CA1311590C

CA1311590C - Apparatus and method for extruding and expanding polytetrafluoroethylene tubing and the products produced thereby

Info

Publication number: CA1311590C
Application number: CA 547153
Authority: CA
Inventors: Michael Lee Campbell; Benjamin Griffith Williams; Rob Gordon Riffle; Charles Edward Biggerstaff
Original assignee: WL Gore and Associates Inc
Current assignee: WL Gore and Associates Inc
Priority date: 1986-11-13
Filing date: 1987-09-17
Publication date: 1992-12-22
Anticipated expiration: 2009-12-22
Also published as: US4743480A; JPH0715022B2; GB2197870A; EP0267719A2; DK590087D0; AU7417687A; ATE75654T1; JPS63139926A; EP0267719A3; DK590087A; AU600730B2; GB8725612D0; EP0267719B1; DE3778831D1; AU5682990A

Abstract

ABSTRACT OF THE INVENTION
Apparatus, a method for extruding and expanding tubular products of polytetrafluoroethylene (PTFE) and the products produced thereby are provided. In the apparatus, at least one helical groove is placed in the wall of the extruded tip or die. Preferably, at least one groove is machined in the tip having a pitch angle of 135 degrees and at least one groove is machined in the die having a pitch opposite that (45 degrees) of the tip groove. In the expanded products produced according to the invention, at at least one radial position within the wall of a tube substantially all of the longitudinal axes of the nodes are oriented at an angle between about 85 degrees and about 15 degrees with respect to the longitudinal axis of the tube. In a preferred embodiment, substantially all of the longitudinal axes of the nodes located adjacent the inside wall of the tube are oriented at an angle between about 85 degrees and about 15 degrees with respect to the longitudinal axis of the tube, and substantially all of the longitudinal axes of the nodes which are located adjacent the outside wall of the tube are oriented at an angle between about 15 degrees and about 165 degrees with respect to the longitudinal axis of the tube. For the preferred product, its hoop strength is increased over an otherwise identical tube extruded using smooth surfaced, nongrooved extrusion tip and die.

Description

1 3 1 ~ 59~

BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to the extrusion of eolytetra-fluoroethylene (PTFE) tubing utilizing, in combination, a helically grooved extrusion tip and/or a helically g~ooved extrusion die. Such tubing, after removal of lubricant extcusion aid, can be expanded by stretching to produce an expanded, pocous PTF~ product having an angular orientation of the node and fibcil microstructure. Products having increased strength compared to conventionally extruded and expanded tubes can be produced. Articles made by this process and apparatus ace particulacly suitable for use in the medical field, but they also have uses in nonmedical fields.

2. Description of the Prior ~ct The products of this invention de~ive fcom paste-formed, extruded tubula~ products of PTFE. Paste extrusion or easte focming techniques are well known and consist of mixing a coagulated dispersion of PTFE resin with a liquid lubricant, pcefocming a tubular billet thereof, and foccing the mixtuce through an annular extrusion orifice in a ram-type batch extruder to form a coherent, extruded, unsinteced PTFE tube.
The lubricant is then removed, usually by a heat drying process, to form a po~ous, unsintered PTFE tube having a density usually within the range of 1.5 to 1.7 gm/cc. Such densities correspond to porosities of appcoximately 39% to 26%, _ 2 -131 15qO

