Polyimide insulated coaxial electric cable

A high temperature and radiation resistant all polyimide insulated coaxial cable and process of manufacture. Perforated polyimide tape wrapping yields low dielectric constant cable.

FIELD OF THE INVENTION 
The field of the invention is coaxial electric cables which resist high 
temperature and radiation and at the same time have reduced size and 
excellent electrical properties. 
BACKGROUND OF THE INVENTION 
There has been a continuing need for high temperature resistant radiation 
resistant insulated wire products. One of the best materials for this type 
of application is polyimide polymer insulation which has the chemical 
composition to withstand both high temperature and radiation better than 
most polymeric materials. Typical useful materials are polyimides 
disclosed and claimed in U.S. Pat. No. 3,129,634 wherein organic aromatic 
tetravalent acids react with at least one organic divalent benzenoid 
diamine to give preferably an all aromatic ring structured polyamide-acid 
intermediate. These intermediates can be made into films or solutions 
which, after the solvent is removed, can be cured by heating above 
50.degree. C. to the fully aromatic polyimide. The polyamide-acid in the 
form of wire enamel is made by fully curing by baking the polyamide-acids 
and similar abrasion-resistant baked wire coatings on other insulation and 
layered with fluorocarbon adhesives as tape wrap wire insulation. 
Two problems exist, however, which limit the use of the material in these 
forms. First, the dielectric constant of the films is high (3.5) as 
compared to expanded, stretched, or foamed alternative materials 
(1.3-2.2). Second, where a fluorocarbon thermoplastic adhesive is used in 
combination with polyimide tape or film, such as disclosed in U.S. Pat. 
Nos. 3,168,417, 3,352,714 and 3,40B,715, the fluorocarbon is not radiation 
resistant, and the advantage of radiation resistance is nullified for 
these tapes. Alternative adhesives which could be substituted, such as 
polyester, polyurethane, or acrylic, are limited in temperature 
resistance, however, so that solution is not fully satisfactory. 
SUMMARY OF THE INVENTION 
To overcome the perceived problems inherent in use of baked polyamide-acid 
enamels and coatings and fluorocarbon adhesive-backed polyimide tapes, 
liquid polyamide-acid adhesives are coated as adhesive layers or coatings 
onto perforated fully cured polyimide tapes. A metal center conductor is 
wrapped with such a polyamide-acid coated polyimide tape by standard cable 
making machinery to the desired thickness, and the tape-wrapped wire 
passed through an oven above 50.degree. C. for a time sufficient to fully 
convert the polyamide-acid to polyimide. This cured construction is now 
wrapped with polyamide-acid adhesive coated polyimide binder tape to bind 
and seal the porous insulation covering the wire and this binder or 
sealing layer also cured above 50.degree. C. in like manner to the 
previous polyamide-acid layer. 
The bound cable is now shielded by a layer of conductive shielding by a 
method known in the cabling art which may be metal wire braiding, braided 
metal foil, served metal tape, or metallized polyimide tape, which has an 
adhesive coating of polyamide-acid. The latter tape, if used, is cured as 
described above. 
The shielded cable is now completed by dipping one or more times in a 
liquid coating of the polyamide-acid adhesive solution, which is dried 
between coats, to build up a protective jacket of desired thickness, which 
is cured by baking above 50.degree. C. as above or as many layers as 
needed of polyamide-acid adhesive coated polyimide tape is wrapped around 
the cable to give a suitable protective jacket when it has been fully 
cured above 50.degree. C. for an adequate period of time. A combination 
tape and dip-coated jacket may be used alternatively. 
A final cable product where 50% of the area of the first layer of the tape 
has been removed by punching out small holes evenly across the area so 
that 50% of the polyimide is replaced by air will have a dielectric 
constant of about 1.8 versus 3.5 for an equal thickness of solid 
polyimide. The percent air content can be varied somewhat by changing 
perforation hole size, numbers, and spacing to taylor the material for 
particular applications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the figures, a detailed description of the invention is 
now made including the processes used for making the cables. FIG. 1 is a 
cross-section of a cured polyimide cable of the invention wherein the 
conductive metal center conductor 1 is surrounded by porous polyimide 
insulation 2. Insulation 2 has been formed about conductor 1 by wrapping 
conductor 1 with a perforated polyimide tape which has coated on it a thin 
layer of polyamide-acid adhesive, which has been applied from a solution 
of the amide-acid in a solvent, much as one of those listed in U.S. Pat. 
No. 3,179,634 above, examples of which are dimethylformamide and 
dimethylacetamide. When the desired thickness of insulation 2 has been 
achieved, a binder tape 3 of solid polyimide tape coated with the same or 
similar polyamide-acid adhesive as used on the perforated tape is wrapped 
around insulation 2 to bind it in place and to seal the porosity into the 
cable. 
At this point in the process, the cable is heated above 50.degree. C. for a 
period long enough to completely convert any polyamide-acid present to 
polyimide, the amide-acid groups present splitting out water to leave an 
imide group in a newly closed aromatic ring. This adds greatly to product 
stability and improves physical properties. 
The all-polyimide insulated cable is now shielded by a conductive shielding 
4 by one of the methods known in the art for shielding electrical cables 
or forming coaxial electric cables, such as wrapping the cable with a 
served conductive metal foil or a metallized polyimide polymer tape or 
braiding a conductive wire or tape shield about the cable by an art known 
braiding means or mechanism. 
The shielded cable is wrapped with a protective layer 5 of polyamide-acid 
adhesive coated tape which is heated above 50.degree. C. for a sufficient 
period of time to effect complete conversion of the adhesive to polyimide 
or the cable is dipped, spray coated, or otherwise coated with 
polyamide-acid in solvent to build up a selected thickness of coating and 
heated similarly above 50.degree. C. to convert this coating completely to 
polyimide. 
The process yields a small light weight, radiation-resistant, all-polyimide 
insulated and coated cable of improved electrical performance such as 
increased velocity of propagation and reduced capacitance. The cable will 
also have a dielectric constant of about 1.8-1.9 if about 50% of the 
volume of polyimide is punched out of the tape forming the main insulation 
of the cable to be replaced by air. Solid polyimide has a dielectric 
constant of about 3.5. The sealing and air retention in the insulation is 
equivalent to that typical for use of standard processes. 
The polyimide tape is hole-punched or perforated by a combination 
male/female punch roll system which allows continuous longitudinal 
perforation of the film. This method is preferred if the tape is to be 
used subsequently for tape wrapping. Long lengths yield maximum 
productivity and minimum costs and the method is a standard in the 
industry for films and foils. 
Alternative to heating above 50.degree. C. for a period of time to convert 
the polyamide-acid to polyimide, the amide-acid can be heated or 
dehydrated chemically in acetic anhydride and pyridine at 
200.degree.-250.degree. C. It has also been found that if the amide-acid 
has been converted to polyimide at less than 300.degree. C., the thermal 
and hydrolytic stability properties of the polyimide may be improved by 
heating between 250.degree. and 500.degree. C. for 15 seconds to 2 hours. 
The cable is expected to find utility in nuclear power plants and around 
other radiation sources, military nuclear power applications, satellite 
and space vehicle or station exposed or lightly shielded wiring, and high 
temperature applications where polyimide would be used but reduced size is 
important, and other uses such as the above for digital signal application 
requiring resistance to heat and/or radiation. 
While the invention has been disclosed in terms of certain embodiments and 
detailed descriptions, it will be clear to one skilled in the art that 
modifications or variations of such details may be made without deviating 
from the scope of the invention, which is limited only by the claims 
appended below.