Method for producing polytetrafluoroethylene molded body, method for producing polytetrafluoroethylene film, and method for producing laminate

Abstract

To provide a production method of a PTFE molded product with excellent abrasion resistance, in which the generation of cracks is suppressed in a production step.SOLUTION: A production method of a polytetrafluoroethylene molded product is a production method of a PTFE molded product having: (1) a first step to obtain a first molding by heating a molding material including PTFE with melt viscosity at 380°C of 7×105 Pa s or lower to a crystalline melting point of PTFE or higher; and (2) a second step to obtain a second molding after the first step by making the first molding be irradiated with ionizing radiation in a condition of no oxygen and at the crystalline melting point or higher, in which when conducting the second step after the first step, (a) after the first step, the second step is conducted while maintaining temperature of the first molding to be not lower than the crystalline melting point, or (b) after the first step, at least in a temperature range equal to or lower than the crystalline melting point and 300°C or higher, the first molding is cooled at a cooling rate of less than 2°C / minute, then the second step is conducted by heating the first molding to the crystalline melting point or higher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a polytetrafluoroethylene molded article, a method for producing a polytetrafluoroethylene membrane, and a method for producing a laminate provided with a tetrafluoroethylene membrane. [Background technology]

[0002] As a method for obtaining a molded polytetrafluoroethylene body having excellent mechanical properties such as abrasion resistance, for example, Patent Document 1 discloses a method for obtaining a molded polytetrafluoroethylene body having excellent mechanical properties such as abrasion resistance, the method comprising a step of irradiating a molding material containing polytetrafluoroethylene with ionizing radiation, and the melt viscosity of the polytetrafluoroethylene at 380° C. is 7×10 5 A method for producing a polytetrafluoroethylene molded article having a compressive strength of 0.1 Pa·s or less has been proposed. The use of ionizing radiation in producing a polytetrafluoroethylene molded article is also described in, for example, Patent Documents 2 to 4. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 043372 [Patent Document 2] JP 2010-94579 A [Patent Document 3] JP 2007-77323 A [Patent Document 4] JP 2012-197779 A Summary of the Invention [Problem to be solved by the invention]

[0004] When manufacturing a molded product of polytetrafluoroethylene (hereinafter also referred to as PTFE), cracks may occur in the molded product during manufacturing. If a crack occurs in the molded product during manufacturing, the subsequent process cannot be carried out, and the manufacturing process up to that point becomes a waste. For example, when manufacturing a PTFE membrane, if a molding material containing PTFE (such as a dispersion liquid containing PTFE) is applied to a support, and then heated to the crystalline melting point of PTFE or higher and then cooled, the cooled membrane-like molding may crack or peel off from the support. In particular, when an attempt is made to produce a PTFE membrane using PTFE having a small molecular weight, such cracking and peeling during cooling is likely to occur. [Means for solving the problem]

[0005] The present invention has been made in consideration of the above circumstances, and has an object to provide a method for producing a polytetrafluoroethylene molded article that suppresses the occurrence of cracks and the like in the molded article during production.

[0006] One aspect of the present invention is a method for producing a composition comprising the steps of: (1) The melt viscosity at 380°C is 7×10 5 a first step of heating a molding material containing polytetrafluoroethylene having a molecular weight of 0.01 Pa·s or less to a crystalline melting point of the polytetrafluoroethylene or more to obtain a first molded product; (2) a second step of irradiating the first molded product with ionizing radiation under conditions of an oxygen-free condition and a temperature equal to or higher than the crystalline melting point of the polytetrafluoroethylene to obtain a second molded product; A method for producing a polytetrafluoroethylene molded article comprising the steps of: After the first process, when carrying out the second process, (a) after the first step, a second step is carried out while maintaining the temperature of the first molded product so as not to fall below the crystalline melting point, or (b) after the first step, the temperature of the first molded product is cooled at a cooling rate of less than 2°C / min at least in a temperature range of 300°C or more below the crystalline melting point of the polytetrafluoroethylene, and then the polytetrafluoroethylene is heated again to a temperature above the crystalline melting point to perform the second step; A method for producing a polytetrafluoroethylene molded article.

[0007] Another aspect of the present invention is In the method for producing a polytetrafluoroethylene membrane, a polytetrafluoroethylene membrane is formed on a support by the above-mentioned method for producing a membrane as a molded article, and then the support is removed.

[0008] Yet another aspect of the present invention is A method for producing a laminate including a substrate and a polytetrafluoroethylene film provided on a surface of the substrate, comprising the steps of: This is a method for producing a laminate, in which a polytetrafluoroethylene membrane is produced as a molded article by the above-mentioned method for producing a polytetrafluoroethylene molded article. Effect of the Invention

