Polytetrafluoroethylene biaxially oriented film as well as preparation method and application thereof

Abstract

The invention provides a polytetrafluoroethylene biaxially oriented film as well as a preparation method and application thereof. The preparation method comprises the following steps: carrying out pushing treatment on a blank containing polytetrafluoroethylene resin and an extrusion aid to obtain a sheet; the sheet is subjected to orientation, degreasing and two-way stretching treatment, and the polytetrafluoroethylene two-way stretching film is obtained. The polytetrafluoroethylene biaxially oriented film prepared by the method is good in thickness and uniformity.

Description

Technical Field

[0001] The present invention relates to the technical field of polytetrafluoroethylene films, and particularly relates to a biaxially stretched polytetrafluoroethylene film, a preparation method thereof, and an application thereof. Background Art

[0002] Expanded polytetrafluoroethylene has good corrosion resistance, weather resistance, is non-toxic, pollution-free, and resistant to high and low temperatures. It is an ideal sealing material and has been widely used in fields such as aerospace, medicine, and petrochemical industry. In the field of aerospace industry, aerospace equipment usually has numerous covers and large-area composite material panels for the use of system equipment and daily inspection and maintenance, etc. When connecting such structures, specific sealants or gaskets are required, usually various rubbers, polycarbonates, vulcanized rubbers, etc. These sealing materials have many deficiencies in their own properties, such as large weight, narrow applicable temperature range, easy aging, poor oil resistance, poor flame retardancy, poor mechanical properties, inconvenient installation, short service life, and high maintenance requirements. Compared with other traditional sealing materials, expanded polytetrafluoroethylene has a flexible structure, can fit complex shapes and irregular surfaces, does not require curing, is easy to install, easy to disassemble, has a low density, and can be reused.

[0003] According to the aerospace material standard SAE AMS3255(B), expanded polytetrafluoroethylene is mainly divided into standard type, high-strength type, and fuel-resistant type. Among them, the standard-strength type is a uniaxially stretched structure, and its strength in the stretching direction is relatively high; both the high-strength type and the fuel-resistant type are biaxially stretched structures, and their tensile strengths in any direction are equivalent, so the dimensional stability is better and the application is more extensive. The biaxially stretched polytetrafluoroethylene film is the basis for producing high-strength type and fuel-resistant type expanded polytetrafluoroethylene materials. The uniformity of the thickness and density of the biaxially stretched polytetrafluoroethylene film plays a fundamental role in the performance of high-strength type and fuel-resistant type expanded polytetrafluoroethylene materials.

[0004] The existing preparation method of the biaxially stretched polytetrafluoroethylene film often adopts the rod extrusion stretching method. Its basic preparation process includes steps such as mixing, preforming, pushing, calendering, degreasing, longitudinal stretching, and transverse stretching. However, during the calendering process of the rod, the center is subjected to the greatest force and the longest time, so the thickness and density of the sheet are greater in the area closer to the center; during the transverse stretching process, it is necessary to heat the polytetrafluoroethylene film while stretching it for stretching. Existing heating equipment usually adopts hot air ovens, resistive heaters, infrared heating equipment, etc. The temperature after heating is often uneven, resulting in the problem that the polytetrafluoroethylene film sheet cannot be uniformly stretched, and the uniformity of the thickness and density of the prepared biaxially stretched polytetrafluoroethylene film is poor. Summary of the Invention

[0005] In order to solve the above technical problems existing in the prior art, the present invention provides a biaxially stretched polytetrafluoroethylene film, a preparation method thereof, and an application thereof.

[0006] In a first aspect, the present invention provides a method for preparing a biaxially stretched polytetrafluoroethylene film, comprising:

[0007] Subjecting a blank containing polytetrafluoroethylene resin and an extrusion aid to a pushing process to obtain a sheet;

[0008] Subjecting the sheet to orientation, degreasing, and biaxial stretching processes to obtain a biaxially stretched polytetrafluoroethylene film.

[0009] According to some embodiments of the present invention, the blank containing polytetrafluoroethylene resin and an extrusion aid is prepared by the following method: mixing polytetrafluoroethylene resin and an extrusion aid and heating to obtain a mixed material; subjecting the mixed material to pre-pressing to obtain a blank.

