TWI916632B - Fluoropolymer pads and their manufacturing methods - Google Patents

Abstract

A fluoropolymer gasket that ensures high airtightness (sealing) between the gasket and the flange even when the gasket thickness is less than 0.8 mm, contains fluoropolymer and filler, has a thickness of less than 0.8 mm, a surface roughness Ra of less than 2.1 µm, and a density of 0.920 or higher.

Description

Fluoropolymer pads and their manufacturing methods

This invention relates to fluoropolymer gaskets and methods for manufacturing the same. More specifically, it relates to fluoropolymer gaskets and methods for manufacturing the same, which are suitable for applications requiring gaskets of thin thickness, such as in pumps, valves, etc.

Fluoropolymer gaskets are used for flanges and other applications due to their chemical resistance, heat resistance, non-adhesiveness, and low abrasion resistance.

In recent years, as a method for manufacturing fluororesin sheets that exhibit high stress mitigation and high airtightness (sealing) even with low fluororesin filling rates and high filler filling rates, a method for manufacturing fluororesin sheets containing fillers has been proposed (see, for example, patents 1 and 2). In this method, a sheet-forming resin composition containing fluororesin, fillers, and processing aids is calendered using calendering rollers at a roller temperature of 40-80°C. The aforementioned processing aids are prepared by fractionation containing 30% by weight or more of a distillate. A processing aid for petroleum-based hydrocarbon solvents at temperatures below 120°C, and a method for manufacturing a fluoropolymer sheet containing filler (see, for example, Patent 3), wherein in the steps of preparing a sheet-forming resin composition by mixing fluoropolymer, powder filler A, powder filler B, and processing aid, and calendering the aforementioned sheet-forming resin composition using a calendering roller with a roller temperature of 40-80°C, the particle size D when the cumulative number of particles in the particle size distribution of powder filler A is 50% is specified. The particle size _DB (50) of _A (50) and powder filler B when the cumulative number of particles in their particle size distribution is 50% satisfies the relationship expressed by equation (1): _DB (50)≦ _0.73DA (50) (1); The volume _VA of powder filler A and the volume _VB of powder filler B satisfy the relationship expressed by equation (2): 1≦ _VA / _VB ≦3 (2); and the aforementioned processing aid uses a processing aid containing more than 30% by mass of a petroleum hydrocarbon solvent with a fractionation temperature of less than 120°C. [Prior Art Documents]

[Patent Documents] [Patent Document 1] Japanese Invention Patent No. 4213167 [Patent Document 2] Japanese Invention Patent No. 4777389 [Patent Document 3] Japanese Invention Patent No. 5226938

[The problem that the invention aims to solve]

According to the aforementioned method for manufacturing fluoropolymer sheets containing fillers, it is possible to manufacture fluoropolymer sheets that still exhibit excellent stress relief and airtightness (sealing) even with a high filler content.

However, in recent years, fluoropolymer gaskets with a thickness of less than 1 mm have been used in equipment such as pump housings. When these gaskets are compressed between the flanges, their compression ratio becomes low due to their thinness. Therefore, the airtightness (sealing) of the interface between the gasket and the flange cannot be adequately guaranteed.

The present invention relates to a fluoropolymer gasket suitable for applications requiring thin gaskets, such as pump housings, and aims to provide a fluoropolymer gasket that ensures high airtightness (sealing) between the gasket and the flange even when the gasket thickness is less than 0.8 mm.

This invention relates to: (1) A fluoropolymer gasket containing fluoropolymer and filler, having a thickness of 0.8 mm or less, characterized in that: the surface roughness Ra is 2.1 µm or less, and the density is 0.920 or more. (2) A method for manufacturing a fluoropolymer gasket, comprising the method for manufacturing a fluoropolymer gasket as described in (1) above, characterized in that: a preform containing fluoropolymer and filler, having a thickness of 20 to 200 mm, is repeatedly calendered until its thickness is 0.8 mm or less, its surface roughness Ra is 2.1 µm or less, and its density is 0.920 or more. (3) The method for manufacturing fluoropolymer gaskets as described in (2) above, wherein the surface temperature of the calendering roller during calendering of the preform is 40~80°C. [Invention Benefits]

This invention provides a fluoropolymer gasket that can ensure high airtightness (sealing) between the gasket and the flange even when the gasket thickness is less than 0.8 mm.

