Manufacture of low density, sintered polytetrafluorethylene articles

A method for extruding, stretching and sintering polytetrafluoroethylene in a single pass operation to form tubular articles of low density, low matrix tensile strength, sintered polytetrafluoroethylene and articles such as electric cable having low density, low matrix tensile strength sintered polytetrafluoroethylene insulation applied about a conductor by such method. A high reduction ratio rating polytetrafluoroethylene powder with a lubricant is extruded by an extruder having a low reduction ratio, and the extrudate is heated to remove the lubricant and is stretched prior to sintering of the polytetrafluoroethylene tube which is held in its previously stretched state during sintering.

BACKGROUND OF THE INVENTION 
This invention relates to tetrafluroethylene polymers and in particular to 
manufacture of articles comprising microporous, that is, low density, 
sintered polytetrafluoroethylene. The invention provides a simple process 
for forming elongated articles of low density, sintered 
tetrafluoroethylene polymers having a relatively low tensile strength, for 
example, in the form of electrical insulation disposed about a conductive 
core, to produce a microporous polytetrafluoroethylene article, such as 
polytetrafluoroethylene insulated cable characterized by improved 
strippability of the insulation. 
"Porous" sold dielectric materials have found favor as insulation for 
electric cables used in communications and in the computer industry 
because the reduction in dielectric constant produced by the incorporation 
of air in a solid dielectric greatly improves the electrical 
characteristics of cable made with such insulation. Moreover, the porosity 
introduced into the dielectric reduces the weight of the cable and 
concommitently its cost. The latter is a particularly important 
consideration in the case of relatively expensive dielectric materials 
such as polytetrafluoroethylene. The lower dielectric constant of "porous" 
dielectrics also reduces the overall size of insulation required to 
achieve a given characteristic cable impedence. 
Low density, sintered polytetrafluoroethylene which is microprocess has 
heretofore been described in Japanese Patent Publication No. 13,560/67 and 
U.S. Pat. 3,953,566, for example. It has been made by a process of 
stretching and consequently is also called "stretched" or "expanded" 
polytetrafluoroethylene. As described in the literature, the product is 
made by extruding a paste of extrusion grade unsintered 
polytetrafluoroethylene admixed with an extrusion aid. The extrusion aid 
is removed from the extrudate after extrusion, and the product is then 
stretched in one or more directions and sintered while holding it in 
stretched condition. Stretching causes the polymer in effect to decrease 
in density without significant decrease in dimensions transverse to the 
direction of stretching. Stretched polymer can be produced which is thus 
increased in volume by several hundred percent of its original volume, 
introducing micropores such that the finished product has "porosity" with 
the pore volume often accounting for a major portion of the total product 
volume. Increases in tensile strength of the product in the direction of 
stretch are achieved, which become substantial after sintering while 
holding it in stretched condition. 
The extrusion process itself is conventional as practiced in the industry 
and called "paste extrusion". Typically extrusion aid is admixed with the 
extrusion grade unsintered polytetrafluorethylene powder in a proportion 
of 10 to 35% by volume. Preferably, the extrusion aid is V.M. & P. naptha 
and is employed in 18% by volume. The resultant paste, which is formed by 
tumbling or a similar procedure, is pressed into a preform shaped to be 
received in the barrel of a ram extruder. In the extruder, the preform is 
forced through a die with substantial reduction in cross-section. Sheeting 
dies and calendaring steps are commonly employed in conjunction with 
extrusion of the paste in order to obtain flat stock and in order to 
promote biaxial orientation of the fibers which are produced when 
unsintered polytetrafluoroethylene is extruded. Paste extrusion is also 
employed in manufacture of polytetrafluoroethylene insulated wire. 
In producing low density, sintered polytetrafluoroethylene, the extrusion 
aid is removed from the extruded product by heating at mild temperatures 
to drive off the extrusion aid or by solvent leaching. 
The product is then stretched in at least one direction, for example, by 
passing it in tape form over a roll travelling at a given speed and then 
to a capstan travelling at a faster speed such that the product is placed 
under tension and stretches between the roll and capstan. The stretching 
in length of the tape which takes place between the roll and capstan is 
not accompanied by any significant reduction in cross-section of the tape, 
thus, in effect increasing the volume and lowering the density of the 
material by stretching the spaces between the ibrous particles of 
polytetrafluoroethylene as well as elongating the fibers themselves. 
The resulting low density polytetrafluoroethylene is soft and upon heating 
without restraint shrinks losing the microporosity achieved by stretching. 
It has been found, however, that if low density, unsintered 
polytetrafluoroethylene is heated to sintering temperatures while 
restrained in stretched condition the porosity becomes set and is retained 
after cooling with a significant increase in tensile strength of the 
product in the direction of the stretch over the tensile strength of 
sintered product which has not been stretched. 
In the prior art processes, the resulting low density sintered 
polytetrafluoroethylene has a relatively high longitudinal tensile 
strength and longitudinal matrix tensile strength which is undesirable for 
various applications particularly, for insulation for wire or cable. Thus, 
with insulated wire or cable, adequate tensile strength is provided by the 
wire within the insulation, and it is desirable to have a low longitudinal 
tensile strength for strippability reasons. Thus, when terminating a wire 
or cable, a cut is made in the insulation, but such cut does not extend to 
the outer surface of the wire in order to avoid damage to the wire. 
