Use of a product with improved abrasion resistance

Abstract: A product having reduced friction and improved abrasion resistance comprises a formed resin containing at least one fluorine-contained polymer which is exposed to an ionizing radiation. The formed resin may include from 1 to 100 weight percent of the fluorine-contained polymer in the form of powder, and from 99 to 0 percent of a high molecule polymer which is not exposed to the ionizing radiation. The fluorine-contained polymer is preferably exposed to the ionizing radiation for a total dose of from 1 KGy to 10 MGy in the absence of oxygen at a temperature not lower than the melting point of a starting fluorine-contained polymer.

Description: This invention relates to the use as an abrasion resistant product of a fluorine-containing resin product, for instance as a nonlubricated bearing or a dynamic seal. Fluorine-containing polymers or resins are materials having improved resistance to chemicals and heat, and have been used extensively as both industrial and consumer resin. Conventional fluorine-containing polymers, however, may not be used in an environment of frictional motion due to abrasion or creep deformation of the material. To overcome these shortcomings, a certain filler has been added to the conventional fluorine-containing polymers. Such filler, however, impairs preferable inherent characteristics of fluorine-containing polymers. As a result, it has had limited uses according to circumstances and has not always been satisfactory. JP-A-95213104 discloses a fluorocarbon resin composition, useful for making wire insulation, which is prepared by cross-linking a composition comprising (A) a fluorocarbon elastomer having main repeat units of hexafluorovinylidene, hexafluoropropylene, tetrafluoroethylene and ethylene or propylene, and (B) a fluorocarbon polymer having main repeat units of either fluorovinylidene or both ethylene and tetrafluoroethylene. The cross-linking by electromagnetic ray irradiation is dependent on the presence of multi-functional monomers such as triallylisocyanurate. JP-63041538 discloses a resin composition useful for making bearings, gears and cams, which contains a synthetic resin and low-temperature plasma-treated fluoroplastic powders. The fluoroplastic powder is first treated with the low-temperature plasma, mixed with the synthetic resin, then melt needed, ground and injection moulded to shape. Plasma treatment is said to improve abrasion resistance. US-A-5444103 discloses a process for producing a modified polytetrafluoroethylene (PTFE) comprising exposing a starting PTFE to an ionizing radiation for a total dose of from 100 kGy to 5 MGy in the absence of oxygen at a temperature of from 330°C to 350°C. The modified PTFE is said to have improved radiation resistance and rubber characteristics, rendering it useful not only in radioactive environments, but also in the manufacture of medical tools thereby permitting radiation sterilisation of such tools, and as sealing and packing materials. The examples illustrate that after irradiation treatment the crystal melting point of the resulting PTFE is 317°C or lower and the amount of heat of crystallisation is 43 J/g or lower. It is an object of the invention to provide a resin for use as an abrasion resistant product, which has reduced friction and improved abrasion and load resistance, and which maintains the inherent preferred characteristics of the resin. According to the first feature of the invention a resin containing an ionizing radiation irradiated fluorine-containing polymer is used as an abrasion resistant product as defined in claim 1. The resin may include from 1 to 100 weight percent of the fluorine-containing polymer and from 99 to 0 percent of a polymer which has not been exposed to any ionizing radiation. The resin may be produced by molding a mix of powder by pressure, wherein the mix consists essentially of from 1 to 100 weight percent of the fluorine-containing polymer in the form of powder, and from 99 to 0 percent of a polymer which is not exposed to any ionizing radiation. Optionally, the ionizing radiation irradiated fluorine-containing polymer powder has an average particle diameter of less than 1 mm, and preferably the ionizing radiation irradiated fluorine-containing polymer powder is such that a coefficient of abrasion of the resin which contains less than 99 weight percent of other polymer and more than 1 weight percent of the ionizing radiation irradiated fluorine-containing polymer powder is less than a half of that of a formed resin which contains only other polymer. Preferably, use is made of a formed resin containing ionizing radiation irradiated fluorine-containing polymer, the peaked crystal melting point thereof being lower