As described above, the fluoropolymer gasket of the present invention is a fluoropolymer gasket containing fluoropolymer and filler, with a thickness of 0.8 mm or less, characterized by a surface roughness Ra of 2.1 μm or less and a density of 0.920 or greater. According to the fluoropolymer gasket of the present invention, even with a gasket thickness of 0.8 mm or less, high airtightness (sealing) between the gasket and the flange can be ensured.

When the thickness of the fluoropolymer gasket is less than 0.8 mm, the gasket has a low compression ratio when it is placed between the flanges to secure them. Therefore, the airtightness (sealing) of the interface between the gasket and the flange is inevitably reduced compared to gaskets with a thickness greater than 0.8 mm.

In contrast, the fluororesin gasket of the present invention, even with a thickness of 0.8 mm or less, still has high airtightness (sealing performance) mainly based on the constituent elements that the surface roughness Ra of the gasket is 2.1 μm or less and the density of the gasket is 0.920 or more.

The fluoropolymer gasket of this invention can ensure high airtightness (sealing) even when the thickness of the fluoropolymer gasket is less than 0.8 mm. Therefore, it can be used in applications that require gaskets of thin thickness, such as pumps and valves.

The fluoropolymer gasket of the present invention can be manufactured by, for example, using a preform containing fluoropolymer and filler, with a thickness of 20 to 200 mm, and repeating the calendering of the preform until its thickness is less than 0.8 mm, its surface roughness Ra is less than 2.1 μm, and its density is greater than 0.920.

As a raw material for fluoropolymer gaskets, a gasket forming resin composition containing fluoropolymer and filler can be used.

Examples of fluoropolymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), and chlorotrifluoroethylene-ethylene copolymer (ECTFE). The present invention is not limited to the examples described. These fluoropolymers can be used individually or in combination of two or more. Among these fluoropolymers, polytetrafluoroethylene (PTFE) is preferred from the viewpoint of improving formability and processability.

Fluoropolymers are typically used in powder form, but they can also be used in the form of a dispersion of fluoropolymer powder in a dispersion medium. Dispersions of fluoropolymer powder have the advantage of easily and uniformly dispersing components such as fillers within the dispersion.

Examples of fillers include carbon-based fillers such as graphite, carbon black, expanded graphite, activated carbon, and carbon nanotubes; inorganic powders such as silicon carbide, talc, mica, clay, calcium carbonate, and magnesium oxide; resin powders such as polyphenylene sulfide; and short fibers such as glass fiber, carbon fiber, aramid fiber, and rock wool. However, the present invention is not limited to the examples described. These fillers can be used individually or in combination of two or more.

The particle size of the powdered filler is not particularly limited. From the viewpoint of improving the mechanical strength of fluoropolymer gaskets, it is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. From the viewpoint of improving the smoothness of the surface of fluoropolymer gaskets, it is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 30 μm or less. The particle size of the powdered filler refers to the particle size measured by the Coulter counter method. The Coulter counter method is a method of measuring the particle size by passing an electrolyte with the filler suspended through a micropore and reading the change in voltage pulse proportional to the volume of the filler. The fiber length of the fibrous filler is not particularly limited, but from the viewpoint of improving the mechanical strength and surface smoothness of fluororesin pads, it is preferably less than 15μm, more preferably 3~15μm, and even more preferably 3~10μm.

From the perspective of improving the mechanical strength of fluoropolymer gaskets and the airtightness (sealing) of the interface between fluoropolymer gaskets, the filler material is preferably 10 to 200 parts by weight per 100 parts by weight of fluoropolymer, and more preferably 100 to 180 parts by weight.

In the initial process of calendering a preform obtained by forming the gasket forming resin composition into a rod or strip shape, from the viewpoint of expanding the fluororesin, the gasket forming resin composition preferably contains processing aids.