[0010] In some embodiments, the mass ratio of the polytetrafluoroethylene resin to the extrusion aid is 1:(0.2 - 0.3), such as 1:0.2, 1:0.22, 1:0.25, 1:0.28, 1:0.3, etc. In some embodiments, the number-average molecular weight of the polytetrafluoroethylene resin is 2 million - 4 million. In some embodiments, the extrusion aid includes isoparaffinic solvent oil. The isoparaffinic solvent oil described in the present invention preferably includes C10 - C14 isoparaffinic solvent oil. In some embodiments, the heating temperature of the polytetrafluoroethylene resin and the extrusion aid is 50 - 60°C, such as 50°C, 52°C, 55°C, 58°C, 60°C, etc. In some embodiments, the pre-pressing temperature is 50 - 60°C, such as 50°C, 52°C, 55°C, 58°C, 60°C, etc. In some embodiments, the pre-pressing pressure is 0.5 - 1.5 MPa, such as 0.5 MPa, 0.8 MPa, 1 MPa, 1.2 MPa, 1.5 MPa, etc.

[0011] According to some embodiments of the present invention, the blank is cylindrical. In some embodiments, the diameter of the cylindrical blank is 55 - 65 mm.

[0012] According to some embodiments of the present invention, the temperature of the pushing process is 50 - 65°C, such as 50°C, 52°C, 55°C, 58°C, 60°C, 62°C, 65°C, etc.

[0013] According to some embodiments of the present invention, the pressure of the pushing process is 4.0 - 7.0 MPa, such as 4 MPa, 4.5 MPa, 5 MPa, 5.5 MPa, 6 MPa, 6.5 MPa, 7 MPa, etc.

[0014] According to some embodiments of the present invention, the compression ratio of the pushing process is 30 - 50, such as 30, 35, 40, 45, 50, etc.

[0015] According to some embodiments of the present invention, the pressing process is carried out in a press with a flat die. In some embodiments, the flat die includes a blank inlet, a flat sheet outlet, and an extrusion channel connecting the blank inlet and the sheet outlet. The extrusion channel is used to extrude the blank into a sheet, and its extrusion direction is from the blank inlet to the sheet outlet. In some embodiments, the blank inlet is cylindrical. In some embodiments, the ratio of the width to the height of the flat sheet outlet is (125 - 180):1, such as 125:1, 130:1, 135:1, 140:1, 145:1, 150:1, 155:1, 160:1, 170:1, 180:1, etc.

[0016] According to some embodiments of the present invention, the ratio of the width to the height of the sheet is (125 - 180):1, such as 125:1, 130:1, 135:1, 140:1, 145:1, 150:1, 155:1, 160:1, 170:1, 180:1, etc.

[0017] According to some embodiments of the present invention, the thickness of the sheet is 0.7 - 0.8 mm. According to some embodiments of the present invention, the width of the sheet is 100 - 110 mm.

[0018] According to some embodiments of the present invention, the orientation includes: passing the sheet through two - roll orientation under heating conditions to obtain an oriented film.

[0019] In some embodiments, the heating temperature during the orientation process is 70 - 80 °C.

[0020] In some embodiments, the rate at which the sheet passes through two - roll orientation is 5 - 7 m / min.

[0021] In some embodiments, the thickness of the oriented film is 0.70 - 0.75 mm, and the width is 100 - 110 mm.

[0022] According to some embodiments of the present invention, the degreasing includes: removing the extrusion aid from the oriented film under heating conditions.

[0023] In some embodiments, the heating temperature during degreasing is 180 - 220 °C.

[0024] According to some embodiments of the present invention, the biaxial stretching treatment includes longitudinal stretching and transverse stretching.

[0025] In some embodiments, the temperature of the longitudinal stretching is 240 to 260 °C. In some embodiments, the diameter of the stretching roller for the longitudinal stretching is 300 to 340 mm. In some embodiments, the stretching ratio of the longitudinal stretching is 3.5 to 4.5 times.

[0026] In some embodiments, the conditions for the transverse stretching include: a stretching temperature of 200 to 300 °C and a stretching ratio of 20 to 24 times.

[0027] In some embodiments, the heating method in the transverse stretching includes medium-wave radiation heating.