than 325°C, an amount of heat crystallization thereof being less than 40 J/g, and a coefficient of abrasion thereof being less than a half of that of a formed resin which contains non-radiated fluorine-containing polymer. More preferably, the formed resin is produced by molding ionizing radiation irradiated fluorine-containing polymer powder by pressure. In the context of the invention, when reference is made to a polymer which has not been irradiated with any ionizing radiation it is meant a polymer which has not been irradiated by, for example, γ-rays, electron beams, X-rays, neutron beams and high-energy ions. A process for producing an ionizing radiation irradiated polytetrafluoroethylene polymer (hereinafter referred to as PTFE) which is improved in reduction in the radiation-induced deterioration of breaking extension and breaking strength by exposing a starting PTFE to an ionizing radiation in specific conditions has been proposed in Japanese Published Patent Applications No.6-116423 (corresponding to US Patent No. 5,444,103), No. 7-118423 and No. 7-118424. The fluorine-containing polymer to be used in the invention is PTFE, tetrafluroethylene-perfluoro (alkylvinylether) copolymer (hereinafter referred to as PFA) or tetrafluoroethylene-hexfluoropropylene copolymer (hereinafter referred to as FEP). The PTFE may include a less than 0.2 molar percent of monomers which can be copolymerized therewith, such as perfluoro(alkylvinylether), hexafluoropropylene, perfluoro(alkylethylene), and chlorotrifluoroethylene. If the fluorine-containing polymer comprises the aforementioned comonomers, a small amount of other elements may be included in their molecular structures. According to the invention, the resin for use as an abrasion resistant product may contain not only a single compound of the fluorine-containing polymer but also a mix of two or more compounds thereof. Furthermore, a polymer other than the fluorine-containing polymer, especially a polymer having heat resistance characteristics may be mixed with the fluorine-containing polymer, or the mix of two or more compounds thereof. In this case, the ionizing radiation may be directed on to a mixture of those compounds, or the fluorine-containing polymer solely, which is afterward mixed with the other polymer. The other polymer is preferably a polymer having a heat resistance of more than 300 °C in the absence of oxygen. Such a polymer includes fluorine-containing copolymers such as ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, propylene-tetrafluoroethylene copolymer, and vinylidenefluoride-hexafluoropropylentetrafluoroethylene copolymer; polyimide, aromatic polyamide, polyarylenesulfide, and aromatic polyester. For producing a resin comprising the fluorine-containing polymer powder a process which comprises the steps of preforming or molding a fluorine-containing polymer in a predetermined shape, then firing the preform, and then exposing the preform to an ionizing radiation is generally used. According to circumstances, the abrasion resistant product may be produced by other processes comprising different step(s), such as (a) molding the irradiated fluorine-containing polymer powder, optionally by pressure, (b) molding a mix of irradiated fluorine-containing polymer powder and non-irradiated powder of other polymer by pressure, (C) preforming a mix of fluorine-containing polymer powder and polymer having heat resistance characteristics into a predetermined shape by pressure, and (d) forming a fluorine-containing polymer sheet, exposing the sheet to said ionizing-radiation, then punching the sheet into a predetermined shape. In the invention, the ionizing radiation to be radiated is selected from γ-ray, electron beams, X-ray, neutron beams and high-energy ions. The exposure to the ionizing radiation is carried out in the absence of oxygen, and a total dose of exposure is preferably in the range of between 1 KGy and 10 MGy. Moreover, in view of reducing friction, and improving abrasion resistance and load resistance characteristics, the more preferable total dose of exposure is in the range of between 10KGy and 1500 KGy. When exposing the fluorine-containing polymer to an ionizing radiation, the polymer is heated at no lower than the melting point of the starting fluorine-containing polymer. For example, to obtain an ionizing radiation irradiated PTFE, the starting PTFE is exposed to the ionizing radiation at the temperature which is higher than its crystal melting point of 327 °C. If PFA, or FEP is used as a starting fluorine-containing polymer, they are exposed to the ionizing radiation at temperatures which are higher than their melting points of 327°C, and 275 °C, respectively. Heating the starting fluorine-containing polymer at not lower than