Examples of processing aids include petroleum-based hydrocarbons such as paraffin-based hydrocarbons. The fractionation temperature of the processing aid is not particularly limited, but from the viewpoint of ensuring sufficient residue of the processing aid in the fluoropolymer gasket, it is preferably below 200°C, and more preferably below 180°C. Examples of petroleum-based hydrocarbons with a fractionation temperature of below 200°C as processing aids include paraffin-based hydrocarbons, but the present invention is not limited to the examples described above. Processing aids with a fractionation temperature of below 200°C are readily available commercially. As commercially available processing aids, examples include those manufactured by Exxon Mobil, with trade names such as Isopar C (fractionation temperature: 97~104℃) and Isopar G (fractionation temperature: 158~175℃).

From the viewpoint of allowing the fluoropolymer to expand appropriately during calendering of preforms, the amount of processing aid is preferably 5 to 50 parts by weight per 100 parts by weight of the total weight of fluoropolymer and filler, more preferably 10 to 30 parts by weight, and even more preferably 15 to 30 parts by weight.

To the extent that it does not impede the purpose of the present invention, the resin composition for pad forming may also contain appropriate amounts of tackifiers such as terpene resins, terpene-phenol resins, coumarone resins, coumarone-indene resins, rosin, etc., as well as ultraviolet absorbers, antioxidants, polymerization inhibitors, pigments, dyes, etc.

The resin composition for pad forming can be prepared by mixing fluororesin, filler, processing aids, additives, etc., in any order, either once or in small batches, to achieve a uniform composition. Alternatively, to obtain a resin composition for pad forming with a uniform composition, an excess of processing aid can be added to the resin composition, and after thorough stirring, the excess processing aid can be removed by means such as filtration or volatilization.

Next, a preform is manufactured from the resin composition for forming a gasket. Examples of methods for manufacturing a preform from the resin composition for forming a gasket include extruding the resin composition for forming a preform by extrusion molding and compressing the resin composition for forming a gasket by compression molding, but the present invention is not limited to these examples.

From the perspective of manufacturing fluoropolymer gaskets that can ensure high airtightness (sealing) between the gasket and the flange even when the gasket thickness is less than 0.8mm, the thickness of the preform is adjusted to 20~200mm.

Next, a preform is calendered using calendering rollers. One of the features of this invention is the repeated calendering of the preform until the fluororesin gasket thickness is 0.8 mm or less, the surface roughness Ra is 2.1 μm or less, and the density is 0.920 or greater. Because of this process, a fluororesin gasket with high airtightness (sealing) can be obtained even with a thickness of 0.8 mm or less.

Fluorine resin gaskets with a thickness of less than 0.8 mm, a surface roughness Ra of less than 2.1 μm, and a density of more than 0.920 can be produced by appropriately adjusting the compression ratio and/or compression times of the fluoropolymer sheet during calendering preforms.

The calendering of fluoropolymer sheets can be carried out using calendering rollers such as biaxial rollers. The surface temperature of the calendering rollers can be room temperature, a temperature higher than room temperature, or a temperature lower than room temperature, but from the viewpoint of obtaining fluoropolymer gaskets that can ensure high airtightness (sealing) even with a thickness of less than 0.8 mm, 40~80°C is preferred.

The calendering of the preform is repeated multiple times. During the repeated calendering of the preform, the roller spacing of the calendering rollers is narrowed with each repetition of the calendering, which gradually reduces the thickness of the preform, thereby increasing the compression ratio and density of the fluoropolymer sheet.

The spacing between the calendering rollers during the calendering of preforms varies depending on the thickness of the fluororesin sheet before calendering, the number of calendering cycles, etc., and therefore cannot be determined in general. However, it is usually preferred to be about 40 to 60% of the thickness of the fluororesin sheet before calendering, and the roller speed is preferably about 3 to 10 m/min.

The fluororesin gasket of this invention can be obtained, for example, by calendering a preform with a compression ratio of 60 times or more using a calendering roller, or by calendering a preform with a calendering roller for 7 or more calendering cycles, thereby adjusting the surface roughness Ra to 2.1 μm or less and the density to 0.920 or more. Alternatively, in this invention, either the aforementioned compression ratio or the aforementioned number of calendering cycles can be used, or both can be used.

