Piston ring and method for manufacturing the same

Abstract

The present invention provides a piston ring and a method for manufacturing the same, which have a low sulfur atom content, do not require special exposure equipment, do not require strict safety measures, and are low cost. [Solution] The piston ring 1 is used in a reciprocating compressor that compresses gas, and is made of a resin composition mainly composed of polyetheretherketone resin. In the heating process of differential scanning calorimetry, it has an endothermic peak due to thermal history in the range of 150°C to 330°C, and the sulfur atom content is 250 ppm or less.

Description

[Technical Field]

[0001] The present invention relates to a piston ring for a reciprocating compressor that compresses gas and a method for manufacturing the same, and more particularly to a piston ring for a reciprocating compressor for hydrogen gas used in hydrogen stations and a method for manufacturing the same. [Background technology]

[0002] Generally, a reciprocating compressor has a structure that includes a piston and a cylinder, and is used to compress fluid by the reciprocating motion of the piston relative to the cylinder. In such reciprocating compressors, an annular piston ring has traditionally been used to seal the fluid in the gap between the piston and the cylinder. The piston ring is fitted into an annular groove provided in the piston. In this case, the fluid is sealed by the outer surface of the piston ring contacting the inner surface of the cylinder, and by the side surface of the piston ring contacting the side surface of the annular groove.

[0003] In recent years, reciprocating compressors have also been applied to hydrogen gas compressors used in hydrogen refueling stations. In hydrogen gas compressors, the presence of sulfur components in the compressed hydrogen gas can cause a decrease in fuel cell performance, so a low sulfur atom content in the piston rings is required.

[0004] As an example of a reciprocating compressor for hydrogen gas, Patent Document 1 is disclosed. Patent Document 1 describes a resin ring-shaped sliding member provided on one of the piston member and cylinder liner members, which slides relative to the other member (sliding member). Patent Document 1 states that by forming an amorphous carbon film on the sliding surfaces of both the sliding member and the sliding member, the replacement life due to wear of the sliding member can be extended. The amorphous carbon film has a higher carbon content in the surface portion than in the inner portion. It is preferable that this amorphous carbon film does not contain sulfur. Furthermore, it is preferable that the sliding member is a desulfurized member that has been treated by exposure to a hydrogen atmosphere before being incorporated into the compressor. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6533631 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Patent Document 1, mentioned above, exposes sliding members to a hydrogen atmosphere as an example of a desulfurization treatment method. However, this involves exposure to a special atmosphere rather than air. Therefore, a special exposure device is required, and safety measures against fire and explosion are also necessary due to the handling of hydrogen, resulting in high costs.

[0007] This invention has been made in view of these circumstances, and aims to provide a piston ring and a method for manufacturing the same that have a low sulfur atom content, do not require special exposure equipment, do not require strict safety measures, and are low cost. [Means for solving the problem]

[0008] The piston ring of the present invention is a piston ring used in a reciprocating compressor for compressing gas, wherein the piston ring is made of a resin composition mainly composed of polyetheretherketone (PEEK) resin, and the piston ring has an endothermic peak due to thermal history in the range of 150°C to 330°C during the heating process of differential scanning calorimetry. Furthermore, in the present invention, "gas" is a general concept meaning a gas, and includes gaseous fuels, etc.

[0009] The piston ring described above is characterized by having a sulfur atom content of 250 ppm or less.

[0010] The above resin composition is characterized in that it contains 5% to 25% by volume of carbon fibers and 5% to 25% by volume of a solid lubricant, wherein the solid lubricant is at least one selected from polytetrafluoroethylene (PTFE) resin and graphite.

[0011] The above polyetheretherketone resin is characterized by having a melt viscosity of 200 Pa·s to 550 Pa·s at a shear rate of 1000 / s and a temperature of 400°C, according to a measurement method compliant with ISO 11443.

[0012] The above-mentioned reciprocating compressor is characterized by being a reciprocating compressor for hydrogen gas that compresses hydrogen gas.

[0013] The above resin composition is characterized by not containing polyphenylene sulfide (PPS) resin.

[0014] The present invention relates to a method for manufacturing a piston ring used in a reciprocating compressor for compressing gas, wherein the piston ring is made of a resin composition mainly composed of PEEK resin, and the manufacturing method is characterized by comprising a heat treatment step of heat-treating a molded body of the resin composition at a maximum temperature of 150°C to 330°C. In the present invention, the molded body to be subjected to the heat treatment step includes the molding material and machined parts of the molding material.

[0015] The above-mentioned molded article is characterized in that the sulfur atom content after the heat treatment process is lower than that before the heat treatment process.

[0016] The above-mentioned molded article is obtained by a molding process in which the above-mentioned resin composition is injection molded, and in the molding process, the maximum temperature inside the nozzle or cylinder of the injection molding machine is 380°C or higher based on the measured value of the resin temperature.

