Paper and paper manufacturing method

Abstract

In order to provide paper composed of fluororesin fibers, suitable for use as an insulating member for electronic devices, having sufficient paper strength to withstand paper manufacturing even as lightweight paper, and having excellent resin impregnability, the paper according to the present invention is composed of fluororesin fibers, and among the fluororesin fibers constituting the paper, 10-100 of the fluororesin fibers have streak-shaped recesses on the fiber surface out of 100 randomly extracted fluororesin fibers.

Description

Papermaking and papermaking manufacturing methods

[0001] This invention relates to papermaking and papermaking methods.

[0002] In recent years, the telecommunications industry has seen an explosive increase in data traffic due to the expansion of IoT (Internet of Things), leading to an increase in the number of connected devices and the growth of high-definition video streaming. To address this excessive data traffic, one solution is to utilize high-frequency bands, which offer a significantly higher data transmission rate per unit time compared to existing bandwidths. This has created a demand for communication components suitable for next-generation communication utilizing high-frequency bands. Generally, high frequencies result in high dielectric loss, requiring the use of materials with low dielectric loss tangent in various communication components. For insulating sheets used in communication components such as electromagnetic shielding and printed circuit boards, films made from low-dielectric materials such as glass cloth impregnated with low-dielectric epoxy resin, or liquid crystal polyester and polyimide with improved dielectric properties have been used. Recently, fluoropolymer resins have attracted attention as even lower dielectric materials. Insulating sheets can take the form of film, cloth, or paper. Paper, in particular, is being investigated extensively due to its high flexibility, numerous voids, and excellent breathability and low dielectric properties.

[0003] For example, Patent Document 1 proposes a thin papermaking sheet with excellent mechanical properties, made from two or more types of fluororesin fibers with different melting points, with some of them fused together. Patent Document 2 describes a papermaking sheet with excellent bonding properties to a conductive layer, achieved by applying pressure to a papermaking process that includes fluororesin fibers to smooth it out. Furthermore, Patent Document 3 proposes a fluororesin fiber sheet with excellent adhesion to a conductive layer, achieved by adjusting the average pore size and maximum pore size.

[0004] Japanese Patent Publication No. 2021-25175, Japanese Patent Publication No. 2005-273100, Japanese Patent Publication No. 2003-49387

[0005] Patent Document 1 discloses a method for obtaining thin paper with excellent mechanical properties, which involves manufacturing a mixed paper composed of two or more types of fluororesin fibers with different melting points and fusing a portion of it. In the papermaking process, a Niagara beater is used to mechanically fibrillate the fluororesin fibers. When a Niagara beater is applied to fluororesin fibers using a conventional method, the fluororesin fibers, which have low elastic modulus and strength, are crushed or cut. As a result, the paper strength of the resulting paper is low, and the density of the paper reduces the impregnation of the resin.

[0006] Furthermore, Patent Document 2 describes a papermaking process in which polytetrafluoroethylene (PTFE) fibers and glass fibers are uniformly dispersed in water as fluororesin fibers, and this slurry is wet-processed to form a sheet. The sheet is then dried at 130°C and heat-treated at 350°C to fuse the fibers together, resulting in a basis weight of 130 g / m². ２ This method yields a papermaking sheet primarily composed of fluororesin fibers mixed with glass fibers. However, when attempting to thin the papermaking sheet using this method, the sheet has weak paper strength before fusion, resulting in paper tearing and gaps during the process. Furthermore, because the fiber surface in the papermaking sheet obtained by heat fusion becomes smooth, the anchoring effect with the resin does not work, and impregnation decreases.

[0007] Furthermore, Patent Document 3 describes a fluororesin fiber sheet with excellent adhesion to a conductive layer by adjusting the average pore size and maximum pore size. In papermaking, paper is obtained by (a) stirring undrawn PTFE fibers with a reciprocating agitator to disperse them uniformly and then making paper, or (b) stirring undrawn PTFE fibers and beaten tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA) fibers (beaten with a beater, filtration water content 500 ml (according to JIS P8121)) with a reciprocating agitator to disperse them uniformly and then making wet paper, followed by a sintering treatment to fuse the fibers together. However, the fiber surface of the paper obtained by this method becomes smooth, so the anchoring effect with the resin does not work, and the impregnation properties decrease.

[0008] Therefore, in view of the above problems, the present invention aims to provide a paper machine composed of fluororesin fibers that have sufficient paper strength to withstand papermaking even when thin, and have high resin impregnation properties, making it suitable as an insulating material for electronic devices.

[0009] As a result of diligent research to solve the aforementioned problems, the inventors have found that by providing streaky recesses on the fiber surface, it is possible to obtain paper made from fluororesin fibers that have sufficient paper strength to withstand papermaking even when thin, and have excellent resin impregnation properties, making them suitable for insulating materials for electronic devices.

[0010] This invention was completed based on these findings, and according to this invention, the following inventions are provided.

[0011] (1) A papermaking process in which fluororesin fibers are composed, wherein among the fluororesin fibers constituting the papermaking process, 10 to 100 fluororesin fibers having striated recesses on their surface are included in a randomly selected group of 100 fluororesin fibers.

[0012] (2) The papermaking method according to (1), wherein the thickness of the papermaking is 5 μm or more and 100 μm or less.

[0013] (3) The papermaking method according to any one of (1) to (2), wherein the fluorine-based resin fiber is a polytetrafluoroethylene fiber.

[0014] (4) The papermaking method according to any one of (1) to (3), wherein the degree of crystal orientation of the fluororesin fibers is 90% or more.

[0015] (5) A method for manufacturing paper according to any one of (1) to (4), characterized by comprising a raw material treatment step of subjecting fluororesin fibers having a crystal orientation degree of 90% or more to a disintegration treatment and / or beating treatment that imparts shear stress capable of forming striated recesses on the fiber surface while maintaining the main shape of the fiber, and a papermaking step of making paper from the obtained fluororesin fibers.

[0016] (6) The papermaking method according to (5), characterized in that the papermaking step is a step of mixing binder fibers together with the fluororesin fibers to make mixed paper, and further comprises a binder removal step of heating and pressurizing the mixed paper obtained in the papermaking step to compress the fibers together, and then dissolving and removing the binder fibers in the mixed paper with a solvent.

[0017] According to the present invention, a paper can be obtained that has sufficient paper strength to withstand papermaking even when thin, has high resin impregnation properties, and is composed of fluororesin fibers suitable for insulating materials for electronic devices.