The compression ratio of the fluoropolymer sheet can be calculated using the following formula: [Compression Ratio (times)] = [Initial Preform Thickness (mm)] / [Fluoropolymer Gasket Thickness (mm)]. From the perspective of manufacturing fluoropolymer gaskets that can ensure high airtightness (sealing) between the gasket and the flange even at a thickness of 0.8mm or less, the compression ratio of the preform is preferably 60 times or more. There is no particular upper limit to the compression ratio of the preform; however, from the perspective of efficiently manufacturing the fluoropolymer gasket of this invention, it is preferably 500 times or less, and more preferably 400 times or less. Therefore, the compression ratio of fluororesin tablets is preferably 60 times or higher, more preferably 60 to 500 times, and even more preferably 60 to 400 times.

Furthermore, within the aforementioned range of compression ratios for the preform, when the compression ratio is low in order to obtain a fluoropolymer gasket with a specified surface roughness Ra and a specified density, by increasing the number of calendering cycles of the preform within the scope of the present invention, a fluoropolymer gasket with a specified surface roughness Ra and a specified density can be obtained.

The number of calendering cycles for the preform refers to the number of times the preform is calendered. From the viewpoint of manufacturing fluoropolymer gaskets that can ensure high airtightness (sealing) between the gasket and the flange even at a thickness of 0.8 mm or less, the number of calendering cycles for the preform is preferably 6 or more. There is no particular upper limit to the number of calendering cycles for the preform, but from the viewpoint of efficiently manufacturing the fluoropolymer gasket of the present invention, it is preferably 20 or less, and more preferably 15 or less. Therefore, the number of calendering cycles for the preform is preferably 6 or more, more preferably 6 to 20, and even more preferably 6 to 15.

Furthermore, within the range of the number of calendering cycles for the aforementioned preform, if the number of calendering cycles is low in order to obtain a fluoropolymer gasket with a specified surface roughness Ra and a specified density, the compression ratio of the preform can be increased within the scope of the present invention to obtain a fluoropolymer gasket with a specified surface roughness Ra and a specified density.

As described above, by repeatedly calendering the preform until its thickness is less than 0.8 mm, its surface roughness Ra is less than 2.1 μm, and its density is greater than 0.920, a fluoropolymer gasket with a surface roughness Ra of less than 2.1 μm and a density of greater than 0.920 can be obtained. Furthermore, the thickness, surface roughness Ra, and density of the fluoropolymer gasket are values at room temperature (approximately 25°C).

The surface roughness Ra of fluoropolymer gaskets refers to the arithmetic mean roughness Ra specified in JIS B0601 (2013). From the viewpoint of obtaining fluoropolymer gaskets that can ensure high airtightness (sealing) between the gasket and the flange even when the gasket thickness is 0.8 mm or less, the surface roughness Ra of fluoropolymer gaskets is preferably 2.1 μm or less, and more preferably 2.09 μm or less. There is no particular limitation on the lower limit of the surface roughness Ra of fluoropolymer gaskets, but a smaller value is preferred.

The density of fluoropolymer pads is calculated using the following formula: [density of fluoropolymer pads (-)] = [actual density of fluoropolymer pads (g/cm ³ )] / [density of fluoropolymer pads calculated from the true density of each nonvolatile component contained in the fluoropolymer pads].

Specifically, the aforementioned "density of fluororesin pads calculated from the true density of each non-volatile component contained in the fluororesin pads" refers to the value obtained by the following formula, based on the true density of each non-volatile component contained in the fluororesin pads and the content of each non-volatile component contained in the fluororesin pads: [density of fluororesin pads calculated from the true density of each non-volatile component contained in the fluororesin pads] = Σ(Dm × Cm) / 100. [In the formula, Dm represents the true density of a specific non-volatile component constituting the fluoropolymer gasket, and Cm represents the content (mass %) of the specific non-volatile component among all the non-volatile components constituting the fluoropolymer gasket.] The true density of the aforementioned specific non-volatile component is, for example, 2.2 g/ ^cm³ for polytetrafluoroethylene (PTFE), 2.6 g/ ^cm³ for micronized clay, and 3.2 g/ ^cm³ for silicon carbide.