[0028] In some embodiments, the transverse stretching further includes a shaping treatment, and the conditions for the shaping treatment include: a temperature of 360 to 380 °C and a shaping time of 20 to 30 s.

[0029] According to some embodiments of the present invention, the transverse stretching is carried out on a medium-wave radiation heating and width expanding machine set. In some embodiments, the medium-wave radiation heating and width expanding machine set includes:

[0030] A preheating zone: which is used for preheating the polytetrafluoroethylene film to be stretched;

[0031] A stretching zone: which includes a plurality of medium-wave radiation heating blocks, and the stretching zone is used for heating and stretching the preheated polytetrafluoroethylene film;

[0032] A heat setting zone: which is used for heat setting the stretched polytetrafluoroethylene film; and

[0033] A cooling zone: which is used for cooling the set polytetrafluoroethylene film.

[0034] In some embodiments, the stretching zone includes 80 to 120 medium-wave radiation heating blocks.

[0035] In some embodiments, the total radiation power of the medium-wave radiation heating blocks is 40 KW to 60 KW (kilowatts).

[0036] In some embodiments, the heating temperature of the medium-wave radiation heating blocks is 200 to 300 °C.

[0037] In some embodiments, the medium-wave radiation heating blocks are arranged in a fan shape. In some embodiments, with the symmetry line of the fan shape as the central region, the temperature of the medium-wave radiation heating blocks decreases step by step from the central region to both sides.

[0038] In some embodiments, the heating temperature of the heat setting zone is 360 to 380 °C and the heating time is 20 to 30 s.

[0039] According to some embodiments of the present invention, after the sheet is oriented and degreased, it is first longitudinally stretched to obtain a longitudinally stretched base film; then the longitudinally stretched base film is transversely stretched to obtain a biaxially stretched polytetrafluoroethylene film.

[0040] In a second aspect, the present invention provides a biaxially stretched polytetrafluoroethylene film, which is prepared by the method described in the first aspect of the present invention.

[0041] In some embodiments, the thickness of the biaxially stretched polytetrafluoroethylene film is 0.05 - 0.06 mm.

[0042] In a third aspect, the present invention provides the use of the biaxially stretched polytetrafluoroethylene film described in the second aspect in the preparation of expanded polytetrafluoroethylene materials.

[0043] In a fourth aspect, the present invention provides an expanded polytetrafluoroethylene material, which comprises the biaxially stretched polytetrafluoroethylene film described in the second aspect of the present invention, or is prepared from raw materials comprising the biaxially stretched polytetrafluoroethylene film described in the second aspect of the present invention.

[0044] In some embodiments, the expanded polytetrafluoroethylene material is an expanded polytetrafluoroethylene sheet.

[0045] In some embodiments, the expanded polytetrafluoroethylene material is obtained by coiling and sintering the biaxially stretched polytetrafluoroethylene film.

[0046] Compared with the prior art, in the present invention, the polytetrafluoroethylene resin blank is first pressed into a sheet instead of a traditional rod, and then the sheet is oriented, degreased and biaxially stretched. The obtained biaxially stretched polytetrafluoroethylene film has the characteristics of good thickness and density uniformity, avoiding the unevenness problem caused by the large central stress during the rolling of the rod and the greater thickness and density in the central region of the sheet than those on both sides in the conventional preparation process of the biaxially stretched polytetrafluoroethylene film. In addition, in the present invention, during the transverse stretching stage, medium-wave radiation heating is adopted, overcoming the problems of uneven heating temperature caused by the traditional heating method and the non-uniform stretching of the polytetrafluoroethylene film. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the flat die structure used in Example 1 as observed from the sheet outlet.

[0048] Figure 2 For Figure 1 Cross-sectional view at A - A in

[0049] Figure 3 Top view of the lower die of the flat die structure used in Example 1.

[0050] Figure 4Schematic structural diagram of the medium-wave radiation transverse expansion radiation unit used in Example 1.