its crystal melting point activates movements of molecules in main chains of the fluorine-containing resin, which results in facilitating cross-link reactions between molecules. An excessive heating may cause disconnections of chain between molecules and decomposition. Therefore, the heating temperature ought to be the temperature of between 10 degrees and 30 degrees higher than the crystal melting point of the starting fluorine-containing resin so as to control generation of such depolymerization phenomena. The resin described above, and which has reduced friction and improved abrasion resistance, is suitable for use as a variety of abrasion resistant products, such as a nonlubricated bearing, a dynamic seal, rolls for copy machines, and a bearing pad, for which the conventional fluorine-contained polymer product has been unsuitable. The following examples are provided for the purpose of further illustrating the invention but are in no way to be taken as limiting. EXAMPLE [EXAMPLES 1∼ 6] Commercial PTFE sheets having 0.5 mm thickness were exposed to electron beams for various total doses in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) at an elevated temperature of 340°C to obtain sheet products. The total doses were 50 KGy (EXAMPLE 1), 100 KGy (EXAMPLE 2), 300 KGy (EXAMPLE 3), 500 KGy (EXAMPLE 4), 1000 KGy (EXAMPLE 5), and 1500 KGy (EXAMPLE 6), respectively. The EXAMPLES 1∼ 6 and the commercial PTFE sheet which was not exposed to any ionizing radiation (COMPARISON 1) were subject to friction and abrasion measuring tests, results of which are shown in TABLE 1. A thrust type friction and abrasion testing machine was used for the tests, wherein each sample of these sheets was pressed by a cylindrical ring ( an outer diameter and an inner diameter of which are 25.6mm and 20.6mm, respectively) at 2.5 kg/cm 2 and moved at 0.5m/sec for 2 hours. The PV value (calculated by multiplying pressure by velocity) was 1.25kg · m/cm 2 · sec. After completion of the tests each sample was subject to measuring the amount of decreased weight. The decreased weight of each sample was converted into decreased volume, which was divided by contact area of the cylindrical ring to calculate abrasion depth. The resulting coefficients of abrasion K (m · sec/MPa/m/hrx10 -6 )were calculated by the definition: W=KPVT, where W is abrasion depth (m), P is load (MPa), V is velocity (m/sec), and T is time (hr). [EXAMPLE 7∼10] Commercial PFA films having 50µm thickness (AFLON P-63P, made by ASAHI GLASS CO., LTD.) were adhesively attached to a SUS304 plate and exposed to electron beams at various temperatures for various total doses in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) to obtain film products. The temperatures and total doses were 315°C, 100 KGy (EXAMPLE 7); 315°C, 1000 KGy (EXAMPLE 8); 315 °C, 500 KGy (EXAMPLE 9); and 310°C, 100 KGy (EXAMPLE 10), respectively. The EXAMPLES 7∼ 10 and the commercial PFA film which was not exposed to any ionizing radiation (COMPARISON 2) were subject to friction and abrasion measuring tests, results of which are shown in TABLE 2. The same thrust-type friction and abrasion testing machine was used for the measurement of coefficients of abrasion of EXAMPLES 7 and 8 in the same manner as for EXAMPLES 1∼ 6 in TABLE 1, while a pin-on-disk type friction and abrasion testing machine was used for the measurement of abrasion characteristics of EXAMPLES 9 and 10. The abrasion characteristics was defined by abrasion depths which were measured in the condition that the diameter of a ball was 1/4 inch (6.350 mm), the load was 0.2 kg, the peripheral speed was 5m/min for 10 hours. For COMPARISON 2, a coefficient of friction and an abrasion depth were measured by the same thrust type friction and abrasion testing machine and pin-on-disk type friction and abrasion testing machine, respectively. [EXAMPLE 11] PTFE molding powder (TEFLON 70J, made by DU PONT-MITSUI FLUORO-CHEMICALS CO., LTD), which consists essentially of tetrafluoroethylene and perfluoro(alkylvinylether), wherein the molar ratio of former monomer to latter monomer is 99.9:0.1 and an average particle diameter of the powder is 50 µm, was preliminarily molded and fired to produce a cylindrical block having 100mm diameter and 100mm height. Then, a sheet of sample having 0.5mm thickness was sliced off from the block and exposed to electron beams in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) at an elevated temperature of 340°C for a total dose of 100 KGy to obtain a sheet product. [EXAMPLE 12] PTFE molding powder (FLUON G190, made by ASAHI GLASS CO., LTD.), an average particle diameter of which is 40 µm, was exposed to electron beams in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) at an elevated temperature of 350 °C