[0009] According to the present invention, in the method for producing a polytetrafluoroethylene molded article, it is possible to suppress the occurrence of cracks in the molded article during production. Furthermore, the produced molded article has exceptionally good mechanical properties such as wear resistance. Furthermore, the above-mentioned method for producing a polytetrafluoroethylene molded article can be suitably used for a method for producing a polytetrafluoroethylene membrane, or a method for producing a laminate including a substrate and a polytetrafluoroethylene membrane. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a process diagram of a method A for producing a PTFE molded article according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a process diagram of a method B for producing a PTFE molded article according to an embodiment of the present invention. [Diagram 3] 1 is a microscope image of the PTFE coating film produced in Test Example 1. [Figure 4] 1 is a microscope image of the PTFE coating film produced in Test Example 2. [Diagram 5] 1 is a microscope image of the PTFE coating film produced in Test Example 3. [Figure 6] 1 is a microscope image of the PTFE coating film produced in Test Example 4. [Figure 7]1 is a microscope image of the PTFE coating film produced in Test Example 5. [Figure 8] 1 is a microscope image of the PTFE coating film produced in Test Example 6. [Figure 9] 1 is a graph showing the relationship between the cooling rate and the latent heat of fusion of PTFE. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Summary of the embodiment of the invention] First, the contents of the embodiments of the present invention will be listed and described. [1] A method for producing a polytetrafluoroethylene molded article (PTFE molded article) according to one embodiment of the present invention comprises the steps of: (1) The melt viscosity at 380°C is 7×10 5 a first step of heating a molding material containing polytetrafluoroethylene having a molecular weight of 0.01 Pa·s or less to a crystalline melting point of the polytetrafluoroethylene or more to obtain a first molded product; (2) a second step of irradiating the first molded product with ionizing radiation under conditions of an oxygen-free condition and a temperature equal to or higher than the crystalline melting point of the polytetrafluoroethylene to obtain a second molded product; A method for producing a polytetrafluoroethylene molded article comprising the steps of: After the first process, when carrying out the second process, (a) after the first step, a second step is carried out while maintaining the temperature of the first molded product so as not to fall below the crystalline melting point, or (b) After the first step, the temperature of the first molded product is cooled at a cooling rate of less than 2°C / min, at least in a temperature range of 300°C or more below the crystalline melting point of the polytetrafluoroethylene, and then the polytetrafluoroethylene is heated again to a temperature above the crystalline melting point to carry out the second step.

[0012] In the above-mentioned method for producing a PTFE molded body, when the above-mentioned second step is carried out after the above-mentioned first step, a treatment is carried out so as to satisfy the requirement (a) or (b). Therefore, according to the above-mentioned production method, it is possible to suppress the occurrence of cracks in the molded product during production (particularly the first molded product after the first step and before the second step is carried out). Therefore, the above-mentioned method for producing a PTFE molded body can produce a PTFE molded product with high production efficiency.

[0013] The method that satisfies the above requirement (a) is an effective method when the second step is carried out immediately after the first step. On the other hand, when manufacturing a PTFE molded body, the first and second steps may be carried out independently, for example, by a batch method. In this case, continuing to heat the first molded body after the first step in order to maintain the temperature of the first molded body at a predetermined temperature until the second step results in excessive energy consumption, which is undesirable both environmentally and economically. Therefore, in the above case, it is often more effective to cool the temperature of the first molded body after the first step and before carrying out the second step. In this case, the molded body is treated to satisfy the requirement (b) in order to prevent cracks from occurring during production. Therefore, a method that satisfies the above requirement (b) is an effective method when the above first step and the above second step are carried out independently.

[0014] [2] In the method for producing the PTFE molded article, the melt viscosity is 1×10 2 The melt viscosity of the PTFE used is preferably 1×10 2 By setting the viscosity at Pa·s or more, it is possible to suppress the volatilization of PTFE during irradiation with ionizing radiation, and to obtain a molded article with superior mechanical properties.

[0015] [3] In the above-mentioned method for producing a PTFE molded article, the dose of the ionizing radiation is preferably 10 kGy or more and 2000 kGy or less. [4] In the above-mentioned method for producing a PTFE molded article, the dose of the ionizing radiation is more preferably 30 kGy or more and 200 kGy or less. By setting the irradiation dose of ionizing radiation within the above range, the mechanical properties of the obtained PTFE molded article can be further improved. This is believed to be because the irradiation dose within the above range is more suitable for forming a good crosslinked state.

[0016] [5] In the above-mentioned method for producing a PTFE molded article, the polytetrafluoroethylene molded article is preferably used for a sliding member. A sliding member using the PTFE molded article obtained by this production method and having excellent mechanical properties is suitable for exhibiting high durability.

[0017] [6] In the above-mentioned method for producing a PTFE molded article, in the first step, the first molded article is preferably formed by extrusion molding or injection molding. The PTFE used in this production method has melt flowability and does not have fibrillation properties. Therefore, extrusion molding or injection molding can be suitably performed, and PTFE molded articles of various shapes can be efficiently produced.

[0018] [7] In the method for producing the PTFE molded article, the first molded article is formed by coating the molding material in the first step, and the molding material is preferably a powder or an aqueous dispersion. The PTFE used in this production method has melt flowability and does not have fibrillation properties, so it can be applied by powder coating or as an aqueous dispersion. In this case, a PTFE membrane can be efficiently produced as a molded body.

[0019] [8] In the above-mentioned method for producing a PTFE molded article, the number average molecular weight of the polytetrafluoroethylene contained in the molding material is preferably 600,000 or less. Since low molecular weight PTFE has a short molecular chain and is less entangled with other PTFE molecules, the molded product tends to crack during production. Therefore, low molecular weight PTFE is suitable as a material for producing a PTFE molded product by the method for producing a PTFE molded product according to the embodiment of the present invention. Furthermore, by using a low molecular weight PTFE, a PTFE molded article having superior mechanical properties can be obtained.

[0020] [9] In the above-mentioned method for producing a PTFE molded article, the molded article produced is preferably a membrane. When a PTFE membrane is produced using low molecular weight PTFE, the membrane-shaped molded product tends to easily crack during production. Therefore, the PTFE membrane is particularly suitable as a target for production in the method for producing a PTFE molded product according to the embodiment of the present invention.

[0021]

[10] In another embodiment of the present invention, a method for producing a polytetrafluoroethylene membrane (PTFE membrane) comprises forming a PTFE membrane on a support by the method for producing a PTFE molded body described in [9] above, and then removing the support. According to the above-mentioned method for producing a PTFE membrane, it is possible to suppress the occurrence of cracks in a molded product during production and the peeling of the molded product from the support, and it is also possible to produce a PTFE membrane having excellent mechanical properties.