[0017] The above-mentioned molded article is made from pellets of the above-mentioned resin composition, which are obtained using a melt extruder, and is characterized in that the maximum temperature inside the nozzle or cylinder of the melt extruder is 380°C or higher based on the measured resin temperature.

[0018] The piston ring described above is characterized by having a sulfur atom content of 250 ppm or less.

[0019] The above-mentioned reciprocating compressor is characterized by being a reciprocating compressor for hydrogen gas that compresses hydrogen gas. [Effects of the Invention]

[0020] The piston ring of the present invention is made of a resin composition mainly composed of PEEK resin, and exhibits an endothermic peak in the range of 150°C to 330°C during the heating process of differential scanning calorimetry due to its thermal history. This endothermic peak is due to the fact that the molded body of the resin composition is heat-treated at a predetermined temperature during the manufacturing of the piston ring, thereby reducing the sulfur atom content of the piston ring. As a result, the amount of outgassing (sulfur-containing gas) containing sulfur atoms generated in a hydrogen atmosphere is reduced, making it particularly suitable for use in reciprocating compressors for hydrogen gas. Furthermore, when the above heat treatment is performed in the atmosphere, it does not require heat treatment in a special atmosphere, such as the treatment involving exposure to a hydrogen atmosphere (desulfurization treatment) described in Patent Document 1, for example, thus reducing costs.

[0021] The resin composition contains 5 to 25% by volume of carbon fiber and 5 to 25% by volume of a solid lubricant (at least one selected from PTFE resin and graphite) with respect to the whole resin composition, and thus has excellent friction and wear characteristics, and can be suitably used even in a compressor under non-lubricating conditions without a lubricant such as oil.

[0022] Also, since the melt viscosity of the PEEK resin at a shear rate of 1000 / s and a temperature of 400°C is 200 Pa·s to 550 Pa·s in the measurement method conforming to ISO 11443, it has even better wear resistance under reciprocating sliding conditions and can reduce the wear damage of the mating material.

[0023] Here, in a reciprocating compressor for hydrogen gas, the sulfur component contained in the piston ring may be gasified in the compression process and mixed into the compressed gas (hydrogen gas). If such compressed gas is filled into a fuel cell vehicle or the like, it may have an adverse effect on the fuel cell. In contrast, since the resin composition does not contain PPS resin, it can prevent the sulfur component derived from PPS resin from being mixed into the compressed gas.

[0024] The manufacturing method of the piston ring of the present invention has a step of heat-treating a molded body of the resin composition at a maximum temperature of 150°C to 330°C, since the piston ring is made of a resin composition mainly composed of PEEK resin. As a result, the content of sulfur atoms in the molded body after heat treatment can be made lower than that before heat treatment. As a result, the generation amount of sulfur-containing gas generated in a high-temperature hydrogen atmosphere is reduced, and it is particularly suitable for use in a reciprocating compressor for hydrogen gas. Further, since the above heat treatment step does not require heat treatment in a special atmosphere such as the treatment of exposing to a hydrogen atmosphere (desulfurization treatment) described in Patent Document 1 above, a special exposure device is not required, strict safety measures are not required, and the cost is low.

[0025] Since diphenylsulfone, a sulfur compound, is used as a solvent in the polymerization of PEEK resin, diphenylsulfone remains as an impurity in the PEEK resin. In one embodiment of the piston ring manufacturing method of the present invention, focusing on the fact that the boiling point of diphenylsulfone is 379°C, the maximum temperature inside the nozzle or cylinder of the injection molding machine is set to 380°C or higher (measured value of the resin temperature) in the molding process in which the resin composition is injected, thereby facilitating the removal of diphenylsulfone. Furthermore, by replacing or adding to this molding process configuration, when manufacturing the molding pellets, which are the raw material for the molded body, in a melt extruder, the maximum temperature inside the nozzle or cylinder of the melt extruder is set to 380°C or higher (measured value of the resin temperature) to facilitate the removal of diphenylsulfone. In this way, by adjusting the temperature of the injection molding machine or melt extruder, the amount of sulfur atoms in the piston ring can be further reduced easily and at low cost. [Brief explanation of the drawing]

[0026] [Figure 1] This is a perspective view of an example of a piston ring of the present invention. [Figure 2] This is a cross-sectional view of an example of a reciprocating compressor using the piston ring of the present invention. [Figure 3] This is a schematic diagram of an injection molding machine used in the molding process of the present invention. [Figure 4] This is a schematic diagram of a pin-on-disk testing machine. [Figure 5] This figure shows the relationship between the melt viscosity and specific abrasion rate of PEEK resin. [Figure 6] This figure shows the results of differential scanning calorimetry. [Modes for carrying out the invention]

[0027] The inventors of this invention have conducted extensive research to reduce the sulfur content in piston rings made from a resin composition mainly composed of PEEK resin. As a result, they have discovered that in the manufacturing of piston rings, heat-treating a molded body of the above resin composition at a maximum temperature of 150°C to 330°C results in a lower sulfur atom content in the molded body after heat treatment compared to before heat treatment. This invention is based on these findings.

