Elongate body insulated by means of an insulating covering

An elongate body insulated by means of an insulating covering, said covering being constituted by a tape (2) made of synthetic material taped around said body (1) and covered in a layer of varnish (11) made of synthetic material, the body being characterized in that the taping is such that there exists a groove (4) between the free edge (5) of any portion of a turn (3) that is not covered by the following turns and the uncovered surface (6) of the preceding turn, the width of said groove (4) lying substantially in the range 0% to 5% of the width of said tape (2) and being strictly greater than zero.

The present invention relates to an elongate body insulated by means of a 
covering of insulating material. 
The term "elongate body" is used to cover any body that is long relative to 
its diameter. The term "elongate body" can thus be used, for example, for 
electrical conductors or cables of all kinds, hollow section bars, tubes, 
pipes, and more generally ducting. 
The present invention relates in particular to insulating cables or 
electrical conductors. The specification below relates to electrical 
conductors. 
French patent application FR-2 119 939 describes an electrical conductor 
insulated by means of taping with an overlap percentage of not less than 
50% using a tape made of synthetic material. The tape is then covered in 
varnish made of a dried and cured synthetic material. The preferred 
overlap percentages mentioned in that document lead in practice to taping 
with overlaps or steps that cause the radial thickness of the conductor to 
be irregular, such that it is not possible to obtain a smooth outer 
surface. 
The overlaps also form projecting edges which constitute points of attack 
for external mechanical forces. This may give rise to the insulation being 
torn off at an overlap zone, thereby possibly exposing the conductor to 
high temperatures, to the influence of moisture, or to a chemically 
aggressive medium. 
The same problems as those mentioned above arise when any elongate body is 
to be insulated. 
The object of the present invention is to provide an elongate body having 
taped insulation and a smooth outside appearance. 
To this end, the present invention provides an elongate body insulated by 
means of an insulating covering, said covering being constituted by a tape 
made of synthetic material taped around said body and covered in a layer 
of varnish made of synthetic material, the body being characterized in 
that the taping is such that there exists a groove between the free edge 
of any portion of a turn that is not covered by the following turns and 
the uncovered surface of the preceding turn, the width of said groove 
lying substantially in the range 0% to 5% of the width of said tape and 
being strictly greater than zero. 
Advantageously, the width of said groove is substantially equal to 1% of 
the width of said tape. This value makes its possible for the varnish to 
adhere thoroughly in the grooves before coating the tape. 
The layer of varnish may be deposited by immersion in a bath containing the 
varnish followed by heat treatment such as sintering for eliminating the 
volatile substances contained in the varnish and for curing the varnish. 
According to an additional feature, the varnish contains wetting agents 
suitable to enable it to adhere to the surface which it covers prior to 
the heat treatment. 
The layer of varnish may also be deposited by an electrostatic method or by 
a fluidized bed method. 
The invention may be applied, for example, to the outer insulation of 
electrical cables, electrical conductors, tubes, pipes, or ducts. 
It may also be applied to the manufacture of hollow section bars, in which 
case the elongate body is a support body that is removed once the heat 
treatment has been terminated. 
The electrical cables, electrical conductors, tubes, pipes, ducts, or 
hollow section bars obtained in this way have a smooth outer surface and 
concentricity greater than 90%. 
Other characteristics and advantages of the present invention appear from 
the following description of an electrical conductor whose insulating 
covering is in accordance with the invention, and given by way of 
non-limiting illustration.