[0022]

[11] A method for producing a laminate according to yet another embodiment of the present invention is a method for producing a laminate including a substrate and a PTFE membrane provided on a surface of the substrate, comprising the steps of: The PTFE membrane is formed by using the method for producing a PTFE molded article described in [9] above. According to the above-mentioned method for producing a laminate, a laminate having a PTFE membrane can be produced while suppressing the occurrence of cracks in the molded product and peeling from the substrate during the production process. Moreover, the produced laminate is a laminate provided with a PTFE membrane having excellent mechanical properties.

[0023] [Details of the embodiment of the invention] Specific examples of the embodiments of the present invention will be described below with reference to the drawings as appropriate. Note that the present invention is not limited to these examples. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0024] <Method for manufacturing PTFE molded article> The method for manufacturing a PTFE molded article according to an embodiment of the present invention is as follows. (1) A first step of obtaining a first molded article by heating a molding material containing polytetrafluoroethylene having a melt viscosity at 380 ° C of 7 × 10 5 Pa·s or less to a temperature equal to or higher than the crystal melting point of the polytetrafluoroethylene; (2) A second step of irradiating the first molded article with ionizing radiation under an oxygen-free condition and at a temperature equal to or higher than the crystal melting point of the polytetrafluoroethylene to obtain a second molded article after the first step. It is a method for manufacturing a polytetrafluoroethylene molded article having When performing the second step after the first step, (a) After the first step, the second step is performed while maintaining the temperature of the first molded article so as not to fall below the crystal melting point, or (b) After the first step, in at least the temperature range of 300 ° C or higher and lower than the crystal melting point of the polytetrafluoroethylene, the temperature of the first molded article is cooled at a cooling rate of less than 2 ° C / min, and then the polytetrafluoroethylene is heated again to a temperature equal to or higher than the crystal melting point to perform the second step.

[0025] According to the method for manufacturing the PTFE molded article, by irradiating a molded article containing PTFE having a low melt viscosity, that is, a low molecular weight, with ionizing radiation, a PTFE molded article excellent in mechanical properties such as wear resistance can be obtained. Although the reason for this is not clear, by using PTFE with relatively short molecular chains, when forming a crosslinked structure by irradiation with ionizing radiation, a crosslinked structure in which the molecular chains are intricately intertwined is formed, and it is presumed that the mechanical strength is improved by this intricate intertwining of the molecular chains. In addition, in the NMR spectrum of the obtained PTFE molded article, although there is no difference from the case where high molecular weight PTFE is used, it is presumed that there is a difference in the intertwining of the molecular chains even though the crosslinked structure and crosslink density are the same.

[0026] The manufacturing method of the PTFE molded body described above is carried out so as to satisfy the requirement of (a) above or the requirement of (b) above. Therefore, it is possible to suppress the occurrence of cracks in the first molded body before performing the second step, and the productivity of the PTFE molded body is improved. Hereinafter, an embodiment that satisfies the requirement of (a) is referred to as the manufacturing method A of the PTFE molded body, and an embodiment that satisfies the requirement of (b) is referred to as the manufacturing method B of the PTFE molded body, and each embodiment will be described.

[0027] <<Manufacturing Method A of PTFE Molded Body>> FIG. 1 is a process diagram of the manufacturing method A of the PTFE molded body according to an embodiment of the present invention. (First Step) In the first step, first, a molding material containing polytetrafluoroethylene (PTFE) having a melt viscosity at 380 ° C. of 7×10 5 Pa·s or less is prepared (S11).

[0028] The above molding material is mainly composed of PTFE. Here, being mainly composed of PTFE means that the content of PTFE in the above molding material is 50% by mass or more based on the total solid content.

[0029] The shape of the above PTFE is not particularly limited, but usually can be in the form of particles. Also, the shape of the molding material is not particularly limited, such as powdery, solution-like, dispersion-like, etc.

[0030] The above PTFE may be homopolymer PTFE, modified PTFE, or a mixture of both. Homopolymer PTFE refers to a homopolymer of tetrafluoroethylene (TFE). Modified PTFE refers to a copolymer of TFE and a monomer other than TFE (also referred to as a "modifying monomer"). The upper limit of the content ratio of the structural unit derived from the modifying monomer in the modified PTFE is preferably 1% by mass, more preferably 0.5% by mass. As the modifying monomer, known monomers can be used, and one kind can be used alone or two or more kinds can be used.

[0031] The method for producing the PTFE is not particularly limited. For example, PTFE having a desired melt viscosity can be suitably obtained by known emulsion polymerization or the like. Specifically, the emulsion polymerization can be carried out by emulsion polymerization of tetrafluoroethylene (TFE) in the presence of an aqueous dispersion medium, a surfactant, and a radical polymerization initiator. In addition, commercially available PTFE can also be used.

[0032] The aqueous dispersion medium is water or a mixed dispersion medium of water and a water-soluble organic dispersion medium (such as alcohol). Water is preferred as the aqueous dispersion medium.

[0033] The surfactant is preferably a fluorine-containing surfactant such as perfluoroalkyl carboxylic acid and its salt, fluorine-containing sulfonic acid and its salt, etc. Specific examples of the perfluoroalkyl carboxylic acid include perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorononanoic acid, etc. The fluorine-containing sulfonic acid includes perfluorooctanesulfonic acid, etc. Examples of the salts of these include alkali metal salts such as lithium salt, sodium salt, potassium salt, etc., and ammonium salt.

[0034] Examples of the radical polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble organic peroxides such as disuccinic acid peroxide and tert-butyl hydroperoxide.

[0035] In the above polymerization, it is preferable to use a chain transfer agent in order to control the molecular weight (melt viscosity) of the resulting PTFE. For example, by increasing the amount of the chain transfer agent used, it is possible to obtain PTFE with a lower molecular weight. Examples of the chain transfer agent include saturated hydrocarbons such as methane, ethane, propane, and butane, halogenated saturated hydrocarbons such as chloromethane, dichloromethane, and difluoroethane, and alcohols such as methanol and ethanol.