Thereafter, the portion of the insulation to be removed is removed to bare 
the end of the wire, the insulation parting at the cut. The ease with 
which such portion of the insulation can be removed, or stripped, depends 
upon, and is an inverse function of the tensile strength of the 
insulation. Similar considerations apply to other articles of sintered, 
low density polytetrafluoroethylene which do not require a high tensile 
strength. 
Low density, sintered polytetrafluoroethylene made as described above has 
been used in the manufacture of insulated cable by making low density 
sintered tape as described above, and then winding the tape helically with 
overlap about a conductor. Typically several layers of helical windings 
are positioned over the conductor. While the final insulated cable has the 
advantages of a "porous" dielectric in terms of significant weight 
reduction, size reduction and the like, the product has several drawbacks. 
The surface, naturally, is rough because of overlap in applying the 
helical servings of polytetrafluoroethylene tape. Consequently, it cannot 
be color coded or otherwise marked as well as might be desired. The 
compression on the inner layers of tape caused by the tension imposed 
during winding the outer layers results in partial collapse of the inner 
layers (increase in density) which makes it difficult to control the 
impedance of the cable. Also the cable does not strip cleanly. Crossed 
fibers from the biaxially oriented wrapped tapes resist clean breakage. 
Where the tapes adhere to each other, the dielectric is discontinuous at 
tape boundaries. 
OBJECTS OF THE INVENTION 
It is an important object of the invention to provide a process for the 
manufacture of elongated, low density, sintered, polytetrafluoroethylene 
articles, such as polytetrafluoroethylene insulated wires or cables and 
tubes, which have a longitudinal tensile strength, apparent and matrix, 
which is relatively low compared to such tensile strength of such articles 
obtained with prior art processes. 
It is a further object of the invention to provide a wire or cable covered 
with a layer of low density polytetrafluoride which layer has a relatively 
low tensile strength and which has a signal propagation velocity of at 
least 75% of the speed of light. 
It is a further object of this invention to provide a process for the 
manufacture of elongated, low density, sintered polytetrafluoroethylene 
products, such as polytetrafluoroethylene insulated cables, in which the 
polytetrafluoroethylene is continuous and has a homogeneous, uniform 
density throughout. Further, the surface of, the polytetrafluoroethylene 
product is smooth facilitating electrolysis plating techniques, and, when 
applied to cable as insulation the product can be cleanly stripped without 
pulling crossed fibers or the like. 
It is a further object of this invention to provide a simple process for 
the production of such low density sintered products in an integrated 
operation in which the extrusion, extrusion aid removal, stretching and 
setting operations are carried out in a single pass arrangement. 
It is a further object of this invention to provide a process for 
manufacturing electric cable insulated with low density sintered 
polytetrafluoroethylene in which the stretching of the insulation and its 
application to the cable core are carried out simultaneously. 
SUMMARY OF THE INVENTION 
These and other objects of this invention are achieved utilizing a 
manufacturing process essentially the same as that conventionally used in 
paste extrusion, in that the paste is conventionally preformed and 
thereafter extruded in a process in which the extrudate is passed through 
an extrusion aid removal zone and the resulting product is then passed to 
a sintering zone in a single pass operation. In accordance with the 
present invention, however, the product is drawn through the sintering 
zone at a rate of linear speed substantially in excess of the linear speed 
of extrusion, that is, the speed determined by volumetric rate of 
extrusion divided by the cross-sectional area of the die exit. In order to 
carry out this operation it is also necessary to restrain or otherwise 
control the speed of extrusion at the die exit such that the extrudate is 
extruded at the same volumetric rate as it is fed to the die. In other 
words, pulltrusion is avoided. 
The process of the present invention also differs from prior art processes 
in that polytetrafluoroethylene powders known in the art as high reduction 
ratio polytetrafluoroethylene powders, i.e, a reduction ratio rating of at 
least 1000/1, are employed to form the preform, and the material of the 
preform is extruded at a low reduction ratio, i.e. at less than 1000/1 and 
preferably less than 800/1. It has been found from tests that when such a 
high reduction ratio polytetrafluoroethylene powder is extruded at a 
reduction ratio less than 1000/1, stretched and sintered, the matrix 
tensile strength of the resulting article is relatively low. 
As the extruded product is extruded from the die exit at extrusion speed 
and passes to the sintering zone through which the product is drawn at a 
greater speed, the product is stretched to accommodate the difference in 
speeds as well as being drawn through the extrusion aid removal zone. This 
stretch desirably is made to occur after extrusion aid removal. Generally 
stretching is predetermined to occur after extrusion aid removal by 
passing the unsintered product through a heated stretching zone after the 
extrusion aid removal zone and prior to the sintering zone in which the 
temperature of the product is raised above that in the extrusion aid 
removal zone such that the tensile strength of the product is lower than 
in the extrusion aid removal zone. When the extrusion aid is a volatile 
material removed by heat, the unsintered polytetrafluoroethylene product 
leaving the extrusion aid removal zone should be heated in the stretching 
zone to a temperature higher than that required for extrusion aid removal 
but less than 560.degree. F. (293.degree. C.) at which stretching becomes 
uncontrollable. Preferably temperatures of the product in the stretching 
zone should be between 250.degree. F. (121.degree. C.) and 560.degree. F. 