In FIGS. 1 and 2, a conductor 1 (having a diameter of 1.25 mm) is covered 
in a PTFE tape 2 of thickness equal to 76 .mu.m and of width L equal to 
about 10 mm. More precisely, the tape is taped around the conductor 1 by 
being helically wound at a pitch such that each turn 3 of width L has 49% 
of its width overlapped by the following turn, i.e. it is overlapped over 
a width L'=4.9 mm. It is then said that the overlap coefficient is 49%. 
Consequently, such winding leaves gaps or grooves 4 of width .DELTA.=1%. 
The width between the free edge 5 of any portion of a turn that is not 
covered by the following turns and the uncovered surface 6 of the 
preceding turn is equal to 0.1 mm. 
After taping, the conductor 7 obtained in this way is coated with varnish 
in a varnishing oven 8 (see FIG. 3). Varnishing takes place in three 
passes P.sub.1, P.sub.2, P.sub.3. On each pass, the conductor 7 is 
immersed in a coating trough 9 containing an aqueous dispersion (or 
varnish) of polytetrafluoroethylene (PTFE) to which wetting agents are 
added. These wetting agents are adapted to PTFE varnish such that when the 
conductor 7 is removed from the trough 9, the still-liquid varnish fills 
the grooves 4 and laps the conductor 7 while adhering to the surface of 
the conductor and not "running" as would occur if the width of the gaps 
were ill-adapted and if the surface tension created by the wetting agents 
were not optimized. 
The conductor 7 coated in PTFE varnish is then subjected to heat treatment 
(or sintering) in a vertical tower oven 10 for eliminating the solvents 
and for curing the varnish. 
After the first pass (immersing and sintering), two further similar passes 
P.sub.2 and P.sub.3 are performed so as to obtain a protective layer 
corresponding to the desired specifications for radial thickness and for 
operating voltage. 
The above description of one particular way of applying varnish is 
naturally given purely as an indication. The varnish may be applied in any 
other manner that is conventional in this field; it may thus be applied by 
an electrostatic method or by a fluidized bed method. 
The final cabling wire obtained thus has an outer layer of varnish 11 
coating the taping and included in the grooves 4. The thickness of this 
layer of varnish lies in the range 27 .mu.m to 36 .mu.m. Because of the 
varnish coating and the presence of narrow gaps, the outside appearance of 
the cabling wire is smooth, offering no points of attack for mechanical 
forces which could tear off or open the insulating covering, and retaining 
the good concentricity characteristic of taped conductors, i.e. better 
than 90%. 
Unlike prior art cabling wires, cabling wires of the invention also have 
narrow gaps that enable the varnish to avoid sliding along the tape so 
that it fills the grooves formed prior to the tape being coated, thereby 
establishing a smooth covering. 
Furthermore, the manufacture of cabling wires of the invention does not 
require any complex modification to conventional technology. A 
conventional taping disk is used merely by adapting its taping pitch to 
the desired overlap coefficient, and varnishing is likewise performed 
using a conventional technique. The resulting product is of improved 
quality, and its electrical, chemical, and operating voltage 
characteristics are identical to those of prior art extruded or taped 
insulation cabling wires. In particular, the outer insulation obtained is 
capable of withstanding electrical and mechanical stresses. 
Naturally, the present invention is not limited to the implementation 
described above. 
Firstly, the width of the gap is not necessarily equal to 1% of the width 
of the tape, and it may preferably take any value lying in the range 0% to 
5%, while always being strictly greater than 0%. For a width greater than 
5% of the width of the tape, there is a risk of the varnish sliding over 
the taping without filling its gaps, even when wetting agents are present. 
It would therefore be more difficult to obtain an insulating covering that 
is smooth. 
The description above also relates to a cabling wire in which the tape and 
the varnish are based on the same material, but it is possible for the 
tape and the varnish to be made of different materials, providing those 
materials are mutually compatible and the varnish is capable of adhering 
to the tape. 
For example, a PTFE tape may be used with a varnish comprising an aqueous 
solution of a fluorinated copolymer of ethylene and propylene (FEP), or a 
polyamide tape (such as "Kapton"--registered trademark) with a varnish of 
polyurethane or of polyamide, or finally a polyester tape with a varnish 
of nylon or of polyester. 
In addition, the invention may be applied to a structure other than a 
cabling wire, for example it may be applied to a coaxial cable whose outer 
insulating covering is constituted by a composite tape of polyamide and of 
PTFE and a varnish of PTFE or of FEP. Naturally all of the above-mentioned 
pairs of materials can also be used for making the outer covering of a 
coaxial cable. 
For cables of non-circular section (rectangular or polygonal, for example), 
where it is impossible to obtain a centered outer insulation by extrusion, 
the invention makes it possible to obtain an insulating covering having 
all of the necessary qualities. 
Advantageously, it is possible to coat the cable or the conductor with more 
than three layers of varnish. This makes it possible to fill all of the 
grooves completely, to achieve the desired performance with respect to 
operating voltage of the cable, and to establish a radial thickness of 
insulation that complies with the requested specifications. 
In general, the invention may be applied to insulating any elongate body, 
and in particular to insulating all types of cable, including optical 
fiber cables, all kinds of electrical conductors, and possibly also pipes, 
tubes, or ducts whose operating environments require insulating protection 
having the qualities of the protection provided by the invention. 
The invention may also be applied to manufacturing hollow section bars. 
This can be done merely by applying the insulation to a supporting body or 
former which is removed after the formed section bar has been made. 
Clearly the numerical values given are given merely by way of example and 
they cannot be considered as being limiting under any circumstances. For 
example, the thickness of the tape may lie in the range 50 .mu.m to 200 
.mu.m, and the width of the tape may lie in the range 7 mm to 12 mm. 
Finally, any means may be replaced by equivalent means without going beyond 
the ambit of the invention.