[0036] During the polymerization, an emulsion stabilizer such as paraffin wax or a pH adjuster such as ammonium carbonate can be used, if necessary.

[0037] The PTFE obtained by such emulsion polymerization or the like may be used as a dispersion or as a dried powder. When the PTFE is in a particulate form, the average primary particle size of the PTFE particles may be, for example, 0.1 μm or more and 0.5 μm or less.

[0038] The average primary particle size of the PTFE particles is measured by the following method. First, a calibration curve is prepared between the transmittance per unit length of a projected light having a wavelength of 550 nm for an aqueous dispersion in which the polymer concentration is adjusted to 0.22% by mass, and the average primary particle diameter determined by measuring the unidirectional diameter in a transmission electron microscope photograph. Then, the transmittance is measured for the aqueous dispersion to be measured, and the average primary particle diameter is calculated based on the calibration curve.

[0039] The upper limit of the melt viscosity of PTFE at 380°C is 7×10 5 The upper limit of the melt viscosity is 6×10 5 Pa s is preferred, 5×10 5 Pa·s is more preferred. On the other hand, the lower limit of the melt viscosity is 1×10 2 Pa s is preferred, 1×10 3 Pa s is more preferable, 1×10 4 Pa s is more preferable, and 1×10 5 Pa·s is particularly preferred. If the melt viscosity of the PTFE contained in the molding material exceeds the upper limit, a molded body having excellent mechanical properties cannot be obtained. In addition, PTFE with a melt viscosity exceeding the upper limit does not have good melt fluidity, and on the other hand, has fibrillation properties, so that the processability is reduced. Conversely, if the melt viscosity is below the lower limit, the amount of volatile components at high temperatures increases, and volatilization is likely to occur during irradiation with ionizing radiation or heat treatment in the first step, etc., and the mechanical properties of the obtained first molded body tend to decrease. In addition, the molding material and the obtained molded body may be colored.

[0040] The method for measuring the melt viscosity of PTFE will be described below. (Method X) The melt viscosity of PTFE at 380°C is a value measured by the following method X. First, a molded body is prepared as a sample by the following procedure. 3 g of powdered PTFE is filled into a cylindrical mold with an inner diameter of 30 mm, and compressed until the final pressure is 30 MPa. The compression of 30 MPa is maintained for 2 minutes, resulting in a sample as a disk-shaped molded body (diameter 30 mm, thickness approximately 2 mm). The obtained sample is removed from the mold, cut into a disk shape with a diameter of 27 mm, and then clamped between the parallel plate test table with a diameter of 25 mm of Anton Paar's "Rheometer MCR500". The sample is held at 380°C for 5 minutes. The plate distance is then adjusted to 1.5 mm, the sample protruding from the plate is removed, and the sample is held at 380°C for 30 minutes until the stress of the sample is sufficiently relaxed. The melt viscosity at 380°C is measured in a vibration mode with a deformation amount of 0.2% and a shear rate of 0.1 (1 / s) during measurement. In the case of a parallel plate, the amount of deformation during measurement is the ratio of the vibration amplitude at the outermost periphery of the parallel plate to the sample thickness.

[0041] (Method Y) The melt viscosity of PTFE can also be measured by the following method Y. The measured value by method X is substantially equal to the measured value by method Y. This is also shown, for example, in the examples of the above-mentioned Patent Document 1 (see PTFE Powder A in Table 1 of Patent Document 1).

[0042] Method Y is a measurement method conforming to ASTM D 1238. Specifically, using a Shimadzu flow tester and a 2φ-8L die, a 2g sample that has been preheated to the measurement temperature (380°C) for 5 minutes can be measured under a load of 0.7 MPa while maintaining the sample at the above temperature.

[0043] (Method Z) On the other hand, the melt viscosity of high molecular weight PTFE that does not melt flow can be determined by the following method Z, which determines the melt viscosity from the elongation (creep) in the molten state by thermomechanical analysis (TMA). Specifically, a creep test is performed using a solid viscoelasticity spectrometer (SII NanoTechnology's "EXSTAR 6000DMS") according to the following procedure, and the (specific) melt viscosity can be determined.

[0044] First, a sample is prepared in the following manner. 80 g of powder is filled into a cylindrical mold with an inner diameter of 50 mm, and the final pressure is approximately 352 kg / cm 2 Gradually apply pressure for about 30 seconds until the final pressure is reached. After holding this final pressure for 2 minutes, the molded body is removed from the mold. The resulting cylindrical molded body (diameter 50 mm) is sintered for 90 minutes in an air electric furnace heated to 371°C, and then cooled to 250°C at a rate of 1°C / min. After holding at 250°C for 30 minutes, the sintered body is removed from the furnace. Next, the resulting cylindrical sintered body is cut along the side to obtain a strip-shaped sheet with a thickness of 0.50 mm. A small piece with a width of 5 mm and a length of 15 mm is cut from the resulting strip-shaped sheet, and the width and thickness are accurately measured to calculate the cross-sectional area. Next, attach sample mounting fixtures to both ends of this small piece (sample) so that the mounting distance is 1.0 cm. Furthermore, this metal-sample assembly is placed in a cylindrical furnace, heated from room temperature to 380°C at a rate of 20°C / min, held for about 5 minutes, and then a load of about 15 g is applied. From the curve of the change in elongation over time, the elongation between 60 and 120 minutes after loading is read, and the ratio to the time (60 minutes) is calculated. The (specific) melt viscosity (η) can be calculated from the following relational formula (1).