[0018] Furthermore, papermaking processes using fluororesin fibers exhibit excellent low dielectric properties due to the nature of the fluororesin fibers, and therefore offer promising applications as low-dielectric materials.

[0019] The papermaking method of the present invention is a papermaking method composed of fluororesin fibers, wherein among the fluororesin fibers constituting the papermaking method, 10 to 100 fluororesin fibers having striated recesses on their surface are included in a randomly selected group of 100 fluororesin fibers.

[0020] The components of this invention will be described in detail below, but the present invention is not limited in any way to the scope described below, as long as it does not exceed the gist of the invention.

[0021] [Fluorine-based resin fibers] Any fluorine-based resin fiber can be used in this invention, as long as 90% or more of the repeating structural units of the polymer are composed of monomers containing one or more fluorine atoms in the main chain or side chain. Fibers composed of monomers with a higher number of fluorine atoms are preferred, and examples include polytetrafluoroethylene (PTFE, melting point 327°C), tetrafluoroethylene-hexafluoropropylene copolymer (FEP, melting point 255-275°C), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA, melting point 280-320°C (typically 310°C)), polyvinylidene fluoride (PVDF, melting point 156-178°C), ethylene-tetrafluoroethylene copolymer (ETFE, melting point 220-280°C (typically 270°C)), and ethylene / chlorotrifluoroethylene copolymer (ECTFE, melting point 220-245°C). Among fluororesin fibers, polytetrafluoroethylene fibers are particularly suitable for papermaking in the present invention because they possess excellent dielectric properties and superior chemical and heat resistance.

[0022] The fiber diameter of the fluororesin fibers used in this invention is preferably 1.0 μm to 15.0 μm, more preferably 1.0 μm to 10.0 μm, and even more preferably 2.0 μm to 6.0 μm. A fiber diameter of 1.0 μm or more allows the fibers to intertwine appropriately, making it easier to disperse uniformly and resulting in papermaking with less unevenness and excellent paper strength. On the other hand, a fiber diameter of 15.0 μm or less makes it easier to manufacture thin paper with a thickness of 5 μm to 100 μm, which is a preferred form of this invention. In addition, the increased number of intertwined fibers provides sufficient paper strength, resulting in papermaking that is resistant to tearing. The fiber diameter of the fluororesin fibers used in this invention can be measured by the method described later. Note that the fluororesin fibers may be torn or crushed during the papermaking process, so their shape may change from the original fiber shape before papermaking. Therefore, the fiber diameter of the fluororesin fibers constituting the papermaking process is preferably 0.3 μm to 30.0 μm, and more preferably 0.5 μm to 8.0 μm. The fiber diameter of the fluororesin fibers after papermaking can be confirmed by the method described later.

[0023] Furthermore, the fiber length of the fluororesin fibers used in the present invention is preferably 1.0 mm or more and 15.0 mm or less, more preferably 3.0 mm or more and 12.0 mm or less, and even more preferably 3.0 mm or more and 6.0 mm or less. A length of 1.0 mm or more allows for increased strength of the papermaking due to the entanglement of the fibers. A length of 15.0 mm or less prevents the entanglement of fibers from clumping together and causing unevenness. Note that the fluororesin fibers are torn and crushed during the papermaking process, so their length changes from the original fiber length before papermaking. Therefore, the lower limit of the fiber length of the fluororesin fibers constituting the papermaking is preferably 0.5 mm or more, more preferably 1 mm or more. The upper limit is preferably 15.0 mm or less, and more preferably 8.0 mm or less.

[0024] The degree of crystal orientation of the fluororesin fibers used in this invention is preferably 90% or higher, and more preferably 95% or higher. A degree of crystal orientation of 90% or higher improves the rigidity of the fibers, thereby improving their resistance to shear stress when creating striated recesses on the fiber surface, as described later. An upper limit of 99% or less for the degree of crystal orientation is preferable because it ensures a certain level of toughness that can withstand shear stress. The degree of crystal orientation of the fluororesin fibers in this invention can be measured by the method described later.

[0025] The degree of crystal orientation of fluororesin fibers can be controlled to the above range by firing and stretching under the preferred conditions described later when manufacturing the fluororesin fibers.

[0026] The cross-sectional shape of the fluororesin fibers used in this invention is not particularly limited, and the cross-section may be any shape, such as round, β-shaped, C-shaped, triangular, flat, dogbone-shaped, multi-lobed, or hollow.

[0027] [Papermaking using fluororesin fibers] The papermaking method of the present invention is a papermaking method using fluororesin fibers, wherein among the fluororesin fibers constituting the papermaking, 10 to 100 randomly selected fluororesin fibers have striated recesses on their surface. Generally, because fluororesin fibers have low friction on their surface, the entanglement between fibers is low, resulting in paper with weak paper strength, and especially when the basis weight is low, papermaking becomes difficult. However, in the present invention, it has been found that by providing striated recesses on the fiber surface, the entanglement between fibers is increased, and the paper strength of the papermaking can be improved. Furthermore, by improving the entanglement between fibers, papermaking becomes possible even with a small amount of fiber, and as a result, the resulting paper has many voids, improving the impregnation of the resin. In addition, by providing striated recesses on the fiber surface, the contact area with the resin increases compared to smooth fibers, and the anchoring effect is enhanced, further improving the resin impregnation of the paper. The higher the proportion of fibers having striated recesses, the better the paper strength and resin impregnation properties described later. Therefore, among 100 randomly selected fluororesin fibers, it is preferable that 15 or more fibers have striated recesses, more preferably 20 or more, and most preferably 25 or more. It is preferable that all 100 fibers have striated recesses, but from the viewpoint of ease of manufacturing, it is preferable that 70 or fewer fibers have striated recesses, and more preferably 50 or fewer. The above "having striated recesses" refers to a state in which striated recesses with a length of 10 μm or more in the direction of the fiber axis exist on the fiber surface. Typically, these are crack-like striated recesses on the fiber surface or traces of fiber splitting due to fibrillation. The number of fluororesin fibers having striated recesses is a value measured by the following method.