[0051] The reference numerals are as follows:

[0052] 1. Upper die; 11. First flange; 2. Lower die; 21. Second flange; 3. Positioning pin; 4. Fastener; 5. Cylindrical blank inlet; 6. Flat sheet outlet; 7. Extrusion channel; 8. Preheating zone; 9. Stretching zone; 10. Heat setting zone; 11. Chain clip cooling zone; 12. Cooling zone; 13. Chain clip. Detailed implementation manners

[0053] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments and the accompanying drawings. The specific embodiments described herein are only used to explain the present invention and do not constitute any limitation to the present invention.

[0054] Unless otherwise defined, all technical terms and scientific and technical terms used in the present invention have the same meanings as those commonly used in the field to which the present invention belongs. For the purpose of explaining this specification, the following definitions will be applied, and where appropriate, terms used in the singular form will also include the plural form and vice versa.

[0055] If not otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art, and the raw materials used are all commercially available conventional products.

[0056] In the following examples and comparative examples, the polytetrafluoroethylene resin has the grade CGF216G and is purchased from Zhonghao Chenguang Chemical Research Institute; the extrusion aid used has the grade IP CLEAN LX and is purchased from Idemitsu Kosan Co., Ltd. of Japan.

[0057] The flat die structure of the flat die press used in the following Example 1 is as Figures 1 - 3 shown. The structure of the flat die includes: upper die 1, lower die 2, flanges 11 and 21 connected to the press, cylindrical blank inlet 5, flat sheet outlet 6, and extrusion channel 7 connected between blank inlet 5 and sheet outlet 6. The extrusion direction of the extrusion channel is from the blank inlet to the sheet outlet.

[0058] From Figure 1 it can be seen that the sheet outlet 6 is a flat structure extending horizontally, the width of the sheet outlet 6 is much larger than its height, and the aspect ratio can be any value between (125 - 180):1.

[0059] As Figure 2As shown, the cylindrical blank enters from the inlet 5, passes through the extrusion flow channel 7 and is output from the sheet outlet 6, and is extruded into a sheet of a specific thickness and a specific width. The first flange 11 and the second flange 21 are connected to each other to form a complete circular flange, through which the flat die can be conveniently installed on the blank outlet of the push press.

[0060] like Figure 3 As shown, a positioning pin 3 for positioning and a fastener 4 for fastening are arranged between the upper die 1 and the lower die 2.

[0061] The structure of the medium wave radiation horizontal expansion unit used in the following embodiments is as follows: Figure 4 As shown, it includes: preheating zone 8, stretching zone 9, heat setting zone 10, cooling zone 12, chain clamp 13 and chain clamp cooling zone 11. Among them, the stretching zone 9 adopts medium wave heating, including 100 heating blocks with a total power of 50Kw, which are arranged in a fan shape according to the stretching state of the track. Each heating block can be independently set and controlled in temperature. The fan-shaped symmetry line is the central area. The temperature setting principle is: the central area decreases in steps to both sides. The temperature control is controlled by a touch screen, which is simple to operate and has high temperature control accuracy. Each medium wave plate can be individually controlled in temperature and can be controlled in a row horizontally and vertically. The response time is fast. At the same time, the entire heating system is equipped with fault monitoring and stretching areas.

[0062] Example 1

[0063] A method for preparing a biaxially stretched polytetrafluoroethylene film comprises the following steps:

[0064] (1) The extrusion aid and polytetrafluoroethylene resin were fully mixed in a mass ratio of 0.2:1 and allowed to stand at 55°C for 30 hours;

[0065] (2) Pressing the mixed materials into a cylindrical blank with a diameter of 60 mm in a pre-pressing molding machine at 55° C. and 1.0 MPa;

[0066] (3) The above blank is passed through a flat die ( Figures 1 - 3 The push press (shown) is used to push and extrude a sheet with a thickness of 0.75 mm and a width of 105 mm at 60°C and 5.0 MPa;

[0067] (4) passing the sheet through a calender and orienting at 70° C. at a speed of 5 m / min to obtain an oriented film with a thickness of 0.7 mm and a width of 105 mm;

[0068] (5) removing the extrusion aid from the oriented film at 200° C., and longitudinally stretching the oriented film at 250° C. with a longitudinal stretching ratio of 4.0 times to obtain a longitudinally stretched base film;

[0069] (6) The longitudinally stretched base film is Figure 4The transverse stretching is performed on the medium wave radiation heating transverse expansion unit shown in the figure, the transverse stretching multiple is 20 times, and the shaping is performed at 370°C to prepare a polytetrafluoroethylene biaxially stretched film with a thickness of 0.05-0.06mm;

[0070] (7) The polytetrafluoroethylene biaxially oriented film obtained above was rolled and sintered by a coating unit to obtain an expanded polytetrafluoroethylene sheet with a specification of 1000*1000*2 mm, and the sheet was sampled in a 9-grid manner with a sampling size of 100*100 mm. The sample weight was weighed with an analytical balance (the result was rounded to two decimal places). The results are shown in Table 1 below.