[0045]

number

[0046] In the above formula, W is the tensile load (g), Lr is the length of the sample at 380°C (cm), and g is the gravitational constant (980 cm / s 2 ), dLr / dT is the rate of elongation over time from 60 to 120 minutes (cm / sec), and Ar is the cross-sectional area of ​​the sample at 380°C (cm 2 Here, from a separately obtained thermal expansion measurement, Lr / Ar can be calculated using the following formula (2). Lr / Ar = 0.80 × L (length at room temperature) / A (cross-sectional area at room temperature) (2)

[0047] The melt viscosity at 380°C as described above is 7×10 5 PTFE having a melt viscosity of Pa·s or less has a low molecular weight (low molecular weight PTFE). The number average molecular weight of PTFE is preferably 600,000 or less. On the other hand, the lower limit of the number average molecular weight of PTFE can be, for example, 10,000. The number average molecular weight of PTFE is a value measured according to the method of S. Wu (Polymer Engineering & Science, 1988, Vol. 28, 538; ibid., 1989, Vol. 29, 273).

[0048] Low molecular weight PTFE does not have fibrillation properties due to its low molecular weight. The presence or absence of fibrillation properties of PTFE can be judged by paste extrusion molding. Usually, paste extrusion is possible because high molecular weight PTFE has fibrillation properties. If the unsintered molded product obtained by paste extrusion has no substantial strength or elongation, for example, if the elongation is 0% and it breaks when pulled, it can be considered to have no fibrillation properties.

[0049] The lower limit of the PTFE content in the solid content (non-volatile component) of the molding material is preferably 50% by mass, more preferably 55% by mass. This lower limit may also be 60% by mass, 80% by mass, or 90% by mass. If the PTFE content in the molding material is less than the lower limit, the resulting PTFE molded article may not exhibit excellent mechanical properties. On the other hand, the upper limit of the PTFE content in the solids (non-volatile components) in the above molding material may be 100% by mass, or it may be 90% by mass, 80% by mass, or 70% by mass. Furthermore, when the molding material is a dispersion or solution containing PTFE, the PTFE content can be, for example, from 10% by mass to 60% by mass.

[0050] The lower limit of the PTFE content in the total polymer components in the molding material is preferably 50% by mass, more preferably 70% by mass, and more preferably 90% by mass. By making the PTFE content in the total polymer components equal to or greater than the lower limit, a PTFE molded article having better mechanical properties can be obtained. On the other hand, the upper limit may be 100% by mass or 90% by mass.

[0051] In the above molding material, examples of components that may be contained in addition to PTFE include other polymer components, surfactants, film-forming assistants, antifoaming agents, fillers, pigments, and flame retardants. Examples of the filler include carbon, graphite, glass fiber, and super engineering plastics. Furthermore, when the molding material is in a dispersion state, it contains water, a surfactant, etc. as a dispersion medium, which are substantially all evaporated during the heating in the first step.

[0052] In the first step, next, the molding material is heated to mold a first molded product (S12). Here, the molding material is heated to a temperature equal to or higher than the crystalline melting point of PTFE, which melts the PTFE and causes the PTFE particles to become integrated.

[0053] The crystalline melting point of the PTFE is, for example, 320°C or higher and lower than 350°C. The heating temperature in this first step is not particularly limited as long as it is equal to or higher than the crystalline melting point, but is preferably, for example, 350° C. to 400° C., because this is suitable for melting and integrating the PTFE powder. The heating time in the first step can be, for example, from 1 minute to 1 hour.

[0054] In the present invention, the crystalline melting point of the above-mentioned PTFE is measured by the following method using differential scanning calorimetry. A differential scanning calorimeter (SII NanoTechnology's "X-DSC7000") that has been temperature calibrated in advance using indium and lead as standard samples is used. 3 mg of PTFE is placed in an aluminum pan (crimp container) and heated at a rate of 10°C / min in a temperature range of 230 to 380°C under a nitrogen gas flow of 40 ml / min. The minimum point of the melting peak in the above range is measured and this minimum point is regarded as the crystalline melting point of the PTFE.

[0055] In the first step, the molding material is molded into a predetermined shape prior to heating the molding material. At this time, various molding methods can be selected according to the shape of the PTFE molded article to be produced.

[0056] The molding method may be, for example, extrusion molding, injection molding, etc. Molding by extrusion molding or injection molding is suitable for producing, for example, sliding members and the like. The extrusion molding may be paste extrusion molding, but melt extrusion molding is preferred. 5For PTFE with a viscosity of less than Pa·s, paste extrusion molding may be difficult. On the other hand, low molecular weight PTFE has melt fluidity in the temperature range above its crystalline melting point. Therefore, good molding can be achieved by melt extrusion molding or injection molding. The extrusion molding or injection molding can be carried out by a known method.

[0057] As the molding method, for example, coating of the molding material can be adopted. In this case, the molding material (PTFE) to be applied is preferably a powder or a water dispersion. Coating is suitable as a molding method for obtaining a film-like first molded product. As described above, low molecular weight PTFE does not have fibrillation properties, so the molding material can be effectively used as a powder coating. The specific method of the coating is not particularly limited, and a known method can be adopted.

[0058] In addition, when a membrane-like first molded product is obtained as the first molded product, the first molded product may be molded on a support. In this case, after the second step described below, the support is removed to form a PTFE membrane. This method is particularly suitable for producing a thin PTFE film. The support is, for example, a metal support, and its material and shape are not particularly limited as long as it is not deformed or significantly deteriorated by the heating or irradiation with ionizing radiation in this first step and the second step described below. A specific example of the metal that is the material of the support is preferably stainless steel, for example, because it makes it easy to peel off the PTFE film after the second step is completed, is not easily oxidized when heated, and has excellent corrosion resistance.