[0028] Specifically, small sample pieces are randomly taken from the papermaking process, and a scanning electron microscope (e.g., Hitachi TM3030Plus Miniscope) is used to take photographs of the fiber surface at an observation magnification expressed by the following formula, depending on the fiber diameter after papermaking. 100 fluororesin fibers are randomly selected from all fibers within the field of view, and the presence or absence of continuous striated recesses of 10 μm or more in the fiber axis direction is observed. The number of fibers with striated recesses is then measured. The observation magnification is rounded to the nearest ten. The above-mentioned "continuous striated recesses of 10 μm or more in the fiber axis direction" is synonymous with striated recesses having a length of 10 μm or more in the fiber axis direction, and "continuous" means that the striated recesses are formed continuously without interruption over the above length. Observation magnification (Z) = 25,000 / post-papermaking fiber diameter (μm)

[0029] Furthermore, even if a single fiber in a fiber surface photograph has multiple striated recesses, it will be counted as one fiber. If a fiber end extends outside the field of view and is cut, even if it is connected outside the field of view, each end will be counted as a separate fiber. If the number of fibers in one field of view is less than 100, the field of view will be increased and observations will be made until the total number of measured fibers reaches 100.

[0030] In this invention, by making the surface of the fluororesin fibers moderately rough so that, among 100 randomly selected fluororesin fibers constituting the papermaking process, as measured by the above method, 10 or more fluororesin fibers have striated recesses on their surface, the frictional force acting between fluororesin fibers, which normally have a low coefficient of friction between fibers and are not easily entangled, increases, improving entanglement. The more fluororesin fibers that have striated recesses on their surface, the better the entanglement between fibers and the stronger the paper becomes.

[0031] In the present invention, the thickness of the paper made of fluororesin fibers is preferably not less than 5 µm and not more than 100 µm, more preferably not less than 5 µm and not more than 50 µm. When the thickness of the paper made of fluororesin fibers according to the present invention is 5 µm or more, the strength and rigidity of the obtained paper can be ensured, and the process passability in the subsequent resin impregnation step can be improved. On the other hand, paper having a thickness of preferably 100 µm or less, more preferably 50 µm or less, is excellent in resin impregnation properties and improves mountability on various communication devices that require higher density and miniaturization. The thickness mentioned above is a value measured by the method described later.

[0032] In the present invention, the basis weight of the paper made of fluororesin fibers is 3 g / m ２ or more and 100 g / m ２ or less, more preferably 15 g / m ２ or more and 50 g / m ２ or less. When the basis weight is 3 g / m ２ or more, strength sufficient for process passage can be obtained, and when the basis weight is 100 g / m ２ or less, in addition to excellent resin impregnation properties, the process passability of wet paper in actual production is improved. Generally, in the paper production process, the formed wet paper proceeds through the process while being transferred between a plurality of felt conveyors. Here, felt transfer is performed by the action of surface tension caused by moisture in the wet paper, but when the basis weight is high, the wet paper easily falls from the felt due to its own weight. In particular, paper made of fluororesin fibers has high hydrophobicity and low water retention, so it is difficult to transfer to a felt, and it is preferable to set the basis weight within an appropriate range.

[0033] In the present invention, the apparent density of the paper made of fluororesin fibers is 0.20 g / cm ３ or more and 2.00 g / cm ３ or less, more preferably 0.30 g / cm ３ or more and 1.00 g / cm ３ or less, further preferably 0.40 g / cm ３ or more and 0.95 g / cm ３is as follows. Setting the value within the above range is preferable because it provides particularly excellent resin impregnation. The apparent density can be calculated by dividing the basis weight by the thickness. Specifically, the apparent density (g / cm ３ ) = basis weight (g / m ２ ) / [thickness (μm)].

[0034] In the present invention, the paper made of fluororesin fibers may be formed by entanglement between the constituent fibers, but the fibers constituting the paper may also be spot-bonded to enhance paper strength. Spot bonding between the fibers constituting the paper can ensure the strength and rigidity of the obtained paper, and further improve the process passability in the subsequent resin impregnation step and copper foil lamination step.

[0035] [Method for producing fluororesin fiber] The method for producing the fluororesin fiber in the present invention can be appropriately selected from a split peeling method, a paste extrusion method, a melt spinning method, and a matrix spinning method.

[0036] The split peeling method is a production method in which fluororesin powder is compressed in a cylinder, sintered, split and peeled, and then stretched. The paste extrusion method is a production method in which fluororesin powder is kneaded with a waxy lubricant, molded into a rod shape or a film shape, then the lubricant is removed, followed by stretching and firing (firing may be omitted in some cases).

[0037] However, with these two production methods, due to the characteristics of the production methods, the cross-section of the final fibrous product obtained by slitting into thin pieces is flat, and moreover, the yarn width is random, resulting in poor uniformity, and variations in tear strength and other properties after papermaking are prone to occur. Therefore, in the papermaking process, it is preferable to adjust the dispersibility of fibers in water or vibrate the fibers when forming the paper in the wire section to make the formation uniform.

[0038] Furthermore, melt spinning is a manufacturing method in which fluororesin powder is heated to a temperature above its melting point, and the molten resin is spun out of a die to form fibers. This method yields highly uniform fluororesin fibers by spinning them out of a die, but it is unsuitable for PTFE and other materials that have high melting points and exhibit almost no fluidity even above the melting point. To ensure spinnability, the melt viscosity of the fluororesin used can be appropriately adjusted. As a means of adjustment, the die temperature may be adjusted or multiple fluororesins with different melting points may be combined.

[0039] Compared to the above method, the fluororesin fibers of the present invention can be spun using the matrix spinning method, which is suitable for thinning papermaking because it allows for the spinning of fluororesins that exhibit almost no fluidity even above their melting point, such as PTFE, and the fiber diameter can be reduced by adjusting the die design and spinning / drawing conditions. The matrix spinning method is a spinning method in which a mixture of a fluororesin aqueous dispersion and a matrix such as viscose is extruded from a die into a coagulation bath to form fibers, which are then scouring and calcined. By calcining above the melting point of the fluororesin, the fluororesin is melted and the particles are fused together while most of the matrix polymer is calcined and scattered, resulting in undrawn yarn with subsequent drawability. The undrawn yarn can be directly drawn in one or more steps to obtain fluororesin fibers.

[0040] The maximum firing temperature in the firing process is preferably 320°C to 350°C. A maximum firing temperature of 320°C or higher is preferable because it ensures sufficient fusion of PTFE particles within the fiber, resulting in excellent stretchability after firing, suppression of yarn breakage, and superior fiber strength. On the other hand, a maximum firing temperature of 350°C or lower suppresses deformation of the fiber cross-sectional shape, making it easier to obtain a uniform cross-sectional shape, suppressing inter-fiber adhesion, and resulting in a fiber with high decomposition properties. It is also possible to perform offline sintering using a separate high-temperature furnace.