(293.degree. C.). 
Extrusion aid removal is accomplished in a conventional manner. While this 
conceivably might be by solvent leaching it is more practical to remove a 
volatile extrusion aid, such as naptha, by the use of heat. Heat can be 
supplied by the use of a hot air furnace or by the use of an electrical 
resistance heater such as CALROD electrical resistance heater elements or 
lamps. Whatever, the temperature of the extrudate should be raised 
sufficiently high to ensure a volatilization of most of the extrusion aid 
in the length of time that the extrudate remains in the extrusion removal 
zone. The temperatures which must be achieved are, of course, dependent 
upon the thickness of the extrudate as well as the extrusion aid employed. 
Oven temperatures of 560.degree. F. (293.degree. C.) are typical when 
removing an extrusion aid such as V.M. & P. naphtha having a boiling range 
of about 246.degree.-290.degree. F. (119.degree. C.-143.degree. C.). With 
the use of volatile extrusion aids the heat of volatilization tends to 
keep the temperature of the paste at or below the boiling temperature of 
the extrusion aid until it is almost entirely removed. 
The heat required to promote stretching can be supplied in similar fashion, 
and can be provided from the standpoint of apparatus by simple extension 
in length of the vaporizing oven used for removal of volatile extrusion 
aid. Oven temperatures of 500.degree. F. are again appropriate. 
After extrusion aid removal and stretching the low density extrudate must 
be passed through a sintering zone at which the temperature of the low 
density unsintered polytetrafluoroethylene is raised above the so-called 
sintering temperature, which normally is 327.degree. C. (620.6.degree. 
F.). Oven temperatures on the order of 700.degree. F. are ordinarily 
satisfactory to achieve sintering temperature under typical conditions. 
In drawing the product through the sintering zone it is necessary that the 
forces placed on the product to draw it should be applied in a manner that 
the product does not stretch during the gel state in the sintering zone. 
The tensile strength of polytetrafluoroethylene decreases as a function of 
temperature. At 500.degree. F. the tensile strength is approximately 
one-half that at room temperature. 
Thus, in accordance with this invention, tension is relieved on the product 
as it passes through the sintering zone by drawing the unsintered product 
after extrusion aid removal and stretching before entering the sintering 
zone at the same linear speed as the sintered product is drawn from the 
sintering zone. Thus,the only tension placed on the product in the 
sintering zone is essentially that required to keep it in stretched 
condition. 
The stretch of the polytetrafluoroethylene product which occurs is equal in 
length per unit time to the difference in linear rate of speed of the 
extrudate at the die exit and that of product passing through the 
sintering zone. If the linear rate of extrusion is 50 feet per minute and 
the drawing rate is 100 feet per minute, the stretch is 50 feet per minute 
or 100%. No loss in volume occurs as a result of extrusion aid removal, 
and no significant decrease in dimensions transverse to the direction of 
stretch occurs. It should be understood that normally in sintering, a 
polytetrafluoroethylene product which has been formed by a process 
involving paste extrusion there is an increase in density from about 1.8 
to about 2.2 with accompanying dimensional decrease. This increase still 
occurs in the process of this invention. What is to be noted is that no 
significant dimensional decrease attributable to stretch occurs. 
The process of forming low density sintered polytetrafluoroethylene 
articles in accordance with this invention is particularly adaptable to 
the formation of tubing and,in particular, to the application of low 
density sintered polytetrafluoroethylene about a wire conductor in the 
manufacture of electric cable. Conventional in-line extrusion equipment 
for application of polytetrafluoroethylene to a conductive core is 
utilized with the additional difference, however, that an opening for the 
core must be formed such that, as the extrudate and core exit the die the 
conductive core can slide within the extrudate. In conventional paste 
extrusion about a metallic wire conductor, no such opening is formed and 
the extruded material is supplied with pressure to the outer surface of 
the conductive core. The formation of the necessary opening in accordance 
with this invention is accomplished using conventional extrusion apparatus 
for extruding polytetrafluoroethylene paste about a central conductor. In 
this apparatus, a guide tube having a needle tip is employed to pass the 
conductor through the center of the preform out of contact with the paste 
until close to the point of entry of the conductor and paste into the land 
of the die. Conventionally the conductor and extruding paste are brought 
together as their speeds become approximately equal. 
In extruding and stretching polytetrafluoroethylene about a conductor in 
accordance with this invention the needle tip is adjusted in position such 
that the tip is in the land of the die. In such position an opening is 
formed in the extrudate through which the central conductor slides. (The 
central conductor travels at the final, speed of the sintered product 
which usually is greater and in excess of the extrusion speed).

DETAILED DESCRIPTION OF THE INVENTION 
Except for FIG. 5, the drawings illustrate the apparatus and cable 
illustrated and described in said applications Ser. Nos. 552,496, now U.S. 