[0028] An example of a piston ring and a reciprocating compressor to which the piston ring is applied according to the present invention will be described with reference to Figures 1 and 2. Figure 1 is a perspective view showing an example of a piston ring according to the present invention. As shown in Figure 1, the piston ring 1 is an annular body with a substantially rectangular cross-section. The corners between the inner circumferential surface 1b of the ring and both sides 1c of the ring may be provided with straight or curved chamfers, and when the seal ring is manufactured by injection molding, a stepped portion that protrudes from the mold may be provided in this portion.

[0029] Furthermore, the piston ring 1 is a cut-type ring having a single gap 1a, and expands in diameter due to elastic deformation to fit into the annular groove of the piston. Because the piston ring 1 has a gap 1a, it expands in diameter due to the gas pressure during use, and its outer surface 1d comes into close contact with the inner surface of the cylinder. The shape of the gap 1a is not limited, and it can be a straight cut type, an angle cut type, etc., but it is preferable to adopt the composite step cut type shown in Figure 1 because it has excellent sealing properties.

[0030] Furthermore, the piston ring of the present invention is not limited to a piston ring consisting of a single component as shown in Figure 1, but may also be a piston ring that forms an annular shape by combining multiple components.

[0031] Figure 2 is a cross-sectional view of an example of a reciprocating compressor using the piston ring of the present invention. The compression mechanism 2 of the reciprocating compressor consists of a cylinder 3 and a piston 4, and the piston 4 is connected to a piston rod 5. Multiple annular grooves for mounting piston rings 1 are arranged on the outer circumferential surface of the piston 4, and the piston rings 1 expand in diameter by elastic deformation and are fitted into each annular groove. The number of piston rings mounted on the piston is not particularly limited, and in Figure 2, six seal rings are mounted. Gas is introduced into the compression chamber 6, compressed by the reciprocating motion of the piston 4 relative to the cylinder 3, and then discharged to the outside.

[0032] In the present invention, the gas compressed by the reciprocating compressor is not particularly limited, and hydrogen gas is one example. A reciprocating compressor for hydrogen gas is installed at hydrogen stations and used for filling fuel cell vehicles, hydrogen engine vehicles, and the like with hydrogen gas.

[0033] The resin composition used in the piston ring of the present invention will be described below.

[0034] The piston ring of the present invention is made of a resin composition with PEEK resin as the base resin. The average molecular weight and molecular weight distribution of the PEEK resin that can be used as the base resin of the above resin composition are not particularly limited, and for example, commercially available products such as PEEK 90P, PEEK 150P, PEEK 380P, PEEK 450P, and PEEK 650P manufactured by Victrex Japan Co., Ltd. can be used. The melt viscosity of the PEEK resin at a shear rate of 1000 / s and a temperature of 400°C is not particularly limited, but it is preferably 200 Pa·s to 550 Pa·s according to the measurement method in accordance with ISO 11443. By keeping it within this range, it is easier to ensure sufficient wear resistance as a piston ring used in a reciprocating compressor while enabling molding by injection molding. The above melt viscosity is preferably 270 Pa·s to 550 Pa·s, more preferably 350 Pa·s to 550 Pa·s, and even more preferably 350 Pa·s to 500 Pa·s. Furthermore, while multiple PEEK resins with different melt viscosities may be mixed and used, it is preferable that the total melt viscosity of the mixed PEEK resins satisfies the above numerical range.

[0035] The resin composition used in the present invention preferably contains 50% to 90% by volume of PEEK resin, more preferably 60% to 90% by volume, and even more preferably 70% to 80% by volume, based on the total volume of the resin composition.

[0036] The above resin composition preferably contains 5% to 25% by volume of carbon fibers. If the carbon fiber content is less than 5% by volume, it is difficult to obtain the effect of improving abrasion resistance, and if it exceeds 25% by volume, the melt viscosity of the resin composition becomes high, making injection molding difficult. A carbon fiber content of 10% to 20% by volume is more preferable.

[0037] The average fiber length of the carbon fibers is not particularly limited, but it is preferably short fibers between 20 μm and 200 μm. If the average fiber length is less than 20 μm, it is difficult to obtain the effect of improving wear resistance, and if it exceeds 200 μm, broken carbon fibers during sliding are more likely to penetrate the sliding surface, causing damage and wear to cylinders and the like. In this specification, the average fiber length refers to the number-average fiber length.

[0038] The carbon fibers incorporated into the resin composition may be either pitch-based or PAN-based, as classified from their raw materials. The firing temperature is not limited and may be either graphitized by firing at a high temperature of 2000°C or higher, or carbonized by firing at approximately 1000-1500°C. Examples of commercially available milled fibers that can be used in this invention include pitch-based carbon fibers such as Kureha Corporation's Kureca M-101S, M-101F, M107T, and M-201S. Examples of PAN-based carbon fibers include Teijin Limited's HT M800 160MU and HT M100 40MU, and Toray Industries, Inc.'s Torayca MLD-30 and MLD-300.

