Stock material or miscellaneous articles – Composite – Of polyisocyanurate
Patent
1997-09-09
2000-09-12
Davis, Jenna
Stock material or miscellaneous articles
Composite
Of polyisocyanurate
428910, B32B 516
Patent
active
061175560
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an unsintered polytetrafluoroethylene (PTFE) tape for screw joint sealing. In particular, the present invention relates to an unsintered PTFE tape for screw joint sealing, which contains a specific inorganic powder.
DESCRIPTION OF THE PRIOR ART
Unsintered tapes prepared from PTFE fine powder are widely used as screw joint sealing tapes, since they have excellent sealing properties, and cleaning of sealed areas of the screw joints is easy when pipes are disassembled, cleaned and reassembled.
JP-B-48-44664 discloses a method for producing a PTFE tape. According to the disclosure of this publication, when a conventional unsintered PTFE tape is stretched in a longitudinal direction by two times or more, the tape width does not decrease substantially, and the thickness does not decrease to a measurable extent, and thus, the 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 the shape of a screw. Furthermore, it is possible to provide a PTFE tape which maintains the sealing properties comparable to those 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 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 voids in the tape increase. The sealing of the screw joint is achieved by filling a clearance between a male screw and a female screw with the tape. Thus, if the voids of the tape are 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, and therefore 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 .mu.m as disclosed in JP-B-48-44664. Even if the tape is rolled to such a thin thickness, both edges of the rolled film are heavily corrugated, and a yield decreases. When the specific gravity is decreased by stretching, the above film cannot be stretched even by about two times. Furthermore when the tape is used as the 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 has excellent heat stability and sealing properties like the conventional PTFE tape for screw joint sealing, which can decrease the required amount of PTFE by compounding a specific inorganic filler, which does not suffer from limitation on stretching conditions, and which does not abrade or damage a screw when used as a screw joint sealing tape.
According to the present invention, an unsintered polytetrafluoroethylene tape for screw joint sealing is provided, which consists 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 (hardness of talc being 1 and that of diamond being 15) of 3 or less and substantially no water absorption.
In one preferred
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Tamaru Shinji
Yamamoto Katsutoshi
Daikin Industries Ltd.
Davis Jenna
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