Pat. No. 4,529,564 and 410,491 now abandoned and the apparatus and its 
operations are described hereinafter. 
Referring more specifically to FIG. 1, the reference numeral 10 indicates 
the overall extrusion apparatus utilized to extrude 
polytetrafluoroethylene about a conductive core in the formation of low 
density sintered polytetrafluoroethylene insulation in accordance with 
this invention. Apparatus 10 basically includes an extruder 12 having a 
die 14, a vaporizing oven 16, a stretching oven 17 and a sintering oven 
18. 
Extruder 12 is a conventional ram extruder for inline extrusion of 
polytetrafluoroethylene extrusion paste, hereinafter described, shaped 
into an annular cylindrical preform P. Conductor wire W is fed through the 
center of the barrel of extruder 12 in which preform P is located and out 
through die 14 located at one end of extruder 12. 
Extruder 12 is vertically positioned with die 14 at its upper end, such 
that the conductor W and extruded tubing T of extrusion paste which 
overlies conductor W are drawn upwardly through tubular vaporizing oven 16 
and tubular stretching oven 17 which are aligned vertically above extruder 
12. For practical reasons, sintering oven 18, which is also tubular, is 
positioned parallel to vaporizing oven 16 and stretching oven 17. For this 
reason, as tubing T and wire W issue from the upper end of oven 17, they 
are carried around a turnaround wheel 20 to reverse the direction of 
travel of the extruder tubing T and wire W and also to offset the path of 
travel to bring tubing T and wire W vertically downward through the center 
of sintering oven 18. 
Ovens 16, 17 and 18, which are convection ovens, are provided with internal 
electrical resistance heating units 16a, 17a, and 18a, respectively, which 
can be controlled to produce internal oven air temperatures in excess of 
700.degree. F. Typically, ovens 16 and 17 are operated at 500.degree. F., 
and oven 18 is operated at 700.degree. F. such that volatile extrusion aid 
in the extrusion paste is driven off in oven 16, and the remaining 
polytetrafluoroethylene is super heated to 250.degree.-450.degree. F. in 
oven 17 and is sintered in oven 18. 
At the lower end of sintering oven 18 the low density sintered 
polytetrafluoroethylene insulated cable C taken from oven 18 is drawn by a 
fleeter capstan 22. Capstan 22 has a drum 23 and fleeter wheel 24, drum 23 
being driven by a motor 25. Drum 23 and wheel 24 are mounted to rotate on 
parallel axes with their surfaces spaced apart slightly. Cable C is wound 
in FIG. 8 fashion in peripheral grooves in drum 23 and wheel 24 and then 
taken off to a storage reel or the like. 
Apparatus 10, as described above, except for the addition of oven 17, is 
conventionally employed in the extrusion of polytetrafluoroethylene 
coatings about wire conductors. The rate of speed at which the final 
insulated conductor C is drawn by capstan 22 is conventionally the same as 
the extrusion rate of paste in the form of tubing T, and of course, is the 
same as the speed of wire W. 
In order to adapt the conventional equipment to produce low density 
sintered polytetrafluoroethylene insulated cable in accordance with this 
invention the apparatus is modified by adding a pair of pinch rolls 30 and 
31 which are driven in counterrotation by a motor 32. Rolls 30 and 31 are 
peripherally grooved, as indicated by the reference numerals 33 and 34, 
respectively, to receive the exterior of extruded tubing T in the nip 
formed between rolls 30 and 31. Motor 32 is connected to drive rolls 30 
and 31 at the same counterrotating speeds such that the confronting 
surfaces of grooves 33 and 34 also travel at the same speed in the same 
direction in the nip between rolls 30 and 31. 
Rolls 30 and 31 are positioned adjacent the exit of die 14 between die 14 
and vaporizing oven 16 and are aligned with the path of travel of extruded 
tubing T such that tubing T is carried between grooves 33 and 34 which 
lightly contact the surface of tubing T, as can be seen in cross-sectional 
view FIG. 4, to nip and control the speed of tubing T. The pressure, 
however, mus be less than would restrain the relative movement of wire W 
and tubing T. 
In the conventional extruder 12, there is normally a long sleeve 13, known 
as a guide tube, sized to receive the conductor core to be coated, in this 
instance wire W, and to carry it through the center of preform P. Sleeve 
13 normally terminates with a needle tip 13a within die 14 short of the 
land 15 of the die. Until wire W reaches land 15, its rate of speed is 
usually greater in excess of the rate of speed of the paste from preform P 
as the latter is extruded toward die 14. Sleeve 13 functions to permit the 
relatively higher speed of wire W to be unimpeded by the slower movement 
of paste. 
In accordance with this invention, however, the sleeve 13 is also utilized 
to form an opening in the extruded paste as the latter enters land 15 thus 
forming a tubing T of the extruded paste and functioning as a mandrel, so 
to speak. As seen in FIG. 3, which is a longitudinal section through the 
die, needle tip 13a of sleeve 13 is located well within die land 15, and, 
because of its slightly greater thickness than wire W, forms a bore in 
tubing T which has a diameter greater than the outside diameter of wire W. 