[0039] Furthermore, carbon fibers may contain sulfur as an impurity. In the case of pitch-based carbon fibers, the pitch used as a raw material contains sulfur as an impurity. Also, in the case of PAN-based carbon fibers, if sulfuric acid is used for surface treatment, sulfur may remain.

[0040] Furthermore, it is preferable that the above resin composition contains at least one of the solid lubricants selected from PTFE resin and graphite in a total amount of 5% to 25% by volume. If the total amount of the solid lubricant selected from PTFE resin and graphite is less than 5% by volume, it is difficult to obtain an improvement in friction and wear characteristics under unlubricated conditions, and if it exceeds 25% by volume, the tensile elongation characteristics of the resin composition may decrease. If the tensile elongation characteristics decrease, the piston ring may break when it is enlarged and fitted into the annular groove of the piston. Therefore, it is more preferable that the amount of the solid lubricant is 10% to 20% by volume.

[0041] PTFE resin is a solid lubricant and can improve the friction and wear characteristics of resin compositions under unlubricated conditions. Any of the following can be used as the PTFE resin: molding powder produced by suspension polymerization, fine powder produced by emulsion polymerization, or recycled PTFE. To stabilize the fluidity of the resin composition, it is preferable to use recycled PTFE, which is less prone to fiber formation due to shearing during molding and less likely to increase melt viscosity. Recycled PTFE refers to heat-treated powder (with a thermal history), powder irradiated with gamma rays or electron beams, etc. For example, there are types such as powder obtained by heat-treating molding powder or fine powder, powder obtained by further irradiating this powder with gamma rays or electron beams, powder obtained by crushing a molded body of molding powder or fine powder, powder obtained by subsequently irradiating with gamma rays or electron beams, and powder obtained by irradiating molding powder or fine powder with gamma rays or electron beams. There are also types that undergo further heat treatment after irradiation with gamma rays or electron beams. The 50% particle size of the PTFE resin is not particularly limited, but it is more preferable to be between 10 μm and 50 μm.

[0042] Examples of commercially available PTFE resins that can be used in the present invention include: Kitamura Corporation: KTL-610, KTL-450, KTL-350, KTL-8N, KTL-400H; Mitsui Chemours Fluoroproducts Co., Ltd.: Teflon® 7-J, TLP-10; AGC Inc.: Fluon G163, L150J, L169J, L170J, L172J, L173J, L182J; Daikin Industries, Ltd.: Polyflon M-15; 3M Japan Limited: Dynion TF9205, TF9207, etc. Alternatively, PTFE resins modified with perfluoroalkyl ether groups, fluoroalkyl groups, or other fluoroalkyl side chain groups may also be used. Among the above, examples of PTFE resins irradiated with gamma rays or electron beams include: Kitamura Co., Ltd.: KTL-610, KTL-450, KTL-350, KTL-8N, KTL-8F; and AGC Inc.: Fullon L169J, L170J, L172J, L173J, L182J.

[0043] Graphite is a solid lubricant and, like PTFE resin, can improve friction and wear characteristics under unlubricated conditions. Either natural or artificial graphite may be used. The particle shape can be flaky, granular, or spherical, and any of these is acceptable. Examples of natural graphite include ACP from Nippon Graphite Industries Co., Ltd., and examples of artificial graphite include KS-6, KS-25, and KS-44 from Imerys GC Japan Co., Ltd. The 50% particle size of the graphite is not limited, but 3 μm to 50 μm is preferred, and 10 μm to 30 μm is more preferred. If it exceeds 50 μm, the tensile elongation properties of the resin composition may decrease. If the tensile elongation properties decrease, there is a risk of breakage when the piston ring is enlarged and fitted into the annular groove of the piston. Both natural and artificial graphite contain sulfur as an impurity.

[0044] The PTFE resin and graphite used in this invention have a 50% particle size (D 50 ) is the particle size at the point where the cumulative value of the particle size distribution reaches 50%, and can be measured using, for example, a particle size distribution measuring device that utilizes laser light scattering.

[0045] The above resin composition may contain a combination of carbon fibers and the above-mentioned solid lubricant, or it may contain only carbon fibers or only a solid lubricant.

[0046] The resin composition may be blended with well-known resin additives that intentionally do not contain sulfur, to the extent that they do not impair the effects of the present invention. Examples of additives include inorganic substances (such as mica, talc, calcium carbonate, and boron nitride), whiskers (such as calcium carbonate and potassium titanate), colorants (such as carbon black, iron oxide, and titanium dioxide), and other resin components. Note that the above resin additives may contain sulfur as an impurity. For example, in carbon black, sulfur exists in a state where it is bonded to polycyclic aromatic hydrocarbons.