[0041] Furthermore, the stretchability of the fibers can be controlled by the amount of heat history during the firing process. If the heat history is low, the fusion of fluororesin particles will not progress, and the stretchability of the fibers after firing will decrease. On the other hand, if the heat history is high, the fusion between fluororesin particles will progress, and the fibers after firing will acquire high stretchability. The stretch ratio is preferably 5 to 15 times, and more preferably 7 to 12 times. By setting the stretch ratio to 5 times or more, more preferably 7 times or more, the degree of crystal orientation of the resulting fluororesin fibers will increase, improving strength, rigidity, and thermal dimensional stability. On the other hand, by setting the stretch ratio to 15 times or less, more preferably 12 times or less, yarn breakage during the stretching process can be reduced, ensuring operability. The stretching temperature is preferably 300°C to 380°C. Even more preferably 310°C to 370°C. By setting the temperature to 300°C or higher, stretching breakage does not occur frequently, process troubles are suppressed, and the yield is good. By keeping the temperature below 380°C, the decomposition of fluororesins can be suppressed, and the decrease in fiber strength can also be suppressed.

[0042] In the present invention, the fiber diameter of the fluororesin fiber can be adjusted not only by the stretching conditions mentioned above, but also by appropriately selecting and combining the diameter of the discharge hole of the die to obtain a fluororesin fiber of the desired fiber diameter.

[0043] Furthermore, the degree of crystal orientation of fluororesin fibers can be controlled within the above range by firing and stretching under the preferred conditions described above. However, increasing the stretching ratio tends to increase the degree of crystal orientation, and decreasing the stretching ratio tends to decrease it, so adjustments should be made as appropriate. Also, as mentioned above, the stretchability of the fibers depends on the amount of heat history in the firing process, so when increasing the stretching ratio, adjustments such as increasing the amount of heat history in the firing process should be made as needed to produce unstretched yarn with high stretchability (that is less likely to break even when the stretching ratio is increased).

[0044] Furthermore, when spinning the fluororesin fibers of the present invention, various pore shapes of spinning nozzles can be used, and by selecting a suitable pore shape, fibers with cross-sectional shapes such as round, β-shaped, C-shaped, triangular, flat, dogbone-shaped, multi-lobed, and hollow can be produced. However, since round cross-sectional fibers generally have higher tensile strength compared to non-round, irregularly shaped cross-sectional fibers, the strength of the resulting paper is also higher, so it is preferable to use round cross-sectional fibers.

[0045] Subsequently, the obtained fluororesin fibers can be cut to a predetermined fiber length to obtain the fluororesin fibers used in the present invention.

[0046] [Method for manufacturing paper made from fluororesin fibers] As a result of diligent research by the present inventors, it has been found that by appropriately adjusting the firing and stretching conditions in the manufacturing process of fluororesin fibers, and then appropriately applying shear stress in the papermaking process, it is possible to efficiently produce the characteristic striated recesses on the fiber surface of the present invention while maintaining the fiber shape.

[0047] In particular, this method is preferable because it allows for the extremely efficient formation of streaky recesses on the surface of fluororesin fibers by performing a raw material treatment step, which involves applying a disintegration treatment and / or beating treatment to fluororesin fibers having a crystal orientation degree of 90% or more, thereby imparting shear stress that can form fine streaky recesses on the fiber surface while maintaining the main shape of the fiber, and then performing a papermaking step to form paper from the obtained fluororesin fibers. The above-mentioned "fine streaky recesses" refers to streaky recesses having a length of 10 μm or more in the fiber axis direction.

[0048] The above raw material processing process will be explained below.

[0049] The above raw material processing step involves subjecting fluororesin fibers having a crystal orientation of 90% or more to a disintegration and / or beating treatment that imparts shear stress capable of forming fine striated recesses on the fiber surface while maintaining the main shape of the fiber, thereby imparting striated recesses to the fiber surface while maintaining the fiber shape of the fluororesin fiber. Here, "a disintegration and / or beating treatment that imparts shear stress capable of forming fine striated recesses on the fiber surface while maintaining the main shape of the fiber" means a disintegration and / or beating treatment performed by controlling the conditions so that shear stress capable of forming fine striated recesses on the fiber surface is imparted while maintaining the main shape of the fiber. Since these conditions vary depending on the equipment, dispersion, and other conditions used, it is difficult to specify them in general, but it is possible to carry out the process without excessive trial and error by controlling them by referring to the following.

[0050] Generally, soft fibers with low elastic modulus, such as fluoropolymer fibers, have low durability against shear stress. Conventional beating processes frequently result in fiber breakage and crushing, further damaging the structure even after the formation of striated recesses. For example, when using a Niagara beater, fiber breakage and crushing will occur when using it with a normal load. The process of imparting striated recesses to the surface of fluoropolymer fibers while maintaining the main shape of the fiber is not particularly limited as long as it can form striated recesses while minimizing fiber breakage and crushing. Examples include applying a fluid such as water to the fluoropolymer fibers, applying mechanical shear to the fibers with a lower-than-normal load, or causing frictional contact between fibers (hereinafter, these disintegration and / or beating processes that impart shear stress capable of forming striated recesses on the fiber surface will be referred to as "appropriate disintegration and beating processes" for convenience). "Beatening" is a process that typically uses devices such as beaters and refiners to beat or grind fibers to create fibrils, while "disintegration" is a process that uses devices such as pulpers and agitators to loosen clumps of fibers (such as those that are bonded, compressed, or tangled) and separate them into single fiber units. In the present invention, any of the following processes may be selected, as long as the papermaking process specified in the present invention is obtained: disintegration only, beatening only, or a combination of disintegration and beatening. As means of applying shear stress, in addition to methods such as stirring with a pulper, and mechanical methods such as Niagara beaters, homogenizers, disc refiners, raika machines, and mortars and pestles, methods that utilize the water flow of pulpers, Niagara beaters, and water jet punches can be mentioned. However, it is preferable to use each method individually or in combination, taking into consideration productivity, while keeping the conditions within a range that can form and maintain striated recesses in the fibers.

