Title of Invention

AN UNSINTERED POLYTETRAFLUOROETHYLENE TAPE

Abstract

An unsintered polytetrafluoroethylene tape for screw joint sealing obtained by uniaxially stretching an unsintered body of a composition comprising 75 to 25 wt. parts of polytetrafluoro- ethylene fine powder and 25 to 75 wt. parts of inorganic powder which has new Mohs hardness of 3 or less and substantially no water absorption, which tape is excellent in heat stability and sealing properties as the conventional polytetrafluoroethylene tape for screw joint sealing, a used amount of which can be decreased. Further, by compounding a specific inorganic filler, the tape does not suffer from limitation on stretching conditions, and does not abrade or damage a screw.

Full Text

TITLE OF THE INVENTION SCREW JOINT SEALING TAPE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an unsintered polytetra-fluoroethylene (PTFE) tape for screw joint sealing. In particular, the present invention relates to a screw joint sealing unsintered PTFE tape containing specific inorganic powder. Description of the Related Art An unsintered tape prepared from PTFE fine powder is widely used as a screw joint sealing tape, since it has excellent sealing property, and cleaning of a sealed area of the screw joint is easy when pipes are disassennbled, cleaned and reassembled. JP-B-48-44664 discloses a method for producing a PTFE tape. According to the disclosure of this publication, when a conven¬tional unsintered PTFE tape is stretched in a longitudinal direction by two times or more, a tape width is not decreased substantially, and a thickness is not decreased to a measurable extent, and as the result, an stretched tape having the same cross sectional area as that of the original tape is obtained. The obtained tape has a larger coefficient of friction and better self tack than the conventionally used PTFE tapes, and has improved followability to a shape of a screw. Further, it is possible to provide a PTFE tape which main¬tains the sealing property comparable to that of the conventional tape when it is used in a smaller amount than before. In the above prior patent, a specific gravity of the PTFE tape is decreased by stretching, and in turn an amount of PTFE to be used per unit volume is decreased to reduce usage of PTFE which is one of materials which are expensive and difficult to be recovered or recycled. The reduction of the specific gravity by stretching means that a void in the tape increases. The sealing of the screw joint is achieved by filling the tape in a clearance between a male screw and a female screw. Then, if the void of the tape is too large due to the small specific gravity, sufficient sealing properties are not attained unless the tape thickness is increased, or the number of windings of the tape is increased, so that the tape lacks the utility or serviceability. JP-A-1-198675 discloses a sheet form soft gasket. In the invention of this publication, to maintain strength of the sheet form gasket, an inorganic filler is compounded in an unsintered PTFE resin which can be fibrillated, and mixed under application of mechanical force such as shear force while fibrillating the PTFE resin powder, the compound is extruded with an extruder, and shaped in a sheet form by rolling or calendering to obtain the soft sheet gasket. But, when the PTFE resin powder is fibrillated prior to sheeting as disclosed in the above publication, it is difficult to obtain a continuous ribbon-form tape having a thickness of about 100 |im as disclosed in JP-B-48-44664. Even if the tape is rolled to such thin thickness, both edges of the rolled film are heavily corrugated, so that a yield is decreased. When the specific gravity is decreased by stretching, the above film cannot be stretched even by about two times. Further, when the tape is used as a sealing tape, the screw may be abraded or damaged according to the kind of the inorganic filler material. SUMMARY OF THE INVENTION An object of the present invention is to provide a PTFE tape which is excellent in heat stability and sealing properties as the conventional PTFE tape for screw joint sealing which can decrease a used amount of PTFE by compounding a specific inorganic filler, which does not suffer from limitation on stretching condi¬tions, and which does not abrade or damage a screw when used as a screw joint sealing tape. According to the present invention, there is provided an unsintered polytetrafluoroethylene tape for screw joint sealing consisting of a uniaxially stretched unsintered body of a composi¬tion comprising 75 to 25 wt. parts of polytetrafluoroethylene fine powder and 25 to 75 wt. parts of inorganic powder which has new Mohs hardness (hardness of talc being 1 and that of diamond being 15) of 3 or less and substantially no water absorption. In one preferred embodiment, the inorganic filler is at least one inorganic powder selected from the group consisting of talc, calcium carbonate and magnesium carbonate. In another preferred embodiment, the unsintered poly¬tetrafluoroethylene tape for screw joint sealing of the present invention is produced by uniaxially stretching the above unsintered body at a temperature lower than a melting point of sintered polytetrafluoroethylene so that a specific gravity of the stretched body is decreased to less than 1.5, and heat treating the stretched body. Accordingly, the present invention provides an unsintered polytetrafluoroethylene tape for screw joint sealing consisting of a uniaxially stretched unsintered body of a composition comprising 75 to 25 wt. parts of polytetrafluoroethylene fine powder and 25 to 75 wt. Parts of inorganic powder which has new M ohs hardness of 3 or less and substantially no water absorption. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows an apparatus used in heating-cooling cycle test in Experiment 1, and Fig. 2 is a cross sectional view of an apparatus used for checking damage/abrasion of a screw thread in Experiment 2. DETAILED DESCRIPTION OF THE INVENTION The inorganic powder filler used in the present invention has Mohs hardness of 3 or less, and substantially no water absorp¬tion. Other properties required for the inorganic powder filler are as follows: (1) It is excellent in heat resistance, and does not suffer from physical and chemical degradation when heated to the melting point of PTFE. (2) It is chemically stable, and it is resistant to general acids or alkalis. (3) Its specific gravity is not large, and close to that of PTFE (about 2.2). (4) It is white, so that it does not deteriorate cleanness of the PTFE tape, and does not impair color tone when a pigment is added to PTFE. (5) It has a good slip property, does not adhere to the screw threads, and the PTFE tape containing the filler is easily peeled off. (6) Compounding of the inorganic filler does not inter¬fere with the stretching of the PTFE film. Among the inorganic powder fillers having such properties, talc, calcium carbonate and magnesium carbonate are preferred, and talc is most preferred in view of the hardness. In the present invention a weight ratio of the PTFE fine powder to the inorganic powder (PTFE fine powder : inorganic powder) is from 75:25 to 25:75, preferably from 65:35 to 35:65, more preferably from 55:45 to 45:55. The PTFE fine powder and the inorganic powder can be mixed by any of conventional mixing methods. For example, an aqueous dispersion of the PTFE fine powder and the inorganic powder are mixed to co-precipitate them. Alternatively, the precipi¬tated PTFE fine powder and the inorganic powder are mixed at a low temperature with suppressing fibrillation of PTFE. To obtain a more homogeneous mixed powder, the co-precipitation is preferred. In the present invention, the' PTFE fine powder includes a fine powder of a tetrafluoroethylene homopolymer and also a fine powder of a modified PTFE comprising at least one other comonomer in such a small amount that melt flowability is not imparted (for example, 0.001 to 2.0 wt. %, preferably 0.001 to 0.5 wt. %). As the inorganic powder used in the present invention, a commercially available one (average particle size D50: about 1 to 10 μm) may be used. When the inorganic powder is co-precipitated with the PTFE fine powder, the inorganic powder which is made hydrophobic by surface treatment is preferably used. As a surface treating agent, a silane or titanium coupling agent, an organo-siloxane, a fluorine-containing surface treating agent, and so on can be used preferably. Herein, the term "substantially no water absorption" is intended to mean that a weight increase is less than 1 %, preferably less than 0.5 % when the PTFE tape containing 50 wt. % of the inorganic powder is subjected to the below described water absor¬bing test. Herein, the "unsintered" body is intended to mean that the PTFE body has not been heated to the melting point of PTFE or higher. The unsintered PTFE body is obtained by a per se conven- tional molding method, and its shape is usually a sheet or a film. In general, the unsintered body is uniaxially stretched under the following conditions: Temperature: 150 to 300°C Stretching rate: 5 to 1000 %/sec. Stretching ratio: 1.5 to 5 times based on the length of an originally (unstretched) body. Usually, the sheet produced by the uniaxial stretching is heat treated to relax local strain caused by the uniaxial stretching, whereby dimensional stability of the stretched film is improved. This heat treatment is performed usually at a temperature from the stretching temperature to lower than the melting point of PTFE for 5 to 60 seconds. The present invention will be explained further in detail by the following examples. Example 1 Preparation of PTFE fine powder containing talc powder (1) Surface treatment of talc powder Using a henschel mixer, talc powder ("MICROACE* manufacture by Nippon Talc C, Ltd. Average particle size D50: 2.3 μm) (10 kg) and a solution of an aminosilane coupling agent ("A-1100" manufactured by Nippon Unicar Co., Ltd.) (200 g) in water (100 ml) were mixed for 10 minutes to surface treat the talc powder. Then, the surface treated talc powder was dried in a fluidized bed with hot air at 120C. (2) Preparation of mixed powder of PTFE fine powder and talc In an emulsion polymerization liquid of PTFE fine powder F104 (manufactured by Daikin Industries Ltd.) (an aqueous disper- sion of PTFE particles having an average particle size of 0.2 to 0.4 μm), the surface treated talc powder prepared in the step (1) was mixed to co-precipitate them, and the precipitated powder was dried to obtain a raw material powder. Details of the preparation conditions were as follows: (i) In a 150 I precipitation tank, pure water (about 50 I) was charged, and the surface treated talc powder (5 kg) was disper¬sed while agitation with agitation blades. (ii) Then, a 10 wt. % aqueous dispersion of the PTFE powder (50 kg corresponding to 5 kg of the resin) was charged in the tank. As the agitation was continued, the PTFE particles and the talc powder were co-precipitated in the form of uniform coagulated particles, and the dispersion was separated to a liquid phase and a solid phase. (iii) From the liquid phase, the solid phase particles were recovered, spread over a pallet to a thickness of about 3 cm and dried thoroughly in a drier at 150°C. The dried particles had an average particle size of 380μm, and an apparent density of about 400 g/l. Preparation of unsintered stretched film (1) Mixing of a lubricating aid The above prepared raw material mixed powder (5 kg) was charged in a vessel, and then a lubricating aid ("ISOPAR M", a petroleum base lubricating aid manufactured by Exxon) (1.15 kg) was added. The vessel was shaken and rotated so that the whole powder is uniformly wet with the lubricating aid, followed by aging at 25°C for 24 hours. (2) Paste extrusion molding The raw material powder prepared in the step (1) for mixing the aid was pressed in a cylinder having a diameter of 89 mm under pressure of 10 kg/cm2 for 10 minutes to obtain a premolded article. The premolded article was extruded through a past extruding mold to produce a PTFE sheet containing the lubricating aid and having a thickness of 3 mm and a width of 200 mm. (3) Calendering and drying of the lubricating aid The PTFE sheet produced in the past extrusion molding step (2) was calendered using a pair of calendering rolls each having a diameter of 500 mm and a length of 700 mm which were heated at 70°C and rotated at a peripheral speed of 20 m/min. to obtain a rolled film having a thickness of 120 μm and a width of 210 mm. Then, the lubricating aid was evaporated off from the rolled film on a roll having a diameter of 300 mm and a length of 500 mm which was heated at 200°C and rotated at a peripheral speed of 5 m/min. (4) Production of unsintered stretched PTFE film The rolled film produced in the step (3) was stretched between a roll having a diameter of 300 mm and a length of 500 mm which was heated at 300°C and rotated at a peripheral speed of 2 m/min. and a roll having a diameter of 250 mm and a length of 500 mm which was heated at 300°C and rotated at a peripheral speed of 5 m/min. A distance between tangential lines to respective rolls was about 10 cm. Subsequently, the stretched film was subjected to the heat treatment (heat set) using a roll having a diameter of 300 mm and a length of 500 mm which was heated at 300°C and rotated at a peripheral speed of 5 m/min. The talc powder-containing PTFE film produced in the previous step was cut to a width of 12.5 mm along the longitudinal direction to obtain a ribbon-form film, and its physical properties were measured. The results are as follows: Tape thickness: 120 |j.m Specific gravity: 0.7 Strength at break: 2.2 kg (at a pulling rate of 200 mm/min.) Elongation at break: 30 % Spontaneous shrinkage factor: 1-3 % Experiment 1 As shown in Fig. 1, inner surfaces of both ends of a 20 A socket 1 for gas piping according to JIS and an outer surface of each of pipes 2a and 2b were threaded at PT (tapered screw) 3/4 (not shown). Then, the ribbon-form PTFE film produced in Example 1 was spirally wound around each of the threaded parts of the pipes 2a and 2b with overlapping one quarter of the tape width (not shown), and the threaded parts of the pipes were screwed in the respective ends of the socket 1. Further, the other end of the pipe 2a was sealed at a flange 3, while the other end of the pipe 2b was connected to a nitrogen gas inlet tube with a flange 4 to assemble an apparatus for a heating-cooling cycle test. This apparatus was immersed in a heating medium 5 contained in a tank 6, and an experiment was performed to check leakage at the sealing. That is, the socket 1 including the parts screwed with the pipes 2a and 2b was alternately immersed in cold water at 20 to 25°C and warm water at 70 to 80°C for 30 minutes each, during which the pressure in the pipes was maintained at 10 kg/cm2 with the nitrogen gas. These procedures were repeated 30 days, no gas leakage from the sealed area was