[0047] Furthermore, a resin having a higher glass transition temperature than PEEK resin and intentionally free of sulfur may be added to the resin composition in a smaller amount than that of PEEK resin, so as not to impede the effects of the present invention. This makes it possible to suppress the decrease in elastic modulus in the temperature range higher than the glass transition temperature of PEEK resin (143°C). Examples of such resins include thermoplastic polyimide resins, thermosetting polyimide resins, polyamideimide resins, and polyetherimide resins, and the amount added can be, for example, 1% to 10% by volume relative to the total resin composition.

[0048] In the resin composition used in the present invention, it is preferable that the carbon fibers, PTFE resin, graphite, and other additives blended into the composition do not intentionally contain sulfur atoms (excluding those present as impurities). Specifically, it is preferable that the above resin composition does not contain PPS resin or molybdenum disulfide.

[0049] Based on the above, a particularly preferred form of the resin composition used in the present invention is a PEEK resin as the base resin, wherein the melt viscosity at a shear rate of 1000 / s and a temperature of 400°C is 270 Pa·s to 550 Pa·s according to the measurement method in accordance with ISO 11443, and the resin composition contains 5% to 25% by volume of carbon fibers and 5% to 25% by volume of a solid lubricant (at least one selected from PTFE resin and graphite), and the average fiber length of the carbon fibers is 20 μm to 200 μm. Furthermore, it is preferable that the resin composition contains 10% to 20% by volume of carbon fibers and 10% to 20% by volume of PTFE resin.

[0050] The piston ring of the present invention is manufactured by heat-treating a molded body of the above-mentioned resin composition at a predetermined temperature. Due to the above heat treatment, this piston ring has an endothermic peak in the range of 150°C to 330°C during the heating process of differential scanning calorimetry. In other words, the piston ring has an endothermic peak in the range of 150°C to 330°C, in addition to the endothermic peak originating from the melting point of the PEEK resin (approximately 343°C).

[0051] Here, for example, in a reciprocating compressor for hydrogen gas, if sulfur components are mixed into the compressed hydrogen gas, it can cause a decrease in the performance of the fuel cell. Therefore, a low sulfur atom content in the piston ring is required. From this viewpoint, the sulfur atom content in the piston ring is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, even more preferably 0.025% by mass (250 ppm) or less, and particularly preferably 0.020% by mass (200 ppm) or less, based on the total amount (100% by mass) of the resin composition. This sulfur atom content can be measured, for example, using inductively coupled plasma mass spectrometry (ICP-MS). A triple quadrupole inductively coupled plasma mass spectrometer (ICP-MS / MS), which can perform analysis with higher precision, may also be used. As a pretreatment method for analysis, for example, a method can be used in which the decomposition solution obtained by acid decomposition in a microwave sample pretreatment device is filtered, and the supernatant is obtained as the analytical sample. The absence of sulfur atoms in the decomposition residue can be confirmed by known analytical methods such as X-ray fluorescence analysis.

[0052] Next, the method for manufacturing the piston ring of the present invention will be described below.

[0053] The manufacturing method of the present invention includes a heat treatment step of heat-treating a molded article made of the above-mentioned resin composition. By performing a predetermined heat treatment step on the molded article, the sulfur atom content in the molded article can be reduced.

[0054] (molding process) The present invention provides a method for manufacturing piston rings, in which a molding process can be performed using a resin composition mainly composed of PEEK resin, with the aim of obtaining (1) a ring-shaped molded body, or (2) a molded body (molding material) that can be machined into a ring shape. The molding method can be appropriately selected from compression molding, injection molding, extrusion molding, etc., and among these, injection molding is preferred.

[0055] Figure 3 shows a schematic diagram of an injection molding machine used for injection molding. Molding pellets obtained by kneading the resin composition described above are used as raw materials. These molding pellets are fed into the hopper 12 of the injection molding machine 11 and introduced from the hopper 12 into the cylinder 13. Thereafter, the molding pellets are heated and melted by the heater 14 in the cylinder 13, pushed by the screw 15, and filled into the nozzle 16 side as molten resin for one shot of a molded body after passing through the metering section. Molding is performed by injecting and filling the molten resin into a cavity of the desired shape through the gate from this nozzle 16. The desired shape is the shape of the molded body described in (1) or (2) above.

[0056] When performing injection molding, it is preferable that the maximum temperature inside the nozzle 16 or cylinder 13 of the injection molding machine 11 be 380°C or higher based on the measured resin temperature. Since the boiling point of diphenyl sulfone remaining in PEEK resin is 379°C, setting the maximum temperature inside the nozzle 16 or cylinder 13 to 380°C or higher based on the measured resin temperature makes it easier to remove the diphenyl sulfone remaining in the PEEK resin. This temperature condition is also effective in removing active sulfur contained in fillers that contain sulfur as an impurity, such as carbon fiber, graphite, and carbon black, when the resin composition is blended with these. It is more preferable that the above maximum temperature be between 380°C and 420°C based on the measured resin temperature.

[0057] In injection molding machines, the heat generated by shearing the resin can sometimes cause the measured resin temperature to exceed the set values ​​for the nozzle and cylinder temperatures. In this case, even if the set value is lower than 380°C, it is acceptable as long as the measured temperature is 380°C or higher.