cespectively. The unsinteced tube can be heated to a tempecatuce above its crystalline melt point of about 345C to sintec it theceby coalescing the pocous matecial to focm a nonpocous, sinteced PTFE tube.
U. S. Patent No. 3,008,187, discloses a method of forming PTFE tubing having improved resistance to cracking and seepage of low viscosity fluids. That method consists of extcuding a preformed, annulac billet of a mixtuce of PTFE resin and a volatile extcusion aid through an extruder bacrel pcovided with a centrally located mandrel, thcough a circulac, tapeced entcy region and thence thcough an annular ocifice to form a tube.
80th a portion of the tapeced circulac inner sucface of the tapeced entry cegion and the tapeced circular outer sueface of the mandrel within the entcy region ace pcovided with a plurality of helical gcooves. The grooves displace the fibers of tetrafluoroethylene resin formed at the outer surface layer and the inner sucface layec within the tapeced entrance cegion.
U. S. Patent No. 3,953,566 discloses and claims a process for making porous, expanded PTFE and U. S. Patent No. 4,187,390 discloses and claims products of pocous, expanded PTFE. The microstructure of those porous, uniaxially expanded PTFE
products comprises nodes intecconnected by fibcils in which the long axis of the nodes ace oriented substantially perpendicularly to the direction of uniaxial expansion.
The products of U. S. Patent No. 4,187,390 have high matrix 131 15~0 ten~ile serenqths~ specifically, above 7,300 p6i in at least one di~ection.

SUMMARY OF THE INVENTION
An extruded and expanded tube of a porous material consisting essentially of highly crystalline PTFE is provided, the tube having a longitudinal axis and tube wall having an inside radius and an outside cadius, which PTFE material has a microstcucture characterized by elongated nodes, each node having a longitudinal axis and a transverse axis, which nodes are interconnected by fine fibrils, subfitantially all of the longitudinal axes of the nodes at at least one cadial position vithin the tube wall being ociented at an angle between about 85 degrees and about 15 degrees with respect to the longitudinal axis of the tube. Such tube ~hich has not been heated to a temp~rature above its crystalline ~elt point has a crystallinity greatec than about 95~ and one which has been heated to a te~perature above the crystalline melt point of PTFE has a crystallinity below about 95%. Preferably, ~ubstantially all of the longitudinal axes of the nodes which are located adjacent the inside radius of the tube are oriented at an angle between about 85 degrees and about 15 degrees with respect to the longitudinal axis of the tube, and substantially 2~ all of ~he lonqitudinal axes of the nodes which are located `.`~