for a total dose of 100 KGy. After that, the powder was pulverized into finer powder of an average particle diameter of about 20 µm by a jet mill, then heat-treated at 300 °C for 12 hours to evaporate high-volatile matters (of about 0.1 %). A mix of 10 weight percent of the irradiated finer powder and 90 weight percent non-irradiated PTFE molding powder was prepared, then preliminarily molded and fired to obtain a sheet product. TABLE 3 shows results of coefficient of friction and coefficient of abrasion measuring tests for EXAMPLES 11 and 12. The TABLE 3 also shows a result of coefficient of abrasion test for a sample (COMPARISON 3) which was sliced off from the block of EXAMPLE 11 and not exposed to electron beams. The same thrust type friction and abrasion testing machine was used for the tests as used for EXAMPLES 1∼ 6 in TABLE 1. [EXAMPLE 13] PTFE molding powder (G-163, made by ASAHI GLASS CO., LTD.), wherein an average particle diameter of the powder is 40 µm and a specific weight is 2.16, is exposed to electron beams in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) at an elevated temperature of 350°C for a total dose of 100 KGy. After that, the powder was pulverized into finer powder of an average particle diameter of about 20 µm by a jet mill, then heat-treated at 300 °C for 12 hours to evaporate high-volatile matters (of about 0.1 %). The irradiated PTFE powder was filled into a metallic die and preliminarily molded by press at 200∼ 700 kg/cm 2 , then placed in an electric furnace and fired at the temperature of 350∼ 400 °C to obtain a block product of 50mm(length) x 50mm (width) x 10mm (thickness). The specific weight of the resulting block product was 2.18. [EXAMPLE 14] Fifty weight percent of electron beam irradiated PTFE powder which was produced by the similar process to that for EXAMPLE 13, and 50 weight percent of non-irradiated PTFE powder which was the same as the starting PTFE powder of EXAMPLE 13 were mixed by a mixer. Then, the mix was filled into a metallic die and preliminarily molded by press at 200∼ 700 kg/cm 2 , then placed in an electric furnace and fired at the temperature of 350∼ 400 °C to obtain a block product of 50mm(length) x 50mm (width) x 10mm (thickness). The specific weight of the resulting block product was 2.17. [COMPARISON 4] Non-irradiated PTFE powder (the same as the starting PTFE powder of EXAMPLE 13) was filled into a metallic die and preliminarily molded by press at 200∼ 700 kg/cm 2 , then placed in an electric furnace and fired at the temperature of 350∼ 400 °C to obtain a block product of 50mm(length) x 50mm (width) x 10mm (thickness) for comparison. TABLE. 4 shows results of coefficient of friction and coefficient of abrasion measuring tests for EXAMPLES 13 and 14, and COMPARISON 4. The same thrust-type friction and abrasion testing machine was used for the tests as used for EXAMPLES 1 ∼ 6 in TABLE 1. [EXAMPLE 15, 16] PTFE molding powder (G-163, made by ASAHI GLASS CO., LTD.), wherein an average particle diameter of the powder is 40 µm and a specific weight is 2.16, is exposed to electron beams in vacuum of less than 0.01 Torr (i.e. in the absence of oxygen) at an elevated temperature of 350°C for two different total doses, which were 50 KGy (EXAMPLE 15) and 100 KGy (EXAMPLE 16). After that, each powder was pulverized into finer powder of an average particle diameter of about 20 µm by a jet mill, then heat-treated at 300 °C for 12 hours to evaporate high-volatile matters (of 0.1 %). Each irradiated PTFE powder was filled into a metallic die and preliminarily molded by press at 200∼ 700 kg/cm 2 , then placed in a electric furnace and fired at the temperature of 350∼ 400 °C to obtain block products of 50mm(length) x 50mm (width) x 10mm (thickness). The specific weight of the resulting block products was 2.1∼ 2.3. For EXAMPLE 15 and 16, in addition to testing of coefficient of abrasion by the same thrust type friction and abrasion testing machine as used in EXAMPLE 1∼6 in TABLE 1, about 10 mg of sample was removed from each block, and packed in an exclusive pan to measure melting point and amount of heat of crystallization. For the measuring, a differential scanning calorimetry (DSC7, made by PERKIN-ELMER) was used. The results are shown in TABLE 5. As is apparent from the foregoing results by comparing EXAMPLES with COMPARISONS, the resins for use in accordance with the invention indicate lower coefficients of friction, which proves good lubrication characteristics of the products, and improved abrasion resistance, while each COMPARISON shows abnormal abrasion or substantial abrasion depth. Because of these advantages, the resins for use in accordance with the invention proves very useful for more extensive uses in an environment of frictional motion.