[0059] In addition, when a membrane-like first molded product is obtained as the first molded product, the first molded product may be molded on the surface of a substrate constituting a laminate. In this case, a laminate including a substrate and a PTFE membrane molded on the surface of the substrate can be produced by going through the second step described later. The substrate may be appropriately selected depending on the configuration of the laminate, but substrates that deform or deteriorate significantly when heated or exposed to ionizing radiation in the first step and the second step described below are not suitable.

[0060] In such a first step, a first molded article containing uncrosslinked PTFE as a main component is produced. Here, containing uncrosslinked PTFE as a main component means that uncrosslinked PTFE is a component that accounts for 50% by mass or more of the molded article.

[0061] (2nd process) In the method A for producing a PTFE molded product, after the first step, the first molded product is irradiated with ionizing radiation under conditions of no oxygen and at or above the crystalline melting point of the PTFE to obtain a second molded product (S14). In the present manufacturing method A, when carrying out the present second step after the above-mentioned first step, (a) After the first step, it is important to carry out the second step while maintaining the temperature of the first molded product so as not to fall below the crystalline melting point (S13). This makes it possible to prevent the first molded product from cracking and from peeling off from the support or the base material.

[0062] Examples of a method for performing the second step after the first step while maintaining the temperature of the first molded product so that it does not fall below the crystalline melting point include a method in which the second step is performed immediately after molding the first molded product, and a method in which, after molding the first molded product, the first molded product is held in a furnace in which the atmospheric temperature is maintained at or above the crystalline melting point.

[0063] The above-mentioned ionizing radiation is applied in a substantially oxygen-free environment in order to prevent oxidation during the irradiation. -4 Torr or less) or in an inert gas atmosphere such as nitrogen (oxygen concentration 100 ppm or less).

[0064] The irradiation with the ionizing radiation is performed in a state where the first molded article is heated to a temperature equal to or higher than the crystal melting point of the PTFE. This activates the molecular motion of the molecular chains of the PTFE, enabling an efficient crosslinking reaction to occur. The crystal melting point of the PTFE varies depending on the molecular weight and the like, but is, for example, 320°C or higher and less than 350°C. Therefore, as this heating temperature, for example, it can be set to be equal to or higher than the crystal melting point of the PTFE and 320°C or higher and 360°C or lower.

[0065] Examples of the ionizing radiation include electron beams, γ-rays, X-rays, neutron beams, high-energy ions, and the like.

[0066] As the lower limit of the irradiation dose of the ionizing radiation, 10 kGy is preferable, 30 kGy is more preferable, and 50 kGy is even more preferable. On the other hand, as the upper limit of the irradiation dose, 2000 kGy is preferable, 1000 kGy is more preferable, 400 kGy is even more preferable, 200 kGy is still more preferable, and 125 kGy is particularly preferable. When the irradiation dose of the ionizing radiation is less than the lower limit, a sufficient crosslinking reaction may not proceed, and it may not be possible to obtain a molded article having excellent mechanical properties such as wear resistance. On the other hand, when the irradiation dose exceeds the upper limit, productivity may decrease, and the mechanical properties of the obtained molded article may also decrease. By performing such a second step, a second molded article made of a crosslinked product of PTFE can be obtained.

[0067] <<Manufacturing Method B of PTFE Molded Article>> FIG. 2 is a process diagram of Manufacturing Method B of the PTFE molded article according to an embodiment of the present invention. (First Step) The first step in Manufacturing Method B is performed in the same manner as the first step in Manufacturing Method A described above. That is, first, a molding material containing PTFE having a melt viscosity at 380°C of 7×10 5 Pa·s or less is prepared (S21). Next, the molding material is heated to mold a first molded article (S22).

[0068] (2nd process) In the method B for producing a PTFE molded product, after the first step, the first molded product is irradiated with ionizing radiation under conditions of no oxygen and at or above the crystalline melting point of the PTFE to obtain a second molded product (S25). In the present manufacturing method B, when carrying out the present second step after the above-mentioned first step, (b) After the first step, it is important to cool the temperature of the first molded product at a cooling rate of less than 2°C / min (S23), at least in a temperature range of 300°C or more below the crystalline melting point of the PTFE, and then to heat the PTFE again to a temperature above the crystalline melting point (S24), and to carry out the second step.

[0069] By cooling the temperature of the first molded product at a cooling rate of less than 2°C / min in a temperature range of at least 300°C below the crystalline melting point of the PTFE, it is possible to prevent cracks and the like from occurring in the first molded product during cooling. If the cooling rate is more than 2° C. / min, cracks may occur in the first molded product after cooling.

[0070] The cooling rate is preferably 1.5° C. / min or less, and more preferably 1.0° C. / min or less, from the viewpoint of being more suitable for suppressing the occurrence of cracks when the first molded product is cooled.

[0071] On the other hand, the lower limit of the cooling rate is not particularly limited, but in order to avoid an excessively long time being required for cooling the first molded product, a rate of 0.5° C. / min is preferable. Moreover, even if the cooling rate is slower than 0.5° C. / min, there is almost no improvement in the effect of suppressing the occurrence of cracks and the like in the first molded product.

[0072] In this production method B, the temperature range for controlling the cooling rate is set to at least 300° C. below the crystalline melting point of PTFE, because crystallization of PTFE during the cooling process begins below the melting point and completes at about 300° C., and does not occur at temperatures below 300° C. It is believed that if the cooling rate of the first molded product is increased in this temperature range where PTFE crystallization can occur, the crystallization of PTFE will not progress easily, and cracks will be more likely to occur in the first molded product.

[0073] In the present production method B, the first molded product is cooled to a temperature below 300° C., for example, to room temperature, and then the first molded product is heated again to a temperature equal to or higher than the crystalline melting point of the PTFE.