1 ~1 1 590 adjacent the outside radius of the tube ace oriented at an angle between about 15 degcees and about 165 degrees with respect to the longitudinal axis of the tube. The tube of the invention which has not been heated above its ccystalline melt eoint may have a matcix tensile strength in at least one dicection above about 7,300 psi. The tube which has been heated above its crystalline melt point may have a matrix tensile strength in at least one dicection above about 7,300 psi. The tube may be slit longitudinally theceby focming a sheet. The sheet may be fucthec colled down. The tube may be colled down without slitting theceby forming a sheet.
Pcefecably, substantially all of the longitudinal axes of the nodes at at least one cadial position within the tube wall ace ociented at an angle of about 30 degcees to about 60 degcees with cespect to the longitudinal axis of the tube and, most pcefecably, substantially all of the longitudinal axes of the nodes which ace located adjacent the inside cadius of the tube are ociented at an angle of about 30 degrees to about 50 degcee~ with respect to the longitudinal axis of the tube, and substantially all of the longitudinal axes of the nodes which ~e l~~ted adjacent the outside radius of the tube ace ociented at an angle of about 30 degcees to about 150 degcees with cespect to the longitudinal axis of said tube.
Appa~atus foc extcuding tubing of PTFE is pcovided compcising, in combination, a hollow, cylindcical baccel containing a mandrel centcally positioned within the barcel, the bacrel having a cesin supely cegion, a tapeced cegion leadinq fcom the supely cegion to an annular outflow cegion comerising an external, hollow, circular cylindrical die having positioned centcally therein the circulacly cylindcical tip of the mandcel, the tip and die forming an annulus, at least one of the tip and die having at least one groove in the sucface theeeof, the gcoove being oriented helically with respect to the longitudinal axis of the tip and die, the apparatus including a ram capable o~ reciprocation within the bacrel and sucrounding the mandcel, and means foc pcoviding the cam with cecipcocating movement wheceby, when a pceformed, tubulac billet of lubricated PTFE is placed in the baccel and the ram is activated to focce the PTFE through the annulus, tubing is provided having at least one helically oriented ridge on at least one sucface theceof. Both the tip and the die may have at least one gcoove in the surface thereof, the grooves being oriented helically with respect to the longitudinal axis of the tip and die. The tip pcefecably has at least one gcoove in the surface thereof pitched at an angle between about 15 degcees and about B5 degcees with ceseect to the longitudinal axis of the tip or at an angle between about 95 degcees and about 165 degcees with cespect to the longitudinal axis of the tip. The die erefecably has at least one gcoove in the surface theceof pitched at an angle between about 15 degrees and about 85 1 ~ 1 1 590 degrees with cespect to the longitudinal axis of the die or at an angle between about 95 deqrees and about 165 degrees with respect to the longitudinal axis of the die. The g~oove in the tip is preferably of opposite pitch to the groove in the die.
The groove in the tip most pcefecably has a pitch angle of 45 degrees or of 135 degrees. ~ gcoove in eithec the tip or the die can be focmed by milling the groove into the sucface of the tooling or by adding matecials to the sucface thereby forming ridges which would define gcooves between them, or by other means that would form grooves on the sucface of the tip and die. In one embodiment, the tip has at least one groove in the surface theraof which gradually decreases in depth longitudinally along the tip, the depth appcoaching zero at the exit end of the annulus between the tip and the die, whereby an extruded tube exiting the appacatus has a smooth-surfaced inner boce. The apparatus may include means foc applyinq pressure to the external sucface of the tubing thereby smoothing its surface and substantially eliminating any ridges therein. The means for applying pressure can be roller means applied while the tubing is held on a mandrel extending thcough its bore, or by other means such as additions to or modifications to the extruder tooling that would smooth the outer sucface of the tube.
~ process foc extcusion of PTFE tubing is also pcovided comprising extruding a pceformed billet of a mixture of coagulated dispersion of PTFE and liquid lubricant through an extruder comprising a hollow, cylindcical barcel containing a mandrel centrally positioned within the bacrel, the barrel having a cesin supply region, a tapered region leading from the supply region to an annular outflow cegion comprising an external, hollow, circular cylindrical die having positioned centrally therein the ciccularly cylindrical tip of the mandrel, the tip and die forming an annulus, at least one of the tip and die having at least one groove in the surface thereof, the groove being oriented helically with respect to the longitudinal axis of the tip and die, the apparatus including a ram capable of reciprocation within the barrel and surrounding the mandrel, forcing the PTFE through the annulus, thereby producing extruded, unsintered tubing having at least one helically oriented ridge on at least one sucface thereof.
The process may include applying pressure to the external sucface of the extcuded tubing thereby smoothing its surface and substantially eliminating any ridges therein. The ~ressure may be applied by roller means rolling over the external surface of the extcuded tubing while the tubing is held on a mandrel extending through the bore of the tubing. The pcocess may include expanding the extruded tubing, after removal of liquid lubricant, by stretching the unsintered tubing pre~erably at a rate exceeding about 10% per second and maintaining the tubing at a temperature between about 35C and the ccystalline melt point of the PTFE ducing the stcetching.
The ~ate of stcetch may exceed about 100% pec second and the tubing may be expanded such that its final length in the dicection of expansion is greater than about four times the ociginal length. The final length may be greater than about seven times the ociginal length. The stcetched tube may be heated to a tempecature above the ccystalline melting temperature of PTFE. The peocess may include the subsequent step of slitting the stcetched tubing along its length theceby ecoducing a sheet of expanded PTFE. The sheet may be furthec colled down. The tubing may be colled down without slitting pcoducing a sheet of expanded PTFE.
BRIEF DESCRIPTION OF THE DRAWINGS
Figuce 1 is a schematic cepcesentation of a poction of an extcudec foc extcuding tubes of PTFE, the extrudec having helically gcooved mandcel tip and die.
Figuce 2 is an elevational view of a helically grooved mandcel tip accocding to the invention.
Figuces 3 and 4 ace ccoss-sectional views of alternate embodiments of helically gcooved dies accocding to the invention.
Figuce 5 is a schematic plan view of a section of expanded PTFE known in the pcioc act as seen undec a miccoscope which was extruded pcioc ~o eKpanSion using smooth-sucfaced extrusion tip and die.