[0051] To increase the number of fluororesin fibers with striated recesses on their surface, it is effective to lengthen the processing time for appropriate disintegration and beating, narrow the clearance between plates when using a refiner or Niagara beater, and set the load on the plates to an appropriate size that does not damage the structure of the striated recesses. For example, a Niagara beater typically has a gear on the upper side and a plate on the lower side for beating pulp. The dispersion to be beaten flows between the gear on the upper side and the plate on the lower side, and the convex part of the gear and the plate apply a load to the dispersion (fibers) to beat it. The stress applied to the plate at this time is referred to as the "load on the plate" above. If the clearance between the gear and the plate is increased, the dispersion will be subjected to a load from the water flow, but naturally no load will be applied. If the processing time is excessively long, or if the clearance between plates is too small, or if the load is too large, the fibers may be cut, shortening their length, or the structure of the striated recesses may be damaged due to fiber crushing. As a result, the entanglement between fibers weakens, and paper made from short, crushed fibers becomes excessively dense, reducing resin impregnation. Therefore, when adjusting the number of fluororesin fibers having striated recesses on the surface in this invention, it is desirable to appropriately control the processing time, the clearance between the gear and the plate, and the load.

[0052] As described above, fluoropolymer fibers have low durability against shear stress, so the disintegration and beating treatment conditions must be adjusted appropriately. For example, when using a Niagara beater, it is preferable to perform the treatment without applying load, that is, to limit the treatment to low-load treatment, such as providing an appropriate clearance between the gear and the plate to apply shear stress only through the action of water flow within the beater and friction between the fibers. In this invention, even when using equipment commonly used in beating treatments such as a Niagara beater, if the treatment is performed under conditions without applying load, it is included in the disintegration treatment.

[0053] The fluororesin fibers are subjected to disintegration and / or beating treatment in a form contained in a dispersion. Water is preferably used as the dispersion medium. Specifically, a dispersion can be used obtained by adding fluororesin fibers to water and stirring with a mixer.

[0054] The fiber concentration in the dispersion is preferably 0.01 to 5% by mass, from the viewpoint of fiber dispersibility and processing efficiency, and is particularly preferably 0.1 to 3% by mass.

[0055] The dispersion can further contain dispersants, viscous agents, etc.

[0056] The fluororesin fibers obtained in this way, which have striated recesses on their surface, have a high frictional force between fibers, resulting in a strong entanglement force between fibers during papermaking. This is particularly effective when producing thin paper, such as paper with a thickness of 5 μm to 100 μm, which is a preferred embodiment of the present invention where the amount of fiber in the papermaking process is small.

[0057] Furthermore, when the fiber diameter of fluororesin fibers becomes smaller, individual fibers adhere to each other, and multiple fibers adhere to each other. As a result, the apparent fiber diameter of the fibers increases, and unevenness occurs in the resulting paper. However, by performing appropriate disintegration and beating treatments, it is possible to separate the adhered fibers while maintaining their fiber shape and disperse them appropriately.

[0058] Next, we will explain the process of making paper from the fluororesin fibers obtained in the raw material processing step described above.

[0059] In the production of papermaking using fluororesin fibers of the present invention, only fluororesin fibers that have undergone appropriate disintegration and beating treatment may be used, or untreated fluororesin fibers and fluororesin fibers that have undergone appropriate disintegration and beating treatment may be used in combination, or fluororesin fibers and binder fibers may be used together to further improve passability through the papermaking process.

[0060] The dispersion used for papermaking can be prepared by using the processed solution (hereinafter referred to as the raw material processing solution), which is the dispersion obtained in the raw material processing step, either as is or by adjusting the concentration by diluting it, and further by adding the untreated fluororesin fibers or binder fibers mentioned above to create the dispersion for papermaking.

[0061] When using untreated fluororesin fibers and fluororesin fibers that have undergone appropriate disintegration and beating treatment in combination, the water retention of wet paper and the resulting papermaking strength can be controlled by adjusting the mixing ratio. The mixing ratio should be such that papermaking within the range specified in this invention can be obtained.

[0062] Binder fibers can also be used as a means to bond fluororesin fibers together and improve process strength. When binder fibers are mixed with fluororesin fibers during papermaking, the fluororesin fibers can be bonded to each other via the binder fibers, thereby increasing the web strength after papermaking. Furthermore, if hydrophilic binder fibers are used, the water retention of the wet paper is improved, and process passability is enhanced. However, since the resulting paper contains binder fibers, it is preferable to use binder fibers that can be removed with hot water or the like after papermaking to avoid an increase in dielectric constant and dielectric loss tangent.

[0063] While there are no particular restrictions on the type of binder fiber, polyvinyl alcohol fibers are preferred because they have a lower melting point than fluoropolymer fibers and can be easily removed with alkali or hot water after papermaking.

[0064] The following describes preferred embodiments for manufacturing papermaking using fluororesin fibers according to the present invention, but is not limited thereto. Furthermore, this manufacturing method is applicable when using only fluororesin fibers, or when using fluororesin fibers and binder fibers.

[0065] In the papermaking method using fluororesin fibers according to the present invention, the fibers are first dispersed in water. At this time, the total amount of fluororesin fibers and binder fibers relative to the total mass of the papermaking dispersion is preferably 0.01 to 5% by mass. A total amount of 0.01% by mass or more is preferable because it provides excellent production efficiency and reduces the load of the dewatering process. Conversely, a total amount of 5% by mass or less results in a good dispersion state of fibers, allowing for the production of uniform paper.

[0066] The dispersion of fluororesin fibers is subjected to a disintegration and beating machine (an apparatus for performing disintegration and / or beating treatment) and a moderate disintegration and beating treatment as described above, thereby imparting striated recesses to the surface of the fluororesin fibers.

[0067] The dispersion can be prepared by separately preparing dispersions of fluororesin fibers and binder fibers beforehand and then mixing them, or by directly mixing and dispersing the fluororesin fibers and binder fibers in the same tank.

[0068] The dispersion for papermaking may contain dispersants such as cationic, anionic, or nonionic surfactants, oils, viscous agents to increase the viscosity of the dispersion and prevent aggregation, and defoaming agents to suppress foam generation.

[0069] The papermaking dispersion prepared as described above is used to make paper using a papermaking machine such as a round-mesh type, long-mesh type, or inclined-mesh type, or a hand-operated papermaking machine. This is then dried using a Yankee dryer or rotary dryer to obtain dry paper. Subsequently, it is subjected to a heating and pressurizing treatment to obtain paper with fluororesin fibers bonded to it. In this invention, the simultaneous application of heating and pressurizing is referred to as heating and pressurizing treatment, and is distinguished from treatments that involve only heating without pressurizing, such as drying. Dry paper refers to paper made by drying a wet-processed web and not subjecting it to heating and pressurizing treatment.