In forming low density sintered polytetrafluoroethylene insulated cable in 
accordance with this invention, the peripheral rate of rolls 30 and 31 is 
the same as the linear speed of extrusion of tubing T. The peripheral 
speeds of drum 23 and wheel 24 of capstan 22, however, are at a rate 
substantially in excess such that cable C is drawn at a rate substantially 
exceeding that of extrusion. Turnaround wheel 20 is normally free to turn 
as drawn by tubing T in contact with it. The tension on tubing T is 
sufficient to bind tubing T against wire W such that no relative movement 
between tubing T and wire W at the location of wheel 20 can take place. 
Wheel 20 thus turns with a peripheral speed determined by the speed of 
wire W, and hence of capstan 22. Tubing T is thus stretched between pinch 
rolls 30 and 31 and turnaround wheel 20 by an amount percentage-wise equal 
to the percentage difference in speeds of rolls 30 and 31 and turnaround 
wheel 20. 
While in the illustrated apparatus pinch rolls 30 and 31 are used to 
control the linear speed of tubing T as it is drawn from die 14, it has 
been found that rolls 30 and 31 are not necessarily needed. In many cases 
the linear speed of tubing T exiting die 14 can also be controlled 
satisfactorily by extending needle tip 13a further into land 15 of die 14 
such that the frictional resistance between land 15 and needle tip 13a 
imposes the drag on tubing T necessary to control its linear speed of exit 
from die 14. This has been found effective with solid (single strand) 
conductor wires W, but has not worked well with stranded wires W. 
Tubing T is thus stretched while under tension imposed by the difference in 
linear speeds of tubing T between pinch rolls 30 and 31 or die 14 and 
wheel 20. This introduces porosity which is set upon sintering in oven 18. 
The degree of porosity achieved is directly proportional to the degree of 
stretch. Thus, if the cable is drawn by capstan 22 at a linear rate about 
wheel 20 equal to twice the speed of extrusion and wire W is allowed to 
feed to extruder 12 at twice the rate of extrusion, the resultant cable C, 
as depicted in FIG. 2, is insulated with polytetrafluoroethylene 
insulation F which has 50% volume of micropores. 
When the tubing T is extruded over the wire W, the stretch is determined by 
the feed speed of the relatively unstretchable wire W. If it is desired to 
make tubing T without a wire W inside thereof, the wire W and the opening 
in the needle tip 13a through which the wire W passes may be omitted. To 
provide the stretching of the tube T, a capstan, like the capstan 22 and 
comprising the drum 23a and a wheel 24a, may be substituted for the wheel 
20 as illustrated in FIG. 5. The drum 23a is driven by a motor 25a at a 
speed slightly less than the speed at which the drum 23 is driven by the 
motor 25 so that the tube T is maintained rectilinear as it travels from 
the drum 23a to the drum 23 without any significant stretching of the tube 
T as it travels between the drums 23a and 23. 
It has been found that preheating of the extruder and of the die to 
200.degree. F. (93.degree. C.) to 350.degree. F. (177.degree. C.) raises 
the temperature of the paste to above room temperature and promotes smooth 
cell structure. Preheating, however, should be limited, as temperatures of 
the paste above about 200.degree. F. results in reduction of final 
diameter and hence, lower porosity of the final product. While evaporation 
of extrusion aid serves to keep the temperature of the extrudate below the 
boiling point of the extrusion aid, such evaporation cannot take place in 
the highly pressurized environment of the extruder. 
The reduction ratio at which a polytetrafluoroethylene powder can be 
extruded depends on the manner in which it is manufactured. The extruder 
reduction ratio is determined by the following formula: 
##EQU1## 
where: Ac=the cross-sectional area of the extruder barrel which has a 
diameter substantially equal to the outer diameter of the 
polytetrafluoroethylene preform P. 
A.sub.R =the cross-sectional area occupied by the extruder guide tube 13 
(ignoring the area of any small opening therein). 
A.sub.L =the cross-sectional area of the die 14 opening. 
A.sub.o =the cross-sectional area of the bore (if any) of the article which 
area is equal to the cross-sectional area of the conductor or wire W when 
the polytetrafluoroethylene is extruded on a wire W as shown in the 
drawings. 
With the type of extrusion described in which the length of a wire which 
can be insulated is limited by the volume of the preform, it usually is 
desirable to have a high extruder reduction ratio in order to be able to 
produce long lengths of insulated wire. If a polytetrafluoroethylene 
powder rated at a low reduction ratio were to be used in an extruder with 
a high reduction ratio, the high extrusion pressures involved would damage 
the extruder. Furthermore, the polytetrafluoroethylene fibers are 
fractured causing insulation surface roughness and non-uniform dielectric 
properties along the length of the insulated wire. Accordingly, it is 
conventional practice to use polytetrafluoroethylene powders having high 
reduction ratio ratings, i.e. greater than 1000/1 and an extruder with a 
high reduction ratio, i.e., greater than 1000/1, when extruding insulation 
over a conductor or wire. After stretching and sintering, the resulting 
insulation has a high matrix tensile strength, e.g. greater than 10,000 
p.s.i. 