observed. After finishing the above test, the talc powder-containing PTFE sealing tape was removed. The tape was easily removed without adhesion to the screws like the PTFE sealing tape containing no filler, and the sealing tape could be wound around the screw without any problem. Experiment 2 Damage of the screw threads As shown in Fig. 2, using an aluminum material (kind: 1070) corresponding to JIS H4000, a bolt 7 having a male screw corresponding to a parallel screw PF 1/2 according to JIS B0202 (leg length of 40 mm), and a box nut 8 (depth of 30 mm) having a female screw which was connected to a pressure pipe 9 were produced. Then, the ribbon-form PTFE film produced in Example 1 was spirally wound around the threaded part of the bolt 7 with overlapping one quarter of the tape width (not shown). This bolt 7 which was wrapped by the sealing tape was screwed in the box nut till its tip end reached the deepest part of the box nut, and a pressuring pipe 9 was pressurized with nitrogen gas to 5 kg/cm2. After confirming that no leakage occurred, the pressure was released and the bolt was unscrewed, the used sealing tape was completely removed. These procedures were repeated 100 times, and then the damaged state of the screw threads was observed, but no flaw or damage was found. Experiment 3 By taking the use applications of the sealing tape into account, its water absorbance and acid resistance were measured as follows: Water absorbing test In a 3 wt. % aqueous solution of a surfactant (C7F14COONH4), the tape (100 g) was irfimersed for 48 hours, and pulled out. Then, the tape was dried in a vacuum drier at 90°C for 12 hours, and weighed with an automated balance which weighs a weight at an accuracy of 0.1 g order. A weight change was expressed in terms of a percentage in relation to a weight of the tape before immersion in the above aqueous solution. Acid resistance test In a 10 % hydrochloric acid containing 3 wt. % of a surfactant (C7H14COOH) (2 liters in total), the tape (100 g) was immersed for 48 hours and pulled out. Then, the tape was dried in a vacuum drier at 90°C for 12 hours, and weighed with an automated balance which weighs a weight at an accuracy of 0.1 g order. A weight change was expressed in terms of a percentage in relation to a weight of the tape before immersion in the above acid solution. Examples 2-3 and Comparative Example 1 In the same manner as in Example 1 except that calcium carbonate (Example 2), magnesium carbonate (Example 3), or gypsum (Comparative Example 1) was used in place of talc, a ribbon-form PTFE tape containing an inorganic filler was produced. The results are shown in the Table. The details of the used inorganic fillers are as follows: Calcium carbonate: SUNLITE #1000 manufactured by TAKEHARA Chemical Industries, Ltd. Magnesium carbonate: KINSEI manufactured by KOHJIMA Chemical Industries, Ltd. Gypsum: Anhydrous CaS04 which is prepared by a reac¬tion between CaF2 and H2SO4 was changed to its dihydrate form, and pulverized with a hammer and sieved to obtain particles having an average particle size of 5 μm (maximum 10 μm to minimum 0.05 μm). Comparative Example 2 In the same manner as in Example 1 except that a mixed powder of PTFE fine powder and glass fiber (POLYFLON FPG 1050 manufactured by Daikin Industries Ltd.) was used as a raw material for the production of an unsintered stretched film, a ribbon-form sealing tape was produced. The obtained tape was subjected to the same experiments as above. In Experiment 2, no gas leakage was observed, but gray deposits were adhered to the surface of the used sealing tape. The deposits were elementary analyzed to find that they consisted of aluminum. From this result, it is apparent that the PTFE sealing tape containing the glass fiber abraded the screw part. Effects of the Invention The PTFE sealing tape containing the organic powder according to the present invention is an economical screw joint sealing tape which is excellent in heat stability and sealing properties as the conventional PTFE sealing tape, is easily stretched, and does not abrade or damage a screw. WE CLAIM; 1. An unsintered polytetrafluoroetbylene tape for screw joint sealing consisting of a uniaxially stretched unsintered body of a composition comprising 75 to 25 wt. parts of polytetrafluoroethylene fine powder and 25 to 75 wt. parts of inorganic powder which has new Mohs hardness of 3 or less and substantially no water absorption. 2. The polytetrafluoroethylene tape according to claims 1, wherein said inorganic powder is at least one inorganic powder selected from the group consisting of talc, calcium carbonate and magnesium carbonate.

Documents:

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522-mas-1996 correspondence others.pdf

522-mas-1996 correspondence po.pdf

522-mas-1996 description (complete).pdf

522-mas-1996 drawing.pdf

522-mas-1996 form-2.pdf

522-mas-1996 form-26.pdf

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522-mas-1996 others.pdf

522-mas-1996 petition.pdf