[0071] Table 1

[0072] Number 1 2 3 4 5 6 7 8 9 Weight / g 10.75 11.02 11.15 11.23 10.99 11.35 10.98 10.88 11.02

[0073] R (lightest sample weight / heaviest sample weight) is used to represent the uniformity of the overall density and thickness of the biaxially stretched film. The larger the R, the better the uniformity of the thickness and density of the biaxially stretched film.

[0074] R = 10.75 g / 11.35 g = 0.947

[0075] Example 2

[0076] A method for preparing a biaxially stretched polytetrafluoroethylene film comprises the following steps:

[0077] (1) The extrusion aid and polytetrafluoroethylene resin were fully mixed in a mass ratio of 0.2:1 and allowed to stand at 55°C for 30 hours;

[0078] (2) Pressing the mixed materials into a cylindrical blank with a diameter of 60 mm in a pre-pressing molding machine at 55° C. and 1.0 MPa;

[0079] (3) The above blank is passed through a flat die ( Figures 1 - 3 The push press (shown) is used to push and extrude a sheet with a thickness of 0.7 mm and a width of 100 mm at 50°C and 6.0 MPa;

[0080] (4) passing the sheet through a calender and orienting at 70° C. at a speed of 5 m / min to obtain an oriented film with a thickness of 0.7 mm and a width of 105 mm;

[0081] (5) removing the extrusion aid from the oriented film at 200° C., and longitudinally stretching the oriented film at 250° C. with a longitudinal stretching ratio of 4.0 times to obtain a longitudinally stretched base film;

[0082] (6) The longitudinally stretched base film is Figure 4Transverse stretching is carried out on the medium-wave radiation heating transverse stretching unit shown, with a transverse stretching multiple of 20 times and shaping at 370 °C to prepare a biaxially stretched polytetrafluoroethylene film with a thickness of 0.05 - 0.06 mm;

[0083] (7) The obtained biaxially stretched polytetrafluoroethylene film is wound and sintered through a laminating unit to obtain an expanded polytetrafluoroethylene sheet with specifications of 1000 * 1000 * 2 mm. The sheet is sampled in the form of a 9-grid, with a sampling size of 100 * 100 mm, and the weight of the sample is weighed using an analytical balance (the result is retained to two decimal places). The results are shown in Table 2 below.

[0084] Table 2

[0085] Number 1 2 3 4 5 6 7 8 9 Weight / g 10.93 10.88 11.24 11.54 11.39 10.75 10.66 11.34 10.89

[0086] Use R (the weight of the lightest sample / the weight of the heaviest sample) to represent the density and thickness uniformity of the biaxially stretched film. The larger R is, the better the thickness and density uniformity of the biaxially stretched film.

[0087] R = 10.88 g / 11.39 g = 0.955

[0088] Example 3

[0089] A method for preparing a biaxially stretched polytetrafluoroethylene film, comprising the following steps:

[0090] (1) The extrusion aid and polytetrafluoroethylene resin are fully mixed at a mass ratio of 0.20:1 and left standing at 55 °C for 30 hours;

[0091] (2) The mixed material is pressed into a cylindrical blank with a diameter of 60 mm in a pre-pressing molding machine at 55 °C and 1.0 MPa;

[0092] (3) The above blank is pushed and extruded through a flat die ( Figures 1 - 3 shown) pusher at 60 °C and 5.0 MPa to obtain a sheet with a thickness of 0.75 mm and a width of 105 mm;

[0093] (4) The above sheet is oriented through a calender at 70 °C at a speed of 5 m / min to obtain an oriented film with a thickness of 0.7 mm and a width of 105 mm;