g ~3 1~

Figuce 6 is a schematic plan view of a section of expanded PTFE as seen under a miccoscope which was extruded pcioc to exeansion using the helically grooved extrusion ti~ and die according to the invention.
Figures 7, 8 and 9 ace photomicrogeaphs showing the node-~ibril orientation of a product according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
AND PREFERRED EMBODI~ENTS WITH
REFERENCE TO THE DRAWINGS
Apparatus and a method for extruding and ex~anding tubulac products of polytetrafluoroethylene (PTFE) are provided. The appacatus is distinct from pcioc appacatus by virtue of at least one helical groove being placed in the wall of the extruder tip oc die. Preferably, at least one groove is machined in the tip having a pitch angle of 45 degrees and at least one gcoove is machined in the die having a pitch angle opposite that of the tip groove. The expanded pcoducts produced according to the invention are distinct from pcior expanded PTFE products in that, at at least one radial position within the wall of a tube, substantially all of the longitudinal axes of the nodes ace ociented at an angle between about 85 degrees and about 15 degrees with cespect to the longitudinal axis of the tube. In a prefecced embodiment, substantially all of the longitudinal axes of the nodes located adjacent the inside wall of the tube ace ociented at an angle between about 85 degcees and about 15 degrees with cespect to 1~1 1590 the longitudinal axis o~ the tube, and substantially all of the longitudinal axes of the nodes which are located adjacent the outside wall o~ the tube are oriented at an angle between about lS degcees and about 165 degrees with respect to the longitudinal axis o~ the tube. For the preferred product, its hoop stcength is increased over an otherwise identical tube extruded using smooth-surfaced, nongrooved extrusion tip and die.
A detailed descciption of the invention is best provided with refecence to the drawings wherein Figuce 1 is a schematic repcesentation of a portion of an extruder for extruding tubes of PTFE. Paste extrusion of PTFE consists of mixing the PTFE
resin with a liquid lubricant, ecefo~ming a tubulac billet and foccing the mixture through an annular ori~ice substantially as shown in Figuce 1. Theeein, extcusion apparatus 10 CompLises a cylindrical baccel 12 containing a mandrel 16 centrally positioned within barcel 12 and having a tip section 18 as shown. The annular orifice 30 through which the tube is extruded is focmed by the tip 18 and the die 24 attached to the end o~ barrel L2. Ram 14 ~orces the PTFE/lubricant billet 11 through baccel 12 having inside wall 20, thence through the tapeced entcy cegion having inside wall 22, and thence through the annulac cegion 30 ~o~med by tip 18 and die 24. ~n Figure 1 both tip 18 and die 24 have helically oriented grooves in their respective outec and inner sur~aces, pitched in opposite dicections. Tubes extcuded thcough orifice 30 have corcesponding ~idges in theic inne~ and outec tube ~alls.
Figuee 2 is an elevational view of tip 18 having thread oeans 32 for affixing the tip ~8 to mandrel 16 shown in ~iguce 1. A helical g~oove 28, having a pitch angle of 45 degrees, is shown in Figuce 2. A plurality of such geoove6 may be employed. The g~oove 28 gradually diminishes in depth as it appcoaches the tip's exit end. A tube extruded with such a tip has a smooth-surfaced inside wall.
Figuce 3 is a ceoss-sectional view of the die 24 shown in Figuee 1. This die has a geoove 26 oeiented in the opposite di~ection as the g~oove Z8 in the tip 18 shown in Figure 2. As with the tip, multiple g~ooves may be employed in the die.
Figuee 4 is a ceoss-&ectional view of a die 24 having a gcoove 26 oeiented such that its pitch angle i6 oriented in the same di~ection as that of geooYe 28 in the tip 18 shown in Figuee 2.
Piguee 5 is a ~chematic plan view of a section of a uniaxially expanded PTFE pcoduced by the techniques disclosed in U,S. patent 3,953,566. The section is depicted as seen under a microscope processing many nodes 34 interconnected by many fine fib~ils 36. This miceostcuctuce, in which the longitudinal axes of the nodes 34 ace all substantially perpendicula~ to the longitudinal axis of the tube (the di~ection of uniaxial expansion), c~sults when tubing is extruded pcior to expansion using smooth-surfaced tip and die.
Figure 6 is a schematic plan view of a section of expanded PTFE as seen under a miccoscope which was extruded prior to expansion using the helically grooved extrusion tip and/or die according to the invention. The~ein, nodes 34 are interconnected by fibrils 36 but the longitudinal axes of substantially all nodes 34 ace ociented at an angle ~ to the direction of uniaxial expansion which is the longitudinal axis of the tube. ~epending on the pitch angle of the grooves,~
may vary between about 85 degrees and about about 15 degrees.
Figure 7 is a photomiccograph taken at lOOOX maqnification of the inside wall (lumen) of a PTFE tube extruded and expanded according to the invention. The longitudinal axes of substantially all nodes are ociented at an angle of approximately 60 degcees with respect to the longitudinal axis of the tube. The angle of pitch of the grooves in the tip used in extrusion is indicated to be 45 degrees.
Figure 8 is a photomicrograph taken at lOOOX magnification of the outside wall of the extruded and expanded tube according to the invention. The longitudinal axes of substantially all nodes a~e ociented at an angle of approximately 60 degrees ~The tube was slit and viewed flat for both inside and outside wall surfaces. The inside wall of the specimen was photographed and the specimen then was turned over and the outside wall photographed. Thus, while the nodes in Figure 8 appear to be oriented at 120 degrees, because of the specimen inversion, they are actually oriented at 60 degrees.