[0058] Molding pellets can be obtained by mixing the materials constituting the above-mentioned resin composition using a Henschel mixer, ball mixer, ribbon blender, etc., as needed, and then melt-kneading them in a melt extruder such as a twin-screw compounding extruder. Note that side-feeding may be used when melt-kneading the carbon fiber, PTFE resin, graphite, and the above-mentioned resin additives in a twin-screw extruder.

[0059] When obtaining molding pellets using a melt extruder, it is preferable that the maximum temperature inside the nozzle or cylinder of the melt extruder be 380°C or higher, based on the measured resin temperature, similar to the injection molding method described above. This makes it easier to remove diphenyl sulfone remaining in the PEEK resin. It is more preferable that the maximum temperature is between 380°C and 420°C, based on the measured resin temperature.

[0060] In a melt extruder, the heat generated by shearing the resin may cause the measured resin temperature to exceed the set values ​​for the nozzle and cylinder temperatures. In this case, even if the set value is lower than 380°C, it is acceptable as long as the measured temperature is 380°C or higher.

[0061] Furthermore, the molding pellets of the resin composition used for injection molding may be mixed with recycled material made by crushing spools and runners generated during injection molding. Since recycled material has undergone more thermal history than virgin material without recycling, mixing in recycled material is advantageous in removing diphenyl sulfone.

[0062] (Heat treatment process) The molded articles to be heat-treated may be (1) ring-shaped molded articles or (2) molded articles that can be machined into a ring shape (molding material), as well as machined parts (additional processing or full processing) of the molding material in (2) above. The heat treatment process is a process of heat-treating the molded article (including machined parts, the same applies hereinafter) at a maximum temperature of 150°C to 330°C. The maximum heat treatment temperature is set to 150°C or higher because the melting point of diphenyl sulfone is 127°C, and the molecular chains of PEEK resin move easily above the glass transition temperature (143°C), making it easy to remove diphenyl sulfone by evaporation. If the maximum temperature is less than 150°C, it is difficult to obtain an effect of reducing the sulfur content.

[0063] The maximum temperature in the heat treatment process is preferably 150°C to 250°C, and more preferably 200°C to 250°C. If the maximum temperature exceeds 250°C, deformation of the molded body is more likely to occur. Furthermore, it is more preferable that the temperature be higher than the operating temperature of the piston ring, and even more preferable that it be 30°C or more higher than the operating temperature. The holding time at the maximum temperature is not particularly limited, but for example, it is 4 to 8 hours. This heat treatment is effective in reducing sulfur in the piston ring, and can reduce the sulfur-containing gas generated during the use of the piston ring in advance. In addition to removing diphenyl sulfone remaining in the PEEK resin by evaporation, it is particularly effective in removing active sulfur contained in fillers that contain sulfur as an impurity, such as carbon fiber, graphite, and carbon black, when the resin composition is blended with these.

[0064] When differential scanning calorimetry (DSC) is performed on a molded body after the above heat treatment, an endothermic peak (hereinafter referred to as the endothermic peak due to thermal history) appears during the heating process that is not observed in the case without heat treatment. Since the endothermic peak due to thermal history appears at a temperature equivalent to or slightly higher than the maximum temperature of the heat treatment (within +20 degrees), it is possible to estimate the maximum temperature of the heat treatment. In the piston ring of the present invention, by performing the above heat treatment, the endothermic peak due to thermal history appears in the range of 150°C to 330°C, preferably in the range of 150°C to 250°C, and more preferably in the range of 200°C to 250°C. Note that the DSC measurement can be performed, for example, under conditions of a heating rate of 15 degrees / min and in nitrogen gas.

[0065] Furthermore, it is preferable to perform the above heat treatment in the atmosphere. This eliminates the need for heat treatment in special atmospheres, such as exposure to a hydrogen atmosphere (desulfurization treatment), thus eliminating the need for special exposure equipment and strict safety measures, resulting in lower costs. [Examples]

[0066] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples.

[0067] Examples 1-4, Comparative Example 1 The resin compositions used in Examples 1 to 4 and Comparative Example 1 were prepared by blending the following raw materials in the following proportions: PEEK resin: 84% by volume, carbon fiber: 10% by volume, PTFE resin: 5% by volume, and carbon black: 1% by volume. (1)PEEK resin Victrex Japan Co., Ltd.: PEEK 150P (2) Carbon fiber Kureha Corporation: Kureka M201S (average fiber length 150 μm) (3)PTFE resin Kitamura Co., Ltd.: KTL-450 (50% particle size 22μm) (4) Carbon Black

[0068] Using the above formulation, molding pellets made from the resin composition were produced using a twin-screw compounding extruder. Using the obtained molding pellets, piston rings (outer diameter 45 mm, radial wall thickness 2 mm, width 2 mm) as shown in Figure 1 were produced by injection molding. Table 1 shows the maximum temperature of the twin-screw compounding extruder and injection molding machine, the heat treatment conditions, and the sulfur content of the produced piston rings for Examples 1 to 4 and Comparative Example 1.