[0094] (5) The above oriented film is de-aided at 200 °C and longitudinally stretched at 250 °C with a longitudinal stretching multiple of 4.0 times to obtain a longitudinally stretched base film;

[0095] (6) The above longitudinally stretched base film is at Figure 4Transverse stretching is carried out on the medium-wave radiation heating transverse stretching unit shown, with a transverse stretching multiple of 24 times and setting at 360 °C to prepare a biaxially stretched polytetrafluoroethylene film with a thickness of 0.05 - 0.06 mm;

[0096] (7) The obtained biaxially stretched polytetrafluoroethylene film is coiled and sintered through a laminating unit to obtain an expanded polytetrafluoroethylene sheet with specifications of 1000 * 1000 * 2 mm. The sheet is sampled in the form of a 9-grid, with a sampling size of 100 * 100 mm, and the weight of the sample is weighed using an analytical balance (the result is retained to two decimal places). The results are shown in Table 3 below.

[0097] Table 3

[0098] Number 1 2 3 4 5 6 7 8 9 Weight / g 11.11 10.75 11.37 10.69 11.05 11.09 10.88 11.15 11.20

[0099] Use R (the weight of the lightest sample / the weight of the heaviest sample) to represent the density and thickness uniformity of the biaxially stretched film. The larger R is, the better the thickness and density uniformity of the biaxially stretched film.

[0100] R = 10.69 g / 11.37 g = 0.940

[0101] Example 4

[0102] A method for preparing a biaxially stretched polytetrafluoroethylene film, comprising the following steps:

[0103] (1) The extrusion aid and polytetrafluoroethylene resin are fully mixed at a mass ratio of 0.2:1 and left standing at 55 °C for 30 hours;

[0104] (2) The mixed material is pressed into a cylindrical blank with a diameter of 60 mm in a pre-pressing molding machine at 55 °C and 2 MPa;

[0105] (3) The above blank is pushed and extruded through a flat die ( Figures 1 - 3 shown) pusher at 60 °C and 5.0 MPa to obtain a sheet with a thickness of 0.75 mm and a width of 105 mm;

[0106] (4) The above sheet is oriented through a calender at 70 °C at a speed of 5 m / min to obtain an oriented film with a thickness of 0.7 mm and a width of 105 mm;

[0107] (5) The above oil-based film is deoiled at 200 °C and longitudinally stretched at 260 °C with a longitudinal stretching multiple of 4.0 times to obtain a longitudinally stretched base film;

[0108] (6) The above longitudinally stretched base film is transversely stretched on a conventional transverse stretching unit with a transverse stretching multiple of 20 times and set at 370 °C to prepare a biaxially stretched polytetrafluoroethylene film;

[0109] (7) The obtained polytetrafluoroethylene biaxially stretched film is wound and sintered through a film laminating machine to obtain an expanded polytetrafluoroethylene sheet with a specification of 1000*1000*2 mm. The sheet is sampled in the form of a 9-grid, with a sampling size of 100*100 mm, and the weight of the sample is weighed with an analytical balance (the result is reserved to two decimal places). The results are shown in Table 4 below.

[0110] Table 4

[0111] Number 1 2 3 4 5 6 7 8 9 Weight / g 10.47 11.26 10.88 11.57 11.43 11.37 10.65 10.93 11.64

[0112] R = 10.47 g / 11.64 g = 0.899

[0113] Comparative Example 1

[0114] A preparation method of a polytetrafluoroethylene biaxially stretched film includes the following steps:

[0115] (1) The extrusion aid and polytetrafluoroethylene resin are fully mixed at a mass ratio of 0.2:1 and left standing at 55 °C for 30 hours;

[0116] (2) The mixed material is pressed into a blank with a diameter of 60 mm in a pre-pressing machine at 55 °C and 2 MPa;

[0117] (3) The above blank is pushed out into a tetrafluoro rod with a diameter of 10 mm on a push press at 60 °C and 5.0 MPa;

[0118] (4) The above tetrafluoro rod is calendered into an oil-based film with a thickness of 0.3 mm at a speed of 15 m / min on a calender at 55 °C;