1~1 15qO

with respect to the longitudinal axis of the tube. The angle of pitch of the grooves in the die used in extcusion was opposite that of the grooves in the tip, i.e. the angle of pitch of the die grooves was 135 degrees with respect to the longitudinal axis of the tube. The relationship between the orientation of the nodes at the inside wall of the tubes and at the outside wall aftec expansion is not completely understood.
What is believed is that, if the longitudinal axes of the nodes at the inside wall ace oriented at an angle between about 85 degrees and about 15 degrees with cespect to the longitudinal axis of the tube, the longitudinal axes of substantially all nodes near the external surface of the tube may vary between about 15 degrees and about 165 degrees, depending upon many variables other than but including the he~ical angles of the grooves in the tip and die.
Figure 9 is a photomicrograph taken at 200x magnification of the inside wall of a PTFE tube extruded and expanded according to the invention. The longitudinal axes of substantially all nodes are oriented at an angle of approximately 60 degcees with respect to the longitudinal axis of the tube. The angle of pitch in the single groove in the die used in extrusion was 75 degrees.
~ccording to the invention, the tip and die may have moce oc less than fouc helical gcooves and the angles of the gcooves may vary. Preferably the grooves in the tip cun in a dicection that is opposite to the dicection of the gcooves in the die.
An optional step pcioc to expansion and within the scope of the invention may include the removal of the external cidges on the outside wall of the extruded tube. This can be accomplished by placing the extcudate on a mandcel, mounting the mandcel on a lathe, and at low cevolutions applying pcessuce to the outside surface of the spinning tube with a smooth cod. Othec means such as additions to or modifications to the extcudec tooling that would smooth the outec sucface of the tube may also be used. By using this optional step prioc to expansion, the cesulting tubes aftec expansion appeac as smooth on theic outec sucface as tubes exteuded w;th smooth-sucfaced tip and die, afeec expansion. The examples which follow demonstcate that, ~oc othecwise identical pcocessing conditions, the extcusion of PTFE tubes using helically gcooved tip and die ecoduces acticles which ace genecally stconger than tubes extcuded using smooth-sucfaced tip and die~ Final pcoducts using these techniques may include acticles such as film, tubes, and continuous filaments.
Expanded PTFE tubes have found use as actificial vasculac gcafts. These gcafts must have a cectain minimum hoop stcength. These gcafts ace often wcapped with an outec ceinfoccement in ocdec to achieve the desiced stcength. Tubes of the pcesent invention may have sufficient stcength foc this use without the need foc a ceinfoccement.