Ideally, the density of fluororesin gaskets should be the same as the density calculated from the true density of each non-volatile component contained in the fluororesin gasket. However, in reality, due to the unavoidable gaps between the non-volatile components in the fluororesin gasket, or the inclusion of non-volatile components such as processing aids, the density of fluororesin gaskets is usually lower than the density calculated from the true density of each non-volatile component. The density of fluoropolymer gaskets varies depending on the compression ratio and number of compression cycles of the preform, and tends to increase with higher compression ratios and more compression cycles.

The density of a fluoropolymer pad can be determined by measuring its mass and volume. The density of a fluoropolymer pad, determined by the true density of each non-volatile component, can be obtained by separating the non-volatile components from the fluoropolymer pad, measuring the true density of each non-volatile component, and determining the content of each non-volatile component in the fluoropolymer pad.

From the perspective of obtaining fluoropolymer gaskets that can ensure high airtightness (sealing) between the gasket and the flange even when the gasket thickness is less than 0.8 mm, the density of fluoropolymer gaskets is 0.920 or higher. There is no particular upper limit to the density of fluoropolymer gaskets; the higher the better.

The fluororesin gaskets obtained above can also be placed at room temperature or appropriately heated to allow the processing aids contained in the laminate to evaporate and be removed, as needed.

In addition, fluoropolymer gaskets can be fired by heating them at a temperature above the melting point of the fluoropolymer. The heating temperature varies depending on the type of fluoropolymer, so it cannot be determined in general. From the perspective of uniform firing of fluoropolymer gaskets and avoiding decomposition of the fluoropolymer, a temperature of around 340~370°C is generally preferred.

The fluoropolymer pads of this invention can be used as is, or they can be used after being cut into the desired shape and size.

Furthermore, the thickness of the fluoropolymer gasket of the present invention varies depending on the application of the fluoropolymer gasket, etc. From the viewpoint of fully utilizing the property of the present invention that can ensure high airtightness (sealing) even when the thickness of the fluoropolymer gasket is thin, it is preferably 0.8 mm or less.

As described above, the fluoropolymer gasket of this invention maintains high airtightness (sealing performance) even when its thickness is less than 0.8 mm. Therefore, it is suitable for applications requiring thin gaskets, such as those in pumps and valves. [Example]

The invention will then be described in more detail based on embodiments, but the invention is not limited to these embodiments.

Examples 1-7 and Comparative Examples 1-3 1000g of fine polytetrafluoroethylene (PTFE) powder [manufactured by Daikin Industries Co., Ltd., product number: F104], 1450g of fine clay powder [manufactured by Showa KDE Co., Ltd., product number: NK-300] (used in Examples 1-6 and Comparative Examples 1-3) or 1450g of silicon carbide [manufactured by SHOWA DENKO Co., Ltd., product number: #1200] (used in Example 7), 125g of processing aid [manufactured by Exxon Mobil, trade name: Isopar C], and processing aid [manufactured by Exxon Mobil, trade name: Isopar C]. G]125g was mixed in a kneader for 5 minutes, and then the mixture was extruded through a rectangular extruder with a die opening size of 300mm × 20mm to produce preforms with the thicknesses shown in Table 1.

Next, using biaxial rollers (roller diameter: 700 mm, initial roller spacing: 20 mm, roller surface temperature: 60 °C) as calendering rollers, the preform obtained above is passed through the rollers of the biaxial rollers at a travel speed of 6 m/min. Then, it is passed through the rollers of the biaxial rollers in sequence, with the roller spacing gradually narrowing, and calendered. Finally, it is calendered by passing through the rollers of biaxial rollers with a spacing corresponding to the thickness of the fluoropolymer gasket shown in Table 1, to obtain the fluoropolymer gasket. The number of times the preform is calendered between the rollers of the biaxial rollers (calendering times) and the compression ratio of the preform are shown in the manufacturing conditions column of Table 1.

The thickness, surface roughness, and density of the fluoropolymer pads obtained above are shown in the performance column of Table 1.

Next, the airtightness (sealing) of the aforementioned fluoropolymer gaskets was investigated using the following method. The results are shown in the performance column of Table 1.

