.

200.

5. The polytetrafluoroethylene film according to claim 1, wherein The specific operation of the compression molding in step (1) is as follows: At room temperature, increase the pressure to 3-8 MPa at a rate of 10-100 mm / min, and then retract the press. After 3-6 seconds, re-pressurize in both directions to 20-50 MPa at a pressurization rate of 10-100 mm / min and maintain the pressure for more than 10 minutes.

6. The polytetrafluoroethylene film according to claim 5, wherein The pressurization rate is 10-30 mm / min.

7. The polytetrafluoroethylene film according to claim 1, wherein In step (1), the sintering is performed in a stepwise heating manner, specifically: Heat the temperature from room temperature to 250-300°C at a rate of 1-3°C / min and keep warm for 10-80 minutes. Then, the temperature is raised from 250-300°C to 360-390°C at a heating rate of 1-3°C / min and kept at this temperature for 6-20 hours to completely melt the green body obtained by compression molding; Finally, cool down to 280-290℃ at a rate of 1-3℃ / min, keep warm for at least 30min, and then cool naturally from 280-290℃ to room temperature; The thickness of the turning film obtained by turning in the step (1) is 0.03-0.05 mm.

8. The polytetrafluoroethylene film according to claim 1, wherein In the step (2), the temperature of the rollers for the primary calendering is controlled at above 330°C; the temperature of the rollers for the secondary calendering is controlled at 200-320°C; The speed of the first calendering and the second calendering is 0.5-5m / min; The rolling ratios of the primary rolling and the secondary rolling are 1.1-5.0 times.

9. The polytetrafluoroethylene film according to claim 8, wherein The roller temperature of the primary calendering is 330-400°C.

10. Use of the polytetrafluoroethylene film according to any one of claims 1 to 9, characterized in that: Used for medical films and circuit board protective films.

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