[0069] [Table 1]

[0070] In Table 1, the sulfur content is the value measured by ICP-MS / MS. The analytical sample was obtained by acid-decomposing a piston ring using a microwave sample preparation device, filtering the decomposition solution, and filtering the supernatant. The absence of sulfur atoms in the decomposition residue was confirmed by X-ray fluorescence analysis.

[0071] As shown in Table 1, in Examples 1 to 4, where the piston rings were heat-treated, the sulfur content was 140 ppm to 200 ppm, which tended to be lower than in Comparative Example 1 (260 ppm) where no heat treatment was performed. This indicates that the amount of sulfur atoms can be reduced by performing heat treatment at a predetermined temperature. Furthermore, comparing Example 2 and Example 4, Example 2, where the maximum temperature of the heat treatment process was 200°C, had a lower sulfur content than Example 4, where the maximum temperature was 150°C.

[0072] Comparing Example 1 and Example 2, Example 2, with an injection molding temperature (measured) of 380°C, had a lower sulfur content than Example 1, with an injection molding temperature (measured) of 360°C. Furthermore, comparing Example 2 and Example 3, Example 3, with a maximum temperature (measured) of 380°C in the twin-screw compounding extruder, had a lower sulfur content than Example 2, with a maximum temperature (measured) of 360°C.

[0073] The following describes our findings on resin compositions for use in piston rings.

[0074] Examples 5 to 12 Using PEEK resin compositions formulated according to the mixing ratios (volume %) shown in Table 2, injection-molded materials measuring φ8 × 20 mm were formed. After heat treatment at a maximum temperature of 220°C in air for 4 hours, φ3 × 13 mm pin test specimens were prepared by machining.

[0075] The raw materials used in the PEEK resin composition are listed below. The melt viscosity of PEEK-1 to PEEK-5 was measured according to the ISO 11443 standard at a shear rate of 1000 / s and a temperature of 400°C. (1) PEEK-1 Victrex Japan Co., Ltd.: 90P (Melting viscosity 90 Pa·s) (2) PEEK-2 Victrex Japan Co., Ltd.: 150P (Melting viscosity 130 Pa·s) (3) PEEK-3 Victrex Japan Co., Ltd.: 380P (Melting viscosity 300 Pa·s) (4) PEEK-4 Victrex Japan Co., Ltd.: 450P (Melting viscosity 350 Pa·s) (5) PEEK-5 Victrex Japan Co., Ltd.: 650P (Melting viscosity 500 Pa·s) (6) CF-1 Kureha Corporation: Kureka M201S (average fiber length 150 μm) (7) CF-2 Toray Industries, Inc.: Torayca MLD-30 (average fiber length 30μm) (8) CF-3 Kureha Corporation: Kureka M107T (average fiber length 400μm) (9)PTFE resin Kitamura Co., Ltd.: KTL-450 (50% particle size 22μm) (10) Graphite Nippon Graphite Industries Co., Ltd.: CGB-20 (50% particle size 20μm)

[0076] As shown in Table 2, in Examples 5-7 and 9-12, a resin composition consisting only of PEEK resin with a predetermined melt viscosity, carbon fiber, and PTFE resin (without graphite) was used. In Example 8, a resin composition consisting only of PEEK resin with a predetermined melt viscosity, carbon fiber, PTFE resin, and graphite was used.

[0077] <Friction and wear test> The obtained pin test specimens were subjected to friction and wear tests using the pin-on-disk testing machine shown in Figure 4. As shown in Figure 4, the test surfaces of the three pin test specimens 17 were pressed against the surface of the rotating disk 18 of the testing machine with the following surface pressure, and the rotating disk 18 was rotated at room temperature. The specific test conditions are as follows, and the material of the rotating disk 18 is SUS304. These test conditions are based on the usage conditions of piston rings in a reciprocating compressor for hydrogen gas. (Test conditions) Circumferential speed: 4.8m / min Surface pressure: 4MPa Lubrication: None (Dry) Temperature: room temperature Time: 50 hours

[0078] After the test, the change in the height of the pin test piece 17 before and after the test was measured respectively, and the specific wear rate was calculated from the average value of the three. Also, the wear damage of the mating material (rotating disk 18) was visually confirmed. The results are shown together in Table 2.

Table 1

[0079] As shown in Table 2, in Examples 5 to 10, the specific wear rate was 19×10 -8 mm 3 / (N·m) to 79×10 -8 mm 3 / (N·m). The specific wear rates of Example 11 and Example 12 using PEEK resin with a melt viscosity of less than 200 Pa·s were 413×10 -8 mm 3 / (N·m) and 200×10 -8 mm 3 / (N·m) respectively, and the results were inferior in wear resistance to those of Examples 5 to 10. Also, in Example 9 (average fiber length of carbon fiber: 400 μm), slight wear damage was observed on the mating material.