[Test Method for Airtightness (Sealing Performance)] Airtightness (sealing performance) was investigated according to Appendix C of JIS B 2490 (2008), "Test Method for Sealing Characteristics of Gaskets for Pipe Flanges". Gaskets with an inner diameter of 48 mm and an outer diameter of 67 mm were used as test samples. These gaskets were installed between steel flanges with a diameter of 100 mm, a height of 50 mm, and a surface roughness Rz of 3 μm. A compression tester was used to apply a load, bringing the surface pressure to 19.6 MPa. A pressure of 0.98 MPa of nitrogen was applied to the inner diameter side of the gasket through a pressure inlet hole set on the flange. After 5 minutes, the leakage of nitrogen was determined using a soap film flow meter.

When the aforementioned leakage is below 3.0 × ^10⁻⁴ Pa· ^m³ /sec, the gasket meets the qualified standard for airtightness (sealing).

[Table 1] Implementation examples and comparative examples numbered Thickness of the preform (mm) Manufacturing conditions performance of pads Compression ratio (times) Number of rolling cycles (times) Thickness (mm) Surface roughness (µm) Density (-) Airtightness (sealing) Leakage rate (Pa· ^m³ /sec) Evaluation Implementation Example 1 50 100 8 0.5 1.50 0.967 6.88× ^10⁻⁵ qualified Implementation Example 2 180 360 8 0.5 0.85 0.983 3.44× ^10⁻⁵ qualified Implementation Example 3 50 100 6 0.5 2.09 0.944 2.58× ^10⁻⁴ qualified Implementation Example 4 30 60 12 0.5 0.82 0.920 8.83× ^10⁻⁵ qualified Implementation Example 5 30 100 8 0.3 1.48 0.970 5.52× ^10⁻⁵ qualified Implementation Example 6 80 100 8 0.8 1.52 0.965 7.68× ^10⁻⁵ qualified Implementation Example 7 50 100 8 0.5 1.46 0.978 6.55× ^10⁻⁵ qualified Comparative example 1 180 360 4 0.5 2.78 0.941 4.82× ^10⁻⁴ Unqualified Comparative example 2 50 100 4 0.5 2.65 0.918 6.50× ^10⁻⁴ Unqualified Comparative example 3 30 60 6 0.5 2.05 0.898 7.00× ^10⁻⁴ Unqualified

According to the results shown in Table 1, all the fluoropolymer gaskets obtained in each embodiment, regardless of the type of filler, even if the thickness of the fluoropolymer gasket is less than 0.8 mm, have a surface roughness Ra of less than 2.1 μm and a density of more than 0.920, thus demonstrating excellent airtightness (sealing performance).

without

without

Claims (3)

A fluoropolymer gasket, containing fluoropolymer and filler, has a thickness of 0.8 mm or less. Its characteristics include a surface roughness Ra of 2.1 µm or less and a density of 0.920 or higher. The aforementioned density is calculated using the following formula: [Density of fluoropolymer gasket (g/cm³)] = [Actual density of fluoropolymer gasket (g/ ^cm³ )] / [Density of fluoropolymer gasket (g/ ^cm³ ) calculated from the true density of each non-volatile component contained in the fluoropolymer gasket] The density of the fluororesin pad, which was calculated from the true density of each non-volatile component contained in the fluororesin pad, is based on the true density of each non-volatile component contained in the fluororesin pad and the content of each non-volatile component in the fluororesin pad, and is calculated according to the following formula: [Density of the fluororesin pad calculated from the true density of each non-volatile component contained in the fluororesin pad] = Σ(Dm×Cm)/100 (where Dm represents the true density of a specific non-volatile component constituting the fluororesin pad, and Cm represents the content (mass%) of that specific non-volatile component among all the non-volatile components constituting the fluororesin pad). A method for manufacturing a fluoropolymer gasket, characterized in that: a preform containing fluoropolymer and filler, having a thickness of 20 to 200 mm, is repeatedly calendered until its thickness is less than 0.8 mm, its surface roughness Ra is less than 2.1 µm, and its density is greater than 0.920. The method for manufacturing a fluoropolymer gasket as described in claim 2, wherein the surface temperature of the calendering roller during the calendering of the preform is 40~80°C.