[0070] Any means that can perform heating and pressurizing simultaneously is acceptable for the heating and pressurizing process. For example, a hot press on a flat plate or a calender can be used. Among these, a calender that can process continuously is preferred. The calender rolls can be a combination of metal-metal rolls, metal-paper rolls, metal-rubber rolls, etc.

[0071] The temperature conditions for the heating and pressurizing treatment are preferably between 180°C and 230°C, and more preferably between 180°C and 210°C. Keeping the temperature below 230°C suppresses the adhesion of binder fibers to the calender roll, ensuring process passability. On the other hand, keeping the temperature above 180°C softens the binder fibers, improving the paper strength of the resulting paper.

[0072] Furthermore, when calendering is used as the heating and pressurizing treatment, the linear pressure is preferably 98 to 2000 N / cm, and more preferably 300 N / cm to 1000 N / cm. A pressure of 98 N / cm or higher allows fluororesin fibers to bond together. On the other hand, a pressure of 2000 N / cm or lower prevents tearing of the wet-processed dried paper during the heating and pressurizing treatment process, and prevents film formation due to excessive pressure between fibers, allowing for stable processing. The process speed is preferably 1 to 30 m / min, and more preferably 2 to 20 m / min. A speed of 1 m / min or higher allows for good work efficiency. On the other hand, a speed of 30 m / min or lower allows heat to be conducted to the fibers inside the paper, resulting in effective inter-fiber bonding.

[0073] If binder fibers are used during the papermaking process, the resulting paper will contain these binder fibers. Therefore, after heating and pressurizing the mixed paper obtained in the papermaking process to compress the fibers together, a binder removal process is performed in which the binder fibers in the mixed paper are dissolved and removed using a solvent.

[0074] To remove the binder fibers, if a solution capable of dissolving the binder fibers is used, such as polyvinyl alcohol, the binder fiber components are eluted by immersion in hot water or an alkaline solution. After removing the binder fibers, the paper is washed with water and then dried to obtain paper made of fluororesin fibers. Paper made of fluororesin fibers obtained in this way is particularly preferable because the porosity increases due to the removal of the binder fibers, thereby improving the impregnation of the resin, and furthermore, the dielectric properties of the entire paper are improved by removing binder fibers, which generally have a higher dielectric constant and dielectric loss tangent compared to fluororesin fibers.

[0075] The thickness of the paper made from the fluororesin fibers of the present invention can be appropriately adjusted by the fiber diameter and fiber quantity of the fluororesin fibers, and can also be controlled by heating and pressurizing processes such as calendering and pressing.

[0076] The resulting paper has sufficient strength to withstand papermaking even as a thin paper, and its high resin impregnation properties make it suitable for use as an insulating material for electronic devices.

[0077] The resin used to impregnate the papermaking process described above may be a thermosetting resin, a thermoplastic resin, a thermosetting resin composition, or a thermoplastic resin composition. These may be used individually or, if necessary, diluted with an appropriate organic solvent. Here, the diluted product can be treated as a so-called resin varnish. Examples of the thermosetting resin include epoxy resin, phenolic resin, bismaleimide resin, cyanate ester resin, polyimide resin, unsaturated polyester resin, vinyl ester resin, and polyphenylene ether resin. Examples of the thermoplastic resin include polyether ether ketone, polyether ketone ketone, polyphenylene sulfide, polyetherimide, polyimide, polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), fluoroethylene propylene copolymer (FEP), and other fluorine-based resins. Furthermore, the thermosetting resin composition may contain a curing agent and / or a curing accelerator. Furthermore, the resin composition (including thermosetting resin compositions or thermoplastic resin compositions) may contain flame retardants, softeners, plasticizers, etc. In addition, the resin composition may contain inorganic fillers, and examples of inorganic fillers include silica fillers.

[0078] Next, the present invention will be described in detail based on the examples. However, the present invention is not limited to these examples. Unless otherwise specified, the measurements of each physical property were performed according to the methods described above.

[0079] [Measurement and Evaluation Method] (1) Thickness (μm) In accordance with JIS P8118 (2014), a 20 cm x 20 cm test piece was taken, and the thickness was measured at 10 different locations on the sample using a thickness measuring instrument under pressure of 100 kPa with a 16 mm diameter pressure bar, after waiting for 10 seconds for the thickness to stabilize, and the average value was calculated.

[0080] (2) Basis weight (g / m ２ In accordance with JIS P8124 (2011), three 20cm x 20cm test pieces were taken for every 1m of sample width, and the mass (g) of each piece under standard conditions was measured. The average value was then calculated per 1m. ２ Mass per unit (g / m³) ２ It was represented as ).

[0081] (3) Fiber diameter before papermaking (μm) Fluorine resin fibers were randomly sampled, and surface photographs were taken at 500 to 1000x magnification using a microscope (Keyence Corporation "VHX-D500"). The width (length perpendicular to the fiber's longitudinal direction) of 100 fibers, 10 from each sample, was measured, and the average value was calculated to determine the fiber diameter (μm).

[0082] (4) Fiber diameter after papermaking (μm) Random samples were taken from the papermaking process, and surface photographs were taken at 500 to 1000x magnification using a microscope (Keyence Corporation "VHX-D500"). Ten fibers were taken from each sample, for a total of 100 fibers, and the width of the thickest part was measured. The average value was calculated to determine the fiber diameter (μm). Regarding the thickest part, since the fibers may deform during the papermaking process, in such cases, the measurement was taken at the thickest part in the longitudinal direction of the fiber being measured within the observation field. If the width is the same along the longitudinal direction of a single fiber within the same field of view, any part may be measured. For branched fibers, the width of the thickest part of the branched, thinner section shall be measured. In addition, if single fibers aggregate or adhere to each other to form a bundle, the width of the thickest part of that bundle shall be measured.