[0119] (5) The above oil-based film is de-aided at 200 °C and longitudinally stretched at 260 °C with a longitudinal stretching ratio of 4.0 times to obtain a longitudinally stretched base film;

[0120] (6) The above longitudinally stretched base film is transversely stretched on a conventional transverse stretching unit with a transverse stretching ratio of 20 times and is shaped at 370 °C to prepare a polytetrafluoroethylene biaxially stretched film;

[0121] (7) The obtained polytetrafluoroethylene biaxially stretched film is wound and sintered through a film laminating machine to obtain an expanded polytetrafluoroethylene sheet with a specification of

[0122] 1000*1000*2 mm. The sheet is sampled in the form of a 9-grid, with a sampling size of 100*100 mm, and the weight of the sample is weighed with an analytical balance (the result is reserved to two decimal places). The results are shown in Table 5 below.

[0123] Table 5

[0124] Number 1 2 3 4 5 6 7 8 9 Weight / g 11.26 11.79 11.00 11.10 10.99 12.35 11.98 11.88 10.42

[0125] R = 10.42 g / 12.35 g = 0.844.

[0126] Comparative Example 2

[0127] A method for preparing a biaxially stretched polytetrafluoroethylene film comprises the following steps:

[0128] (1) The extrusion aid and polytetrafluoroethylene resin were fully mixed in a mass ratio of 0.2:1 and allowed to stand at 55°C for 30 hours;

[0129] (2) Pressing the mixed material into a cylindrical blank with a diameter of 60 mm in a pre-pressing molding machine at 55° C. and 2 MPa;

[0130] (3) The blank is pushed out on a push press at 60°C and 5.0 MPa to form a PTFE rod with a diameter of 10 mm;

[0131] (4) rolling the above-mentioned polytetrafluoroethylene rod into a 0.3 mm oil-containing base film on a calender at 55° C. and a speed of 15 m / min;

[0132] (5) removing the extrusion aid from the oil-containing base film at 200° C., and longitudinally stretching the base film at 260° C. at a longitudinal stretching ratio of 4.0 times to obtain a longitudinally stretched base film;

[0133] (6) The longitudinally stretched base film is Figure 4 The transverse stretching is performed on the medium wave radiation heating transverse expansion unit shown in the figure, the transverse stretching multiple is 20 times, and the shaping is performed at 370°C to prepare a polytetrafluoroethylene biaxially stretched film;

[0134] (7) The polytetrafluoroethylene biaxially oriented film obtained above was rolled and sintered by a coating unit to obtain an expanded polytetrafluoroethylene sheet with a specification of 1000*1000*2 mm, and the sheet was sampled in a 9-grid manner with a sampling size of 100*100 mm. The sample weight was weighed with an analytical balance (the result was rounded to two decimal places). The results are shown in Table 6 below.

[0135] Table 6

[0136] Number 1 2 3 4 5 6 7 8 9 Weight / g 10.39 10.85 11.75 10.78 11.23 11.10 10.78 10.95 11.50

[0137] R = 10.39 g / 11.75 g = 0.884.

[0138] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a biaxially stretched polytetrafluoroethylene film, comprising: Subjecting a blank containing polytetrafluoroethylene resin and an extrusion aid to a pressing process to obtain a sheet; Subjecting the sheet to orientation, degreasing, and biaxial stretching processes to obtain a biaxially stretched polytetrafluoroethylene film.

2. The method according to claim 1, wherein The blank containing polytetrafluoroethylene resin and an extrusion aid is prepared by the following method: mixing polytetrafluoroethylene resin with an extrusion aid and heating to obtain a mixed material; subjecting the mixed material to pre-pressing and forming to obtain the blank; Preferably, the mass ratio of the polytetrafluoroethylene resin to the extrusion aid is 1:(0.2 - 0.3); Preferably, the number-average molecular weight of the polytetrafluoroethylene resin is 2 million - 4 million; Preferably, the extrusion aid includes isoparaffinic solvent oil, preferably isoparaffinic solvent oil with C10 - C14; Preferably, the heating temperature is 50 - 60°C; Preferably, the conditions for the pre-pressing and forming include: temperature 50 - 60°C, and / or pressure 0.5 - 1.5 MPa; Preferably, the blank is a cylindrical blank; more preferably, the diameter of the cylindrical blank is 55 - 65 mm.