Contrary to the conventional practice, the process of the present invention 
uses a polytetrafluoroethylene powder of a reduction ratio rating of at 
least 1000/1, and preferably, at least 2500/1, and an extruder having a 
reduction ratio of substantially less than, e.g. preferably not more than 
one-fourth of, the reduction ratio rating of the powder and not greater 
than tetrafluoroethylene preform, and an article having a relatively low 
tensile strength is obtained. Preferably, the extruder reduction ratio is 
in the range from 200/1 to 800/1, and the matrix tensile strength of the 
resulting polytetrafluoroethylene is in the range from 2000 p.s.i. to 8000 
p.s.i. but may be as low as 900 p.s.i. and as high as 12,000 p.s.i. The 
reduction ratio ratings: of the polytetrafluoroethylene powders which are 
used in the process of the invention are always substantially higher than 
the extruder reduction ratio so that when the extruder reduction ratio is 
low the powder reduction ratio rating may be at the low end of the range 
but as the extruder reduction ratio increases, the reduction ratio rating 
of the polytetrafluoroethylene powder should be increased. Preferably, the 
extrusion pressure on the polytetrafluoroethylene mass in the extruder is 
in the range from 2000-4000 p.s.i. With lower pressures the extruded 
product does not properly hold together and with higher pressures, e.g. 
above about 6000 p.s.i., there is an undesirable increase in matrix 
tensile strength of the product and/or a breaking up of the extruded 
product. 
As pointed out in said Pat. No. 3,953,566, the tensile strength of 
conventional extruded polytetrafluoroethylene after sintering, but without 
stretching, is generally considered to be about 3000 psi. While this is a 
relatively low tensile strength, it is desirable to improve the properties 
of the extruded product, particularly for electrical reasons, to increase 
the porosity of the product by stretching it. Thus, as the product is 
stretched, the porosity increases with increases in the amount of 
stretching. In the process of the invention, the extruded product is 
stretched at a low rate prior to sintering and may be stretched in the 
range from 20% to 600%. Preferably, the stretching is in the range from 
100% to 400%, and the percentage of voids in the sintered product is at 
least 50%. As pointed out hereinbefore, the electrical characteristics of 
the sintered polytetrafluoroethylene are improved by increasing the 
porosity thereof. The signal propagation velocity of solid 
polytetrafluoroethylene is about 70% of the speed of light, and by 
increasing the porosity, in accordance with the invention, the signal 
propagation velocity in the sintered, porous polytetrafluoroethylene 
product can be as high as 93% of the speed of light. Preferably, the 
stretching amount is selected so that the signal propagation velocity is 
at least 75%, and preferably, 85-90%, in the product of the invention, and 
the density of the product is not greater than 1.9 g/cm.sup.3 and 
preferably in the range of 0.45-1.9 g/cm.sup.3. 
It has been observed that the product produced with the process of the 
invention has an increased resistance to crushing as compared to wrapped 
insulation because the insulation fibers are oriented in one direction 
rather than in a plurality of directions as is the case with wrapped tape 
constructions. 
The density, specific gravity, break strength, apparent tensile, time delay 
(Td), velocity of propagation (Vp) and impedance of the product produced 
by the process of the invention are determined in a conventional manner. 
The matrix tensile strength is calculated by multiplying the apparent 
tensile strength by the ratio of the specific gravity of solid 
polytetrafluoroethylene (2.15) to the specific gravity of the stretched 
and sintered product. 
In the art, a polytetrafluoroethylene powder is considered to be a high 
reduction ratio powder when the ratio is at least 1000/1. Examples of such 
polytetrafluoroethylene powders which are commercially available are 
TEFLON T6C sold by E.I. duPont de Nemours & Co., Inc., and having a 
reduction ratio rating of about 3000/1, FLUON CD 076, CD 509 and CD 4 sold 
by ICI United States, Inc., Wilmington, Del. and having reduction ratio 
ratings of 2500/1, 3000/1, and 3500/1 respectively and Daikin F-201 sold 
by Daikin Kogyo Co., Ltd., Osaka, Japan and having a reduction ratio 
rating of up to 4000/1. 
The industry has adopted standard tests for determining the reduction ratio 
rating of a polytetrafluoroethylene powder. One such test, which can be 
used to determine the reduction ratio rating of a polytetrafluoroethylene 
powder for the purposes of this invention, has been published by ICI 
Americas, Inc., Wilmington, Del. In this test, a billet of 
polytetrafluoroethylene is preformed in a press and then inserted in a 
Havelock hydraulic pressure extruder and extruded through a die where a 
string is obtained. The amount of pressure exerted to force the 
polytetrafluoroethylene through the die is its extrusion pressure. 
Depending upon the grade of the polytetrafluoroethylene, various die 
openinqs may be used, giving different die reduction ratios. For example, 
the die openings may be as follows for polytetrafluoroethylene powders 
made and sold by ICI Americas,Inc.: 
______________________________________ 
Grade Die Opening - diam. in. 
______________________________________ 
CD 125 0.132 
CD 126 0.078 
CD 123 0.069 
others 0.050 
______________________________________ 
The extrusion pressure at an extruder reduction ratio of 900/1, as well as 
other factors, such as appearance, physical properties, etc., determines 
the reduction ratio rating of a polytetrafluoroethylene powder, and the 
pressures at other reduction ratios may be converted to the pressure at a 
reduction ratio of 900/1 by multiplying by a known factor, e.g. the 
pressure at a reduction ratio of 470:1 will give the pressure at a 
reduction ratio of 900/1 if it is multiplied by 1.5. 