[0083] (5) Degree of crystal orientation of fibers (%) Samples were taken from fluororesin fibers before papermaking. The samples were cut into 40 mm, 20 mg lengths, weighed, and both ends of the yarn bundle were tied together with enameled wire. The prepared samples were fixed to a fiber sample holder and wide-angle X-ray diffraction measurements were performed. The degree of crystal orientation (%) was calculated from the full width at half maximum (H) of the orientation profile obtained by scanning the crystal peaks in the circumferential direction. The calculation formula is as follows: Degree of crystal orientation (%) = 100 × (180 - H) / 180 (%). The measurements were performed using an X-ray diffractometer (SmartLab for polymers) manufactured by Rigaku Corporation, under the following conditions, using the transmission method (β scan of the strongest peak). X-ray source: CuKα (using Ni filter) Output: 40kV 50mA Slit system: IS = 0.5h × 0.55w mm, RS1 = RS2 = 5 mm Detector: D / teX (one-dimensional detector) Scanning method: Continuous scan Measurement range: β = -90 to 270° Step: 0.5° Scanning speed: 15° / min

[0084] (6) Of the fluororesin fibers constituting the papermaking process, the number of fluororesin fibers having striated recesses on their surface was measured using the method described above, out of 100 randomly selected fluororesin fibers. The measuring instrument used was a scanning electron microscope (TM3030Plus Miniscope, Hitachi, Ltd.).

[0085] (7) When peeling the wet paper obtained by papermaking from the process-permeable fiber dispersion from the wire (wet paper peeling), and when drying the wet paper and peeling it from the drying drum (dried paper peeling), we visually checked for any tears.

[0086] <Judgment Criteria> [When peeling off wet paper] A: All 10 sheets could be peeled off the wire. B: 8 to 9 sheets could be peeled off the wire, but the peeled wet paper was slightly torn. C: 5 to 7 sheets could be peeled off the wire, but the peeled wet paper was torn. D: 3 to 4 sheets could be peeled off the wire, but the peeled wet paper was torn. E: 1 to 2 sheets could be peeled off the wire, but the peeled wet paper was torn. [When peeling off dry paper] A: The dry paper could be peeled off the drum without tearing or falling out. B: The dry paper had some torn or missing parts, but the entire amount could be peeled off the drum. C: The drying paper tore, and a portion of it (less than 25% of the total area) stuck to the drum and could not be removed. D: The drying paper tore, and a portion of it (25% or more but less than 50% of the total area) stuck to the drum and could not be removed. E: The drying paper tore, and more than half of it (more than 50% of the total area) stuck to the drum and could not be removed.

[0087] (8) Resin-impregnated release paper with bisphenol A type epoxy resin at 12 g / m² ２The epoxy resin was applied to achieve the specified basis weight. With the papermaking process sandwiched between two release papers coated with this epoxy resin, the epoxy resin was impregnated into the papermaking process by applying a linear pressure of 5 kN / m using a hot roll calender heated to 110°C. The resin-impregnated material obtained was treated in an oven heated to 120°C for 30 minutes, and then pressed with a flat plate heated to 135°C at a surface pressure of 5 MPa for 30 minutes to obtain an FRP plate. Ultrathin sections of the cross-section in the thickness direction of the obtained FRP plate were observed at 1,000x magnification using a microscope (Keyence Corporation "VHX-D500"), and the resin impregnation performance was evaluated according to the following criteria. A smaller number of fibers with areas where the resin is not in contact with the fibers indicates superior resin impregnation, while the presence of areas where the resin is not in contact with the fibers (i.e., voids remaining after resin impregnation) indicates that the resin and fibers have separated. Note that 100 fibers were observed in the cross-section of the FRP, and fibers exposed on the surface and back were not included in the count. If 100 fibers could not be observed in a single field of view, multiple fields of view were used and counted until a total of 100 fibers were reached. Fibers were selected randomly for observation, and when measuring in multiple fields of view, the number of fibers extracted from each field of view was made as equal as possible.

[0088] 《Judgment Criteria》 A: Within the observed field of view, 10 or fewer fibers had a portion where the resin was not in contact with the fiber. B: Within the observed field of view, 11 to 20 fibers had a portion where the resin was not in contact with the fiber. C: Within the observed field of view, 21 to 30 fibers had a portion where the resin was not in contact with the fiber. D: Within the observed field of view, 31 to 50 fibers had a portion where the resin was not in contact with the fiber. E: Within the observed field of view, 51 or more fibers had a portion where the resin was not in contact with the fiber.

[0089] [Manufacturing Method and Equipment] <Handmade Paper Machine> A handmade paper machine measuring 30 cm x 30 cm and 40 cm in height (manufactured by Kumagai Riki Kogyo Co., Ltd.) was used, with a 140 mesh handmade paper screen installed at the bottom.

[0090] Rotary Dryer: A rotary dryer (ROTARY DRYER DR-200, manufactured by Kumagai Riki Kogyo Co., Ltd.) was used for drying the paper after it had been made by hand.

[0091] Heating and pressurizing were performed using a hydraulic three-roll calendering machine (manufactured by Yuri Roll, model IH-type H3RCM) consisting of iron rolls and paper rolls.

[0092] [Example 1] 46% by mass of viscose (cellulose concentration 9.0% by mass, alkali concentration 6.0% by mass) and 54% by mass of a PTFE aqueous dispersion with a concentration of 60% by mass were mixed, and then degassed under reduced pressure of 10 Torr to obtain a spinning mixture in which the cellulose / PTFE ratio in the mixture was 12.8% by mass. This raw solution was discharged into a coagulation bath from a round-hole die with a pore diameter of 50 μm and a hole length of 50 μm and allowed to coagulate to obtain uncalcined yarn. The coagulation bath was a mixed aqueous solution with a sulfuric acid concentration of 10.0% by mass and a sodium sulfate concentration of 11.0% by mass, and the temperature was 10°C. The obtained uncalcined yarn was then washed with warm water at a temperature of 80°C, and then scouring was performed in an alkaline bath containing a 0.12% by mass aqueous solution of caustic soda to completely remove the acidic components. Subsequently, the unfired yarn drawn from the alkaline bath was squeezed with a nip roller, then fired using a firing roller with a maximum temperature of 340°C and taken up at a speed of 30 m / min. Next, fired using a firing roller with a gradually increasing temperature of 280°C to 350°C and taken up at a speed of 30 m / min to obtain undrawn yarn. The undrawn yarn was then heat-stretched 10 times at a temperature of 350°C to obtain PTFE drawn yarn with a round cross-section. The obtained PTFE drawn yarn was cut to 6 mm to obtain fluoropolymer resin fibers with a fiber diameter of 4.9 μm.