[0080] For Examples 5 to 6 and Examples 10 to 12 (combination of CF-1: 10% by volume and PTFE resin: 10% by volume), the relationship between the melt viscosity of the PEEK resin and the specific wear rate is shown in Fig. 5. As shown in Fig. 5, when the melt viscosity of the PEEK resin is below a certain value, the specific wear rate tends to increase rapidly. From such results, by setting the melt viscosity of the PEEK resin in the range of 200 Pa·s to 550 Pa·s, the specific wear rate can be further reduced and the wear resistance can be further improved.

[0081] The injection-molded material used to prepare the pin specimens in Example 5 underwent quantitative analysis of sulfur atoms before and after heat treatment at a maximum temperature of 220°C for 4 hours. The injection-molded material was freeze-dried and acid-decomposed using a microwave sample pretreatment device. The resulting decomposition solution was filtered, and the supernatant was obtained as the analytical sample. Analysis of this analytical sample by ICP-MS / MS revealed that the sulfur content was 220 ppm before heat treatment and 150 ppm after heat treatment. The absence of sulfur atoms in the decomposition residue was confirmed by X-ray fluorescence analysis.

[0082] In addition, an injection-molded resin material, primarily composed of PEEK resin, was heat-treated at a maximum temperature of 210°C for 4 hours. An example of the results of differential scanning calorimetry (DSC) performed after the heat treatment is shown in Figure 6. As shown in Figure 6, an endothermic peak was observed at 223°C during the heating process, which was not observed in the case without heat treatment.

[0083] Thus, the piston ring of the above-described embodiment has an endothermic peak due to its thermal history in the range of 150°C to 330°C during the heating process of differential scanning calorimetry, corresponding to the highest temperature in the heat treatment process. [Industrial applicability]

[0084] The piston ring of the present invention is suitable for use as a piston ring in a reciprocating compressor that compresses gas, and can reduce the generation of outgassing containing sulfur atoms, so it can also be used in reciprocating compressors for hydrogen gas where it is necessary to avoid sulfur contamination. [Explanation of Symbols]

[0085] 1 Piston Ring 2 Compression mechanism 3 cylinders 4 pistons 5 Piston rod 6 Compression Chamber 11 Injection molding machine 12 hoppers 13 cylinders 14 Heater 15. Screw 16 nozzles 17 Pin test piece 18 RPM disc

Claims

1. A piston ring used in a reciprocating compressor that compresses gas, The piston ring is made of a resin composition mainly composed of polyether ether ketone resin. The piston ring is characterized in that, during the heating process of differential scanning calorimetry, it has an endothermic peak due to its thermal history in the range of 150°C to 330°C.

2. The piston ring according to claim 1, characterized in that the sulfur atom content of the piston ring is 250 ppm or less.

3. The resin composition contains 5% to 25% by volume of carbon fibers and 5% to 25% by volume of a solid lubricant, The piston ring according to claim 1 or 2, characterized in that the solid lubricant is at least one selected from polytetrafluoroethylene resin and graphite.

4. The piston ring according to any one of claims 1 to 3, characterized in that the melt viscosity of the polyetheretherketone resin at a shear rate of 1000 / s and a temperature of 400°C is 200 Pa·s to 550 Pa·s according to a measurement method compliant with ISO 11443.

5. The piston ring according to any one of claims 1 to 4, characterized in that the reciprocating compressor is a reciprocating compressor for hydrogen gas that compresses hydrogen gas.

6. The piston ring according to any one of claims 1 to 5, characterized in that the resin composition does not contain polyphenylene sulfide resin.

7. A method for manufacturing piston rings used in a reciprocating compressor that compresses gas, The piston ring is made of a resin composition mainly composed of polyether ether ketone resin. The manufacturing method is characterized by comprising a heat treatment step of heat-treating a molded body of the resin composition at a maximum temperature of 150°C to 330°C.

8. The method for manufacturing a piston ring according to claim 7, characterized in that the sulfur atom content of the molded body after the heat treatment step is lower than that before the heat treatment step.

9. The method for manufacturing a piston ring according to claim 7 or 8, characterized in that the molded body is obtained by a molding step of injection molding the resin composition, and in the molding step, the maximum temperature inside the nozzle or cylinder of the injection molding machine is 380°C or higher based on the measured value of the resin temperature.

10. The method for manufacturing a piston ring according to any one of claims 7 to 9, characterized in that the molded body is made from pellets made of the resin composition, the pellets are obtained using a melt extruder, and the maximum temperature inside the nozzle or cylinder of the melt extruder is 380°C or higher based on the measured resin temperature.

11. A method for manufacturing a piston ring according to any one of claims 7 to 10, characterized in that the sulfur atom content of the piston ring is 250 ppm or less.

12. The method for manufacturing a piston ring according to any one of claims 7 to 11, characterized in that the reciprocating compressor is a reciprocating compressor for hydrogen gas that compresses hydrogen gas.

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