In such test, the die has an included angle of 20.degree. and the preform 
includes 16% by weight of ISO H lubricant. The ram speed is 0.8 
in./min. 
Typical reduction ratio ratings for commercially available 
polytetrafluoroethylene powders are as follows: 
______________________________________ 
.sup.1 CD- 
.sup.1 CD- 
.sup.1 CD 
Properties 
l.sup.1 CD1 
014 076 509 .sup.1 CD-4 
.sup.2 F-201 
.sup.3 T6C 
______________________________________ 
Nominal 14,000 11,000 9,000 
8,000 
7,000 7,000 -- 
extrusion 
pressure-psi 
Reduction 
500 800 2,500 
3,000 
3,500 4,000 3,000 
ratio 
rating 
______________________________________ 
Note .sup.1 grades of polytetrafluoroethylene powder sold by ICI United 
States, Inc., Wilmington, Del. 
Note .sup.2 grade of polytetrafluoroethylene powder sold by Daikin Kogyo 
Co., Ltd., Osaka, Japan 
Note .sup.3 grade of polytetrafluoroethylene powder sold by E.I. duPont 
de Nemours & Co., Inc. 
The extrusion pressure which may be used depends upon the characteristics 
of the polytetrafluoroethylene powder. If the extrusion pressure is too 
low, the physical strength of the extrudate is too low for processing 
purposes, and if the extrusion pressure is too high, the extrudate is 
irregular and breaks up. Similarly, if an attempt is made to extrude a low 
reduction ratio polytetrafluoroethylene powder with a high reduction ratio 
extruder, excessive pressures usually are required and the product usually 
will be unsatisfactory not only from the standpoint of tensile strength 
but also from the standpoint of other physical properties. However, a 
polytetrafluoroethylene powder rated at a high reduction ratio can be 
satisfactorily extruded at a reduction ratio much lower than the reduction 
ratio rating. 
The following specific examples of the process of the invention will 
further illustrate the process and the product obtained thereby. 
An apparatus 10 was set up as described with reference to FIG. 1 with the 
extruder barrel and die at temperatures approximately 100.degree. F. in a 
machine equipped to take a preform 1" in diameter. A paste of "T6C 
Teflon", having a reduction ratio rating of 3000/1 and having 17% by 
weight of V.M & P. naphtha was formed into a preform 1".times.18" in 
length and loaded into the barrel of the extruder 12. The die 14 employed 
had a land 15 inside diameter of 0.041", approximately 3/16" long. Needle 
tip 13a had an outside diameter of 0.020" and an inside diameter of 0.10". 
A 33 AWG silver plated copper conductor (D=0.0071") was employed as wire 
W. The extruder reduction ratio was about 582/1. Grooves 30 and 31 each 
had a radius of 0.020". Ovens 16 and 17 were identical, and each was a 10' 
length having 2" inside diameter. Oven 18 was made up of two such 10' 
lengths. Oven 16 and oven 17 were each heated to 500.degree. F., and oven 
18 was heated to 700.degree. F. 
Extrusion was commenced with both wire W and tubing T being extruded at the 
same speed of 40' per minute. When extrudate T was passing completely 
through the apparatus set up, rolls 30 and 31, which had not theretofore 
been touching the extrudate were brought into contact with it at the 
normal speed of 40' per minute. Capstan 22 was then brought up to 130' per 
minute, over a 20 to 30 second period. The tip 13a of needle 13 was 
adjusted in land 15 to a position at which wire W was just tight in the 
final sintered wire. As extrusion continued, the product cable C which was 
so manufactured had a polytetrafluoroethylene insulation F with 70% voids 
and an outside diameter of 0.032", as compared with 0.035" which would 
have resulted in the absence of any sintering. 
The cable C so manufactured had low density sintered 
polytetrafluoroethylene insulation F which was tightly adhered to wire W. 
Adjustment of the needle tip 13a controlled such tightness. Once the 
position of tip 13a was determined, it was not necessary to reposition it 
for additional runs under the same speed conditions. 
Other runs were also made using the same apparatus set up (but with 
different adjustment of tip 13a) at extrusion speeds of 25 and 30 feet per 
minute and wire W speeds of 80 and 100 feet per minute, respectively. 
The insulation F was found to be continuous, without breaks as in the case 
of wrapped tape, with uniform density from the center out to the surface. 
The surface of insulation F was smooth and of uniform diameter. Although 
containing 70% voids, the appearance in section was homogeneous. 
In another similar test using T6C polytetrafluoroethylene with a reduction 
ratio rating of 3000/1, an extruder reduction ratio of about 584/1 and 
extrusion and wire speeds which provided a stretch of 196%, it was found 
that the matrix tensile strength of the wire insulation was about 11,600 
p.s.i. However, by reducing the extruder reduction ratio, the matrix 
tensile strength can be reduced, e.g., to below 10,000 p.s.i. 