[0074] After heating the first molded product again to a temperature equal to or higher than the crystalline melting point of PTFE, it may be irradiated with ionizing radiation in the same manner as in the method A for producing a PTFE molded product. By carrying out such a second step, a second molded body made of a cross-linked product of PTFE can be obtained.

[0075] (Verification of the effect of this manufacturing method B) (1) PTFE powder: Lubron L5 (melt viscosity at 380°C: 2.7 x 10) manufactured by Daikin Industries, Ltd. 4 An aqueous dispersion of PTFE (hereinafter referred to as PTFE dispersion) was prepared by mixing PTFE (polytetrafluoroethylene (PTFE) polystyrene (PA·s, crystalline melting point: 330°C) with 3% water-soluble cellulose in a weight ratio of 6:4.

[0076] (2) Next, the PTFE dispersion was applied to a plate-shaped substrate made of aluminum using a coater with a gap of 100 μm. The applied PTFE dispersion was then heat-treated at 360°C for 60 minutes, and then cooled from 360°C to room temperature at different cooling rates for each test example to obtain a PTFE coating film.

[0077] The cooling rates in the temperature range of 360°C to 300°C in each test example are as follows. Test example 1: 1℃ / min Test Example 2: 2°C / min Test example 3: 5℃ / min Test Example 4: 10℃ / min Test Example 5: 20℃ / min Test Example 6: 50℃ / min

[0078] After cooling, the PTFE coating film produced in each test example was observed under an optical microscope. 3 to 8 show microscope images of the PTFE coating films according to Test Examples 1 to 6.

[0079] As shown in the results in Figs. 3 to 8, in the method B for producing a PTFE molded body according to an embodiment of the present invention, it was revealed that the occurrence of cracks and the like can be suppressed by setting the cooling rate to less than 2°C / min.

[0080] The reason for this is thought to be that by setting the cooling rate to less than 2° C. / min, crystallization (recrystallization) of PTFE proceeds, making cracks less likely to occur. The fact that the crystallization of PTFE proceeds by setting the cooling rate at less than 2° C. / min is supported by the measurement of the latent heat of fusion described below.

[0081] (Measurement of latent heat of fusion) The relationship between the cooling rate of a PTFE powder (Lubron L5, manufactured by Daikin Industries, Ltd.) and the latent heat of fusion was measured by the following method. Here, measurements were performed using a differential scanning calorimeter (SII NanoTechnology's "X-DSC7000") that had been temperature calibrated in advance using indium and lead as standard samples. 3 mg of PTFE was placed in an aluminum pan (crimp container) and heated at a rate of 10°C / min in a temperature range of 230 to 380°C under a nitrogen gas flow of 40 ml / min. The latent heat of fusion at this time was 45.96 J / g. This latent heat of fusion was used as the initial value of the raw material. Then, the temperature was lowered in the temperature range of 380 to 230°C at the cooling rate shown in Table 1. After further lowering the temperature to 230°C, the temperature was raised again in the temperature range of 230 to 380°C at a heating rate of 10°C / min to measure the latent heat of fusion of the PTFE powder. The results are shown in Table 1 and FIG. 9.

[0082] [Table 1]

[0083] As shown in Table 1 and FIG. 9, when the cooling rate was less than 2° C. / min (1.5° C. / min or less), the latent heat of fusion increased, confirming that crystallization of PTFE was progressing.

[0084] As described above, in the manufacturing method B of the PTFE molded body, the occurrence of cracks and the like was suppressed by setting the cooling rate to less than 2°C / min, and this was thought to be due to the progress of crystallization (recrystallization) of PTFE.

[0085] In addition, the recrystallized PTFE film usually contains small gaps between the crystal grains and does not airtightly cover the entire substrate, resulting in a film with minor defects. Therefore, the first product cooled at a predetermined cooling rate in the above manufacturing method B is also considered to be a molded product with minor defects. On the other hand, in the above-mentioned manufacturing method B, a second step is carried out in which the first product is melted again and irradiated with ionizing radiation to produce a crosslinked product of PTFE. By carrying out this second step, minute defects generated in the first product are eliminated, and the mechanical properties of the product are improved by crosslinking caused by ionizing radiation. Therefore, although the above-mentioned production method B involves a step in which the once-molten PTFE is recrystallized, it is possible to produce a PTFE molded article having no minute defects and high mechanical strength. Such a production method B is suitable for producing a PTFE membrane using low-molecular-weight PTFE, which is difficult to ensure mechanical strength.

[0086] (PTFE molded body) The PTFE molded article obtained by the manufacturing method A or B of the PTFE molded article according to the embodiment of the present invention is a molded article having excellent mechanical properties, and containing as a main component a cross-linked PTFE product obtained by cross-linking low-molecular-weight PTFE with ionizing radiation.

[0087] The lower limit of the limit PV value measured under constant speed conditions in the PTFE molded body is preferably 700 MPa·m / min, more preferably 1000 MPa·m / min, and even more preferably 1300 MPa·m / min. Furthermore, 1600 MPa·m / min is preferable, 1700 MPa·m / min is more preferable, 1800 MPa·m / min is even more preferable, and 1900 MPa·m / min is particularly preferable. On the other hand, the upper limit of the limit PV value is preferably 2500 MPa·m / min, more preferably 2200 MPa·m / min, and even more preferably 2000 MPa·m / min. The "limit PV value" is a value measured in accordance with JIS-K-7218 (1986) Method A (ring-on-disk type thrust wear test) except that a ring with an outer diameter of 11.6 mm and an inner diameter of 7.4 mm is used as the counter material.

[0088] The PTFE molded article has a limit PV value equal to or greater than the lower limit, and therefore has high abrasion resistance. Therefore, the PTFE molded article can be suitably used as a sliding member, etc. On the other hand, if the limit PV value of the PTFE molded article exceeds the upper limit, there is a risk of a decrease in productivity due to irradiation of ionizing radiation to a material containing PTFE.