[0093] A dispersion with a fiber concentration of 1% by mass was prepared by adding fluororesin fibers to water and stirring with a mixer. The obtained dispersion was subjected to a disintegration treatment for 20 minutes without applying any load to a Niagara beater (test Niagara beater manufactured by Kumagai Riki Kogyo Co., Ltd.) with clearances left open. To this disintegration treatment solution, polyvinyl alcohol fibers (fiber length: 3.0 mm, fineness: 1.1 dtex) were added in a ratio of 90 parts by mass of fluororesin fibers to 10 parts by mass of polyvinyl alcohol fibers to prepare a mixed dispersion. This mixed dispersion was diluted to a fiber concentration of 0.4% by mass, and wet paper was produced using this dispersion on a hand-operated paper machine. The web obtained by dewatering with rollers was peeled off the wire to obtain wet paper. The obtained wet paper was dried at 110°C for 70 seconds using a rotary drum dryer. After peeling it off the rotary drum as dry paper, one side was then heated and pressurized at an iron roll surface temperature of 220°C, a linear pressure of 490 N / cm, and a roll rotation speed of 3 m / min to obtain paper. The obtained sample was immersed in hot water heated to 90°C ± 10°C to dissolve the polyvinyl alcohol fibers, washed with water, and dried at 110°C for 70 seconds to obtain paper. Observation of the sample surface revealed that 31 out of every 100 fluororesin fibers had striated recesses on their surface. The obtained paper had a thickness of 44 μm and a basis weight of 38 g / m². ２ That was the case.

[0094] [Example 2] Paper was manufactured in the same manner as in Example 1, except that the fiber concentration of the mixed dispersion was set to 0.8% by mass. 32 fluororesin fibers had striated recesses on their surface per 100 observed fluororesin fibers. The resulting paper had a thickness of 91 μm and a basis weight of 76 g / m². ２ That was the case.

[0095] [Example 3] Paper was manufactured in the same manner as in Example 1, except that the fiber concentration of the mixed dispersion was set to 1.2% by mass. 32 out of every 100 fluororesin fibers observed had striated recesses on their surface. The resulting paper had a thickness of 137 μm and a basis weight of 116 g / m². ２ That was the case.

[0096] [Example 4] Paper was manufactured in the same manner as in Example 1, except that instead of performing dissociation treatment with a Niagara beater on the mixed dispersion, it was stirred at 5,000 rpm for 5 minutes using a homogenizer (T.K. Robomix, manufactured by Primix Corporation). 21 fluororesin fibers had striated recesses on their surface per 100 observed fluororesin fibers. The resulting paper had a thickness of 49 μm and a basis weight of 36 g / m². ２ That was the case.

[0097] [Example 5] Paper was manufactured in the same manner as in Example 4, except that the fiber concentration of the mixed dispersion was changed to 0.5% by mass. 23 out of every 100 fluororesin fibers observed had striated recesses on their surface. The resulting paper had a thickness of 70 μm and a basis weight of 49 g / m². ２ That was the case.

[0098] [Example 6] Paper was manufactured in the same manner as in Example 5, except that the fiber diameter of the fluororesin fibers used for papermaking was changed to 13 μm. 21 out of 100 fluororesin fibers observed had striated recesses on their surface. The resulting paper had a thickness of 79 μm and a basis weight of 51 g / m². ２ That was the case.

[0099] [Example 7] Paper was manufactured in the same manner as in Example 5, except that the stretching ratio of the fluororesin fibers was increased to 5 times. Of the fluororesin fibers observed, 12 had striated recesses on the surface. The resulting paper had a thickness of 84 μm and a basis weight of 51 g / m². ２ That was the case.

[0100] [Comparative Example 1] Paper was manufactured in the same manner as in Example 3, except that a load of 5.5 kg was applied to a Niagara beater (manufactured by Kumagai Riki Kogyo Co., Ltd.) and the beating treatment was performed for 20 minutes. Of the fluororesin fibers observed, 7 had striated recesses on the surface.

[0101] [Comparative Example 2] Paper was manufactured in the same manner as in Example 4, except that a homogenizer was not used. Two fluororesin fibers had striated recesses on their surface per 100 observed fluororesin fibers.

[0102]

[0103] Table 1 shows the results for Examples 1 to 7 and Comparative Examples 1 to 2, including (1) thickness, (2) basis weight, (6) the number of fluororesin fibers having striated recesses on the fiber surface among 100 randomly selected fluororesin fibers constituting the papermaking process, (7) process passability, and (8) resin impregnation. As a result, it was clear that the papermaking process according to the present invention, because it has striated recesses on the fiber surface, is a papermaking process with sufficient strength to withstand papermaking even as a thin paper, and has high resin impregnation.

[0104] Furthermore, the papermaking process of this invention does not require the use of two or more types of fluororesin fibers with different melting points to fuse parts of them together to maintain paper strength; it is possible to construct the papermaking process using only a single type of fiber (although it is also possible to use multiple fibers), resulting in a high degree of freedom in papermaking design. The papermaking process shown in the examples uses only PTFE fibers, and therefore exhibits excellent low dielectric properties as a material.

[0105] The papermaking process of the present invention maintains the material properties of fluororesin fibers while possessing a high porosity, excellent low dielectric properties and resin impregnation properties, and good post-processing properties for resin impregnation, making it suitable for insulating materials for electronic devices.

Claims

1. A papermaking process in which fluororesin fibers are used, wherein, among the fluororesin fibers constituting the papermaking process, 10 to 100 randomly selected fluororesin fibers have striated recesses on their surface.

2. The papermaking method according to claim 1, wherein the thickness of the papermaking is 5 μm or more and 100 μm or less.

3. The papermaking method according to claim 1 or 2, wherein the fluorine-based resin fiber is a polytetrafluoroethylene fiber.

4. The papermaking method according to claim 1 or 2, wherein the degree of crystal orientation of the fluororesin fibers is 90% or more.

5. A method for manufacturing paper according to claim 1 or 2, comprising: a raw material treatment step of subjecting fluororesin fibers having a crystal orientation degree of 90% or more to a disintegration treatment and / or beating treatment that imparts shear stress capable of forming striated recesses on the fiber surface while maintaining the main shape of the fiber; and a papermaking step of forming paper from the obtained fluororesin fibers.

6. The papermaking method according to claim 5, characterized in that the papermaking step is a step of mixing binder fibers together with the fluororesin fibers to make mixed paper, and further comprises a binder removal step of heating and pressurizing the mixed paper obtained in the papermaking step to compress the fibers together, and then dissolving and removing the binder fibers in the mixed paper with a solvent.

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