Although no modification was required of the conventional apparatus to 
enable control of speed of the unsintered product as it entered sintering 
oven 18, this would not necessary be the case. If wire W were not present, 
for example, in the instance of manufacturing coreless tubing, the 
embodiment illustrated in FIG. 5 may be used. Where the ovens are all 
in-line, pinch rolls like pinch rolls 30 and 31, but synchronized with 
capstan 22, can be used to prevent stretching in oven 18. 
Numerous tests of the process described hereinbefore and the following 
tables setting forth the results of some of such tests will illustrate the 
effect of reduction ratio rating on the matrix tensile strength of the 
extruded polytetrafuoroethylene: 
TABLE I 
__________________________________________________________________________ 
Reduction 
Extruder Break Wire 
ptfe Naptha 
Ratio Reduction 
Specific 
Strength size 
Test 
powder 
% Rating 
Ratio Gravity 
lbs. Type inches 
__________________________________________________________________________ 
(1) 
*F-201 
17% 4000 375 0.4681 
3.8 co-ax 0.012 
braided 
shield 
(2) 
F-201 
19% 4000 381 0.6335 
2.0 wire with 
0.0071 
single 
drain wire 
(3) 
**CD-123 
18% 300 584 0.7675 
4.9 three wire 
0.0071 
(4) 
F-201 
19% 4000 584 0.7025 
1.95 
three wire 
0.0071 
(5) 
***T6C 
.apprxeq.18% 
3000 584 -- -- three wire 
-- 
(6) 
F-201 
19% 4000 646 0.5346 
3.4 flat cable 
0.0485 
5 cond. + 
drain wire 
(7) 
F-201 
17% 4000 867 0.8203 
1.2 wire with 
0.0103 
single 
drain wire 
__________________________________________________________________________ 
*F-201 grade polytetrafluoroethylene powder sold by Daikin Kogyo Co., Ltd 
**CD123 grade polytetrafluoroethylene powder sold by ICI United States, 
Inc. 
***Previously identified 
TABLE II 
__________________________________________________________________________ 
Time 
Reduction 
Extruder Apparent 
Matrix 
Delay Imped- 
ptfe Ratio Reduction 
Stretch 
Tensile 
Tensile 
Nanosec 
Velocity 
ance 
Test 
powder 
Rating 
Ratio % psi psi per ft. 
of prop. % 
ohms 
__________________________________________________________________________ 
(1) 
F-201 
4000 375 359 933 
4,186 
1.16 87.6 95 
(2) 
F-201 
4000 381 239 1,561 
5,299 
1.20 84.7 92 
(3) 
CD-123 
300 584 180 7,102 
19,986 
-- -- -- 
(4) 
F-201 
4000 584 206 2,596 
7,649 
1.16 87.6 82 
(5) 
T6C 3000 584 196 -- 11,600 
-- -- -- 
(6) 
F-201 
4000 646 302 1,927 
7,751 
1.15 88.4 85 
(7) 
F-201 
4000 867 162 3,018 
7,726 
1.26 80.8 52 
__________________________________________________________________________ 
From an examination of the foregoing Tables I and II, it will be apparent 
that when the reduction ratio rating of the polytetrafluoroethylene powder 
is high and the extruder reduction ratio is low, matrix tensile strengths 
for the insulation below 12,000 p.s.i. can be obtained. Also, the greater 
the ratio between the reduction ratio rating of the powder to the 
reduction ratio rating of the extruder, the lower the matrix tensile 
strength will be. On the other hand, test No. (3) illustrates that with an 
extruder reduction ratio greater than the reduction ratio rating of the 
polytetrafluoroethylene powder, the matrix tensile strength of the 
insulation far exceeds 12,000 p.s.i., namely, is 19,986 p.s.i. 
In addition, the data in the Tables I and II indicate that with a high 
reduction ratio rating for a polytetrafluoroethylene powder, e.g. 4000/1, 
the extruder reduction ratio preferably is less than one-fourth the 
polytetrafluoroethylene powder reduction ratio, but the matrix tensile 
strength is also affected by the amount of stretching. Compare tests Nos. 
(1) and (2) and tests Nos. (6) and (7). Furthermore, from the data 
presented in Tables I and II, it will be noted that although the amount of 
stretching does not affect a substantial change in the signal time delay, 
it causes a significant effect with respect to the velocity of 
propagation. 
For the foregoing reasons, it is preferred that the extruder reduction 
ratio be less than one-fourth the reduction ratio rating of the 
polytetrafluoroethylene reduction ratio rating and that the amount of 
stretch be at least 150%. 
It has also been observed that the matrix tensile strength of the 
insulation can be reduced by increasing the amount of volatile extrusion 
aid, or lubricant, contained in the preform. However, to avoid other 
difficulties, the lubricant content preferably does not exceed about 24% 
by weight of the weight of the polytetrafluoroethylene. 
Although the invention has been described in connection with sintered 
products, tubing and cable insulation produced in accordance with the 
process of the invention are useful without sintering, and in such case, 
the sintering step would be omitted 
Although preferred embodiments of the present invention have been described 
and illustrated, it will be apparent to those skilled in the art that 
various modifications may be made without departing from the principles of 
the invention.