~31 15qO
The examples below include comparisons of strengths of tubes ~ade by conventional methods and those made according to the present invention. Test parameters included matrix tensile stcength, burst pressure, hoop stress, water entry pressùre, suture retention, ~ibcil length and wall thickness. To compute matrix tensile stcength of a porous specimen, one divides the maximum force cequired to bceak the sample by the cross-sectional area of the pocous sample, and then multiplies this quantity by the ratio of the specific gravity of the solid polymer divided by the specific gravity of the porous specimen. Test results ace ceported at 23C.
To compute burst pcessuce one increases the internal watec pcessure at a fixed cate of about 10 psi/sec. until the test specimen dilitates suf~iciently that a 10 psi decrease in intecnal pressure is detected. The final water pressure at which this phenomenon occurs is the burst pressure. The specimen may or may not actually break during the test. Test results are reported at 23C.
To compute hoop stress (a) on the internal sucface of the tube, one uses the burst pressure (p), the internal radius of the tube (a), and the external radius of the tube (b3 in the following equation: a~ = a p/b -a (1 + b /a ) (Reference: TheorY of ElasticitY, 5. P. Timoshenko and J. N.
Goodier, Engineering Society Monogcaphs, 1970, pp. 65-71.
To compute water entry pressure one subjects a test sample 131 15~0 to an inccementally increasing internal watec pressure of about 0.2 psi/min. until small beads o~ water appear on the outec surface of the tube. The water pcessure cequired to do this is the watec entry pressure.
To compute sutu~e cetPntion, for use of tubes as vascula~
grafts, one dete~mines suture holding strength in both the longitudinal and tcansverse directions. To compute sutuce cetention in the longitudinal direction, a 1200 denier filament is thceaded through both tube walls approximately lmm from the end of the samele tube. The sample is then installed in a tensile testing apearatus and tested to destruction. The maximum force required to pull the filament fcom the sample is cecorded. To compute suture holding ~etention in the transverse direction the tube is cut lonqitudinally and the 1200 denier filament is thceaded thcough one wall approximately ~mm from the cut edge. Similac tensile testing pcocedures to those described above are again applied to test dest~uction in the tcansverse direc~ion.
To compute fibril length one cuts a sample tube lengthwise and photographs the sucface under sufficient magnification.
Su~ficiQnt ~ibcil lengths ace measuced f~om the sample to compute a statistically significant mean fibcil length.
fib~il length is measured from the edge of one node to the edge of an adjacent node. Fibril lengths less than five micLons ace not recorded because of indistinct nodal separation.

To compute wall thickness one subtraces the inner diameter measurement from the outer diametec measurement and divides by two.
The ~ollowing examples which disclose processes and products according to the present invention are illustcative only and are not intended to limit the scope of the present invention in any way.

Expansion Of Tube:
~ie With One Helical Groove PTFE resin (Fluon CD-123 obtained fcom ICI Americas) was blended with 121cc of "Isopar M" odorless solvent (produced by Exxon Corporation) per pound of PTFE, compressed into a tubular billet, heated to 60C and extruded into a 6mm I. D. and 7.7mm O. D. tube in a ram extruder having a reduction ratio of about 123:1 in cross-sectional area from billet to the extruded tube. The extruder tooling consisted of a smooth surfaced tip and a die having a single helical gcoove machined into it a pitch angle of 75 with respect to the longitudinal axis of the die with a right hand twist. The depth of the groove ~as 0.51mm. After removal of lubricant, the extruded tube was used to produce a cylindrical tube of porous, expanded, amorphously locked PTFE having a 6mm internal diameter and outside diameter of 7.7mm by the process described in U. S. Patent No.