Claims: Use as an abrasion resistant product of a resin containing an ionizing radiation irradiated fluorine-containing polymer, said polymer being obtainable by exposure of a fluorine-containing polymer to an ionizing radiation selected from γ-rays, electron beams, X-rays and neutron beams, at a total dose of from 1 KGy to 10 Mgy, in the absence of oxygen and at a temperature in the range from the melting point of the fluorine-containing polymer to 30°C above the melting point of the fluorine-containing polymer, the fluorine-containing polymer being selected from the group consisting of tetrafluoroethylene polymers, tetrafluoroethylene-perfluoro(alkylvinylether) copolymers and tetrafluoroethylene-hexafluoropropylene copolymers. Use according to claim 1, wherein said product is a resin molded from said ionizing radiation irradiated fluorine-containing polymer. Use according to claim 1, wherein said product is obtainable by exposure of the fluorine-containing polymer to ionizing radiation after molding thereof. Use according to any preceding claim, wherein said resin includes from 1 to 100 weight % of a powder of said ionizing radiation irradiated fluorine-containing polymer, and from 99 to 0 weight % of a polymer which has not been exposed to any ionizing radiation. Use according to claim 4, wherein the polymer which has not been exposed to ionizing radiation has a heat resistance of more than 300°C in the absence of oxygen. Use according to claim 4 or claim 5, wherein the polymer powder has an average particle diameter of less than 1 mm. Use according to any of claims 4 to 6, wherein said resin is obtainable by molding a mix of powder by pressure, said mix consisting essentially of from 1 to 100 weight % of said ionizing radiation irradiated fluorine-containing polymer powder, and from 99 to 0 weight % of a powder of said polymer which has not been exposed to any ionizing radiation. Use according to any preceding claim, wherein said resin has a melting point of lower than 325°C, an amount of heat of crystallisation of less than 40 J/G, and a coefficient of abrasion of less than a half of that of a resin which contains fluorine-containing polymer which has not been irradiated with any ionizing radiation. Use according to claim 8, wherein said resin is obtainable by molding a powder of said irradiated fluorine-containing polymer by pressure. Use according to any preceding claim, wherein the abrasion resistant product is selected from the group consisting of a non-lubricated bearing, a dynamic seal, a roll for a copy machine, and a bearing pad.

Inventors: IKEDA SHIGETOSHI, KASAI NOBORU, MOROZUMI MITSUHARU, NAKAYAMA AKINARI, NOMURA MASUMI, OSHIMA AKIHIRO, OTSUHATA KAZUSHIGE, SEGUCHI TADAO, TABATA YONEHO, YAGYU HIDEKI, IKEDA, SHIGETOSHI, KASAI, NOBORU, MOROZUMI, MITSUHARU, NAKAYAMA, AKINARI, NOMURA, MASUMI, OSHIMA, AKIHIRO, OTSUHATA, KAZUSHIGE, SEGUCHI, TADAO, TABATA, YONEHO, YAGYU, HIDEKI
Applicants: ASAHI GLASS CO LTD, HITACHI CABLE, RAYTECH CORP, ASAHI GLASS COMPANY LTD., HITACHI CABLE, LTD., RAYTECH CORPORATION
Application year: 1997
Main classification: C08J3
IPC codes: C08J5, B29C71, C08J3, C08J7