3. The method according to claim 1 or 2, characterized in that, The conditions for the pressing process include: temperature 50 - 65°C, and / or pressure 4.0 - 7.0 MPa, and / or compression ratio 30 - 50; Preferably, the pressing process is carried out in a press with a flat die, the flat die includes a blank inlet, a flat sheet outlet, and an extrusion channel connecting the blank inlet and the sheet outlet, the extrusion channel is used to extrude the blank into a sheet, and its extrusion direction is from the blank inlet to the sheet outlet; Preferably, the blank inlet is cylindrical; Preferably, the ratio of the width to the height of the sheet outlet is (125 - 180):

1.

4. The method according to any one of claims 1 to 3, characterized in that, The ratio of the width to the height of the sheet is (125 - 180):1; Preferably, the thickness of the sheet is 0.7 - 0.8 mm; and / or the width is 100 - 110 mm.

5. The method according to any one of claims 1 to 4, characterized in that, The orientation includes: passing the sheet through two-roll orientation under heating conditions to obtain an oriented film; Preferably, the heating temperature is 70 - 80°C; Preferably, the rate of passing the sheet through two-roll orientation is 5 - 7 m / min; Preferably, the thickness of the oriented film is 0.70 - 0.75 mm and the width is 100 - 110 mm.

6. The method according to any one of claims 1 to 5, characterized in that, The degreasing includes: removing the extrusion aid from the oriented film under heating conditions; and / or the biaxial stretching process includes longitudinal stretching and transverse stretching; Preferably, the heating temperature during degreasing is 180 - 220°C; Preferably, the conditions for the longitudinal stretching include: temperature 240 - 260°C, diameter of the stretching roller 300 - 340 mm, stretching multiple 3.5 - 4.5 times; Preferably, the conditions for the transverse stretching include: stretching temperature 200 - 300°C, stretching multiple 20 - 24 times; Preferably, the heating method for the transverse stretching includes medium-wave radiation heating; Preferably, the transverse stretching further includes a shaping process, and the conditions for the shaping process include: temperature 360 - 380°C, time 20 - 30 s; Preferably, the stretching treatment includes: first performing longitudinal stretching to obtain a longitudinally stretched base film; then performing transverse stretching on the longitudinally stretched base film to obtain a biaxially stretched polytetrafluoroethylene film.

7. The method according to claim 6, wherein The transverse stretching is carried out on a medium-wave radiation heating and width expanding machine set; preferably, the medium-wave radiation heating and width expanding machine set includes: A preheating zone: which is used for preheating the polytetrafluoroethylene film to be stretched; A stretching zone: which includes preferably 80 to 120 medium-wave radiation heating blocks, and the stretching zone is used for heating and stretching the preheated polytetrafluoroethylene film; A heat setting zone: which is used for heat setting the stretched polytetrafluoroethylene film; A cooling zone: which is used for cooling the heat-set polytetrafluoroethylene film; Preferably, the total radiation power of the medium-wave radiation heating blocks is 40KW to 60KW; Preferably, the medium-wave radiation heating blocks are arranged in a fan shape, and more preferably, with the symmetry line of the fan as the central area, the temperature of the medium-wave radiation heating blocks decreases step by step from the central area to both sides; Preferably, the heating temperature of the heat setting zone is 360 to 380 °C, and the setting time is 20 to 30 s.

8. A biaxially stretched polytetrafluoroethylene film, which is prepared by the method according to any one of claims 1 to 7; Preferably, the thickness of the biaxially stretched polytetrafluoroethylene film is 0.05 to 0.06 mm.

9. Use of the biaxially stretched polytetrafluoroethylene film according to claim 8 in the preparation of expanded polytetrafluoroethylene materials.

10. An expanded polytetrafluoroethylene material, which includes the biaxially stretched polytetrafluoroethylene film according to claim 8, or is prepared from raw materials including the biaxially stretched polytetrafluoroethylene film according to claim 8; Preferably, the expanded polytetrafluoroethylene material is an expanded polytetrafluoroethylene plate; Preferably, the expanded polytetrafluoroethylene material is obtained by coiling and sintering the biaxially stretched polytetrafluoroethylene film.

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