[0089] The pencil hardness of the PTFE molded article is preferably B or more, more preferably HB or more, and even more preferably F or more. Such a high hardness enhances the usefulness of the PTFE molded article as a sliding member or the like. On the other hand, the pencil hardness can be, for example, H or less. If the pencil hardness of the PTFE molded article exceeds H, the productivity of such a PTFE molded article may decrease. The "pencil hardness" is a value measured in accordance with JIS-K-5600-5-4 (1999).

[0090] The lower limit of the breaking elongation (tensile breaking elongation) of the PTFE molded article is preferably 20%, more preferably 60%, even more preferably 100%, and particularly preferably 140%. When the breaking elongation of the PTFE molded article is so large, the mechanical strength is further increased, and the usefulness as a sliding member or the like is further increased. On the other hand, the upper limit is, for example, 300%, preferably 250%. When the breaking elongation of the PTFE molded article exceeds the upper limit, the productivity, abrasion resistance, etc. of the PTFE molded article may decrease. The "breaking elongation" is a value measured in accordance with JIS-K-7161 (1994).

[0091] The lower limit of the breaking strength (tensile breaking strength) of the PTFE molded body is 0.5 kg / mm 2 is preferable, and 1kg / mm 2 is more preferable. When the breaking strength of the PTFE molded article is as high as this, the mechanical strength is further increased, and the usefulness as a sliding member, etc. is further increased. The "breaking strength" is a value measured in accordance with JIS-K-7161 (1994).

[0092] The PTFE molded article is, for example, a PTFE membrane. The PTFE membrane is free from cracks and has excellent mechanical properties. The thickness of the PTFE film is not particularly limited, but is usually about 0.01 to 5 mm. From the viewpoint of using the PTFE film as an insulating member, a sliding member, or a coating film making use of the chemical resistance, high insulation, and low friction properties that are characteristic of PTFE, the PTFE film is preferably a thin PTFE film having a thickness of about 20 to 1000 μm. The PTFE membrane may exist in a state in which only the membrane is present with the support removed, or may be a laminate provided on a substrate and used together with the substrate.

[0093] When the PTFE membrane exists only as a membrane, the PTFE membrane can be suitably used as, for example, a packing or a sliding member.

[0094] A specific example of a laminate having the above-mentioned PTFE film is an electric wire having a core wire as a substrate and an insulating coating made of the above-mentioned PTFE film provided around the periphery of the core wire. The laminate can also be suitably used as a sliding member for vehicles, machine tools, home appliances, etc. Specific examples of the sliding member include bearings, gears, crankshafts, slide bearings, pistons, gaskets, transport rollers, pressure rollers, etc. When the laminate is used in the sliding member, the covering layer that comes into sliding contact with the mating material of the sliding member may be the PTFE film.

[0095] The shape of the PTFE molded article is not limited to a film shape, and may be other shapes. The above-mentioned sliding member may be entirely made of the above-mentioned PTFE molded article.

Claims

1. (1) The melt viscosity at 380°C is 7 x 10 ５ A first step of heating a molding material containing polytetrafluoroethylene having a viscosity of 0.01 Pa·s or less to a temperature equal to or higher than the crystalline melting point of the polytetrafluoroethylene to obtain a first molded product; (2) a second step of irradiating the first molded product with ionizing radiation in the absence of oxygen and at a temperature equal to or higher than the crystalline melting point of the polytetrafluoroethylene to obtain a second molded product after the first step; A method for producing a polytetrafluoroethylene molded article comprising the steps of: When performing the second step after the first step, (b) after the first step, at least in a temperature range of 300°C below the crystalline melting point of the polytetrafluoroethylene, the temperature of the first molded product is cooled at a cooling rate of 1.5°C / min or less, and then the polytetrafluoroethylene is heated again to a temperature above the crystalline melting point to perform the second step; A method for producing a polytetrafluoroethylene molded article.

2. The melt viscosity is 1×10 ２ The method for producing a polytetrafluoroethylene molded article according to claim 1, wherein the viscosity is Pa·s or more.

3. The method for producing a polytetrafluoroethylene molded article according to claim 1 or 2, wherein the exposure dose of the ionizing radiation is from 10 kGy to 2000 kGy.

4. The method for producing a polytetrafluoroethylene molded article according to claim 3, wherein the amount of irradiation with the ionizing radiation is 30 kGy or more and 200 kGy or less.

5. The method for producing a polytetrafluoroethylene molded article according to any one of claims 1 to 4, wherein the polytetrafluoroethylene molded article is used for a sliding member.

6. 6. The method for producing a polytetrafluoroethylene molded article according to claim 1, wherein in the first step, the first molded article is molded by extrusion molding or injection molding.

7. In the first step, a first molded product is molded by applying the molding material, The method for producing a polytetrafluoroethylene molded article according to any one of claims 1 to 5, wherein the molding material is in the form of a powder or an aqueous dispersion.

8. The method for producing a polytetrafluoroethylene molded article according to any one of claims 1 to 7, wherein the number average molecular weight of the polytetrafluoroethylene contained in the molding material is 600,000 or less.

9. The method for producing a polytetrafluoroethylene molded article according to any one of claims 1 to 8, wherein the molded article produced is a membrane.

10. A method for producing a polytetrafluoroethylene film, comprising forming a polytetrafluoroethylene film on a support by the method for producing a polytetrafluoroethylene molded article according to claim 9, and then removing the support.

11. A method for producing a laminate including a substrate and a polytetrafluoroethylene film provided on a surface of the substrate, comprising the steps of: A method for producing a laminate, comprising forming a polytetrafluoroethylene film by the method for producing a polytetrafluoroethylene molded article according to claim 9.

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