Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – Making hole or aperture in article
Patent
1993-12-21
1995-12-12
Tentoni, Leo B.
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
Making hole or aperture in article
2642888, B29C 5504, B29C 6720
Patent
active
054747279
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a polytetrafluoroethylene (PTFE) tape of very high porosity, to a porous tape obtained thereby, and to use thereof, in particular in a coaxial cable.
2. Description of the Related Art
Porous PTFE tapes already exist having a density of 0.2 grams per cubic centimeter (g/cm.sup.3), which corresponds to a dielectric constant (.epsilon.) of about 1.09.
Such PTFE tapes are manufactured by conventional stretching methods that include a single and continuous stretching step at temperatures that are well below the melting temperature (freezing point) of PTFE. Such methods encounter a density limit at about 0.3 g/cm.sup.3. Thus, when stretching comes close to this value, the tape breaks.
To achieve lower densities, known methods use at least two successive stretching passes, that imply changes to the structure of the PTFE which goes from a state in which its degree of crystallinity is high to a state in which the amorphous fraction is preponderant.
This sudden change of state is caused by a sintering operation that is performed after the stretching operation and at a temperature close to the melting temperature (freezing point) of the raw PTFE tape.
The structure is locked in the amorphous state by a rapid cooling step.
It should be observed that the dielectric constant of PTFE is related to its porosity, its specific gravity, or its density by the equation .epsilon.=(1+d/3)/(1-d/12.6) where d is specific gravity.
In addition, the relationship between porosity P and specific gravity d (or density in g/cm.sup.3) is the following:
Consequently, one of the major uses of said tape lies in providing insulating coatings for cables, in particular coaxial cables.
SUMMARY OF THE INVENTION
An object of the present invention is to prepare a PTFE tape of high porosity in simplified manner and having a density that is lower than that which has been obtained previously.
According to the invention, this object is achieved by a method of manufacturing a PTFE tape whose density is less than or equal to 0.2 g/cm.sup.3, the method being characterized in that a raw PTFE tape is subjected to a first step of stretching while being heated so as to reduce its density to within the range 0.7 g/cm.sup.3 to 0.30 g/cm.sup.3, after which at least one step is performed of the type including a stage during which the tape is stabilized without changing the state of the PTFE by being passed through an oven at a temperature and for a duration that are functions of the mass to be stabilized while keeping the tape under tension so that its density does not increase, followed by a stage of stretching under heating so that the density of the tape reaches a value lying in the range 0.2 g/cm.sup.3 to 0.12 g/cm.sup.3.
According to an advantageous characteristic of the invention, the stretching passes comprise stretching in the travel direction of the tape.
According to another characteristic, the temperature of the stabilization stage lies in the range 150.degree. C. to 250.degree. C.
The invention also provides a porous PTFE tape having a crystalline structure whose density lies in the range 0.2 g/cm.sup.3 to 0.12 g/cm.sup.3.
The invention also relates to the use of the high porosity tape for coating a coaxial cable.
The method of the invention thus makes it simple to obtain a PTFE tape of high porosity with improved dielectric properties.
This simplification in the method is due in particular to the fact that the invention makes it possible to use as raw material, raw PTFE tapes regardless of the way in which they have been manufactured and regardless of the properties of their constituents (degree of crystallinity of the initial resin, grade of said resin, nature of the lubricants, . . .).
In addition, the method of the invention makes it possible to reduce dimensional variations in the tape due to stretching.
For the same total stretching ratio, i.e. ratio between the speed of the tape at the outlet of the stretching device to the speed
REFERENCES:
patent: 3953566 (1976-04-01), Gore
patent: 4110392 (1978-08-01), Yamazaki
patent: 4707314 (1987-11-01), Kawahigashi et al.
patent: 4710331 (1987-12-01), Nobuo et al.
patent: 4902423 (1990-02-01), Bacino
Abstract of Japan 57-51,450 (Published Mar. 26, 1982).
Axon Cable
Tentoni Leo B.
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