3,953,566. The stretch conditions were:
~Trade mark 1:~1 15~0 Temperatuce: appcoximately 300C
Stretch Ratio: 7.4:1 (640% increase in length) Stretching Rate: approximately 105%/sec. (detecmined by dividing the percent change in length by the ducation of the stcetching operation) The stretched tube was then cestcained fcom shrinking and heated in an oven set at appcoximately 390C for a total of about 7 minutes thereby sintering the tube. This heating step can be perfocmed in a single operation or in multiple stages.
Figure 9 i8 a photomicrograph of the inner surface of this tube. The longitudinal axes of the nodes are angled at approximately 60 degrees with respect to the direction of uniaxial expansion.
EXAMPL~ 2 Expansion Of Tube:
Tip And Die With Four Helical Grooves Used Separately ~nd In Combination PTFE resin (Fluon CD-123 obtained from ICI ~mericas) was blended with 121cc of "Isopar M" odorless solvent ~produced by Exxon Cocpocation) per pound of PTFE, compressed into a tubu]ar billet, heated to 40C and extruded into a 6mm I. D. tube in a ram extruder having a reduction catio of about 123:1 in ccoss-sectional area from billet to the extruded tube. Five combinations of extrudec tooling were used, chosen from two different dies and thcee different tips:
1. a smooth sucfaced tip and smooth sucfaced die, 2. a s~ooth sucfaced tip and a die havinq fouc helical geooves machined into it at a pitch angle of 45 - 19 ~

1 3 1 1 5qO

degcees (a cight hand twis~), 3. a tip with four helical grooves machined into it at an angle of 135 degrees (a left hand twist) and a smooth surfaced die, 4. a tip as in #3 and a die as in #2, that is, opposite twist, 5. a tip with fou~ helical gcooves machined into it at an angle of 45 degrees (a right hand twise~ and a die as in #2, that is, the grooves are eitched in the same direction.
The continuous depth of grooves in the die was 0.51mm. The gcooving in the tips gradually tapered from this depth at the upstream end to zero depth at the exit end of the tlp. This produced tubes with smooth inside walls. The external surface of tubes made with grooved dies had ridges.
The extruded tubing from the various tooling combinations wece then processed into cylindrical tubes of porous, expanded, amorphously locked PTFE having an approximate internal diametee of 6mm by the process described in U. S. Patent No. 3,953,566.
The stretch conditions were:
Tempecature: approximately 300C
5tretch Ratio: 5:1 (400% increase in length) Stcetching Rate: approximately 400%/sec. (detecmined by dividing the peLcent change in length by the duration of the stcetching operation).
The stcetched tubes were then restcained from shrinking and heated in an oven set at approximately 390C for a total of 1 ~1 1 590 about seven minutes thereby sinte~ing the tube. This heating step can be pe~formed in a single operation o~ in multiple stages.
A comparison was made between ce~tain physical prope~ties of tubes made with the various combinations of tooling mentioned above. The eesults are shown in Table 1. These are mean data calculated from five tube samples from each type of extrusion tooling employed.

X ~ ~ q ~
~l ~c ~
-- ~ ~ ~ ~
~C tn cL ~ ~ _ -- o =
~ ~ - - - o -- v~
g~ o~
~l ~c . c J - ~ u~ o r~
E V

Vl ~, L _ ~ ~ G _ ,~ ~ X
._ ~ ~ O,~ -- L
~ C~ ~ ~ ~~ ~ --~
.
O
~ ul C `D -- 11~ ~ r~ c ~
a~ $ -- ~ u~ Ll~ O
~ . C

-1 o - o o ~ o 3 i~ .o~^ ~ o ~ 2 ~ ' ' ~ ~ ~ _ ~_ ~11 c 3 `--i ~

~ 1` ~ E
3 ~ _ O o O o O ~ ~
._ ~- o IL ~I
_ L I ~ O

~ ~-1 ` ` ` ` ` ~3-._ c ~ 111 CL~ 3 ~ O ~1 g o`- i~ 1-- - O O ~ O
a~, O ~ o ,.~o - O ~, ._ f Q 8 8 OE f CL I ~ c IX g ~ ~ L tD ~ 3 -o U~
U~

Expansion Of Tube:
Tip And Die With Fouc Helical Grooves PTFE cesin*(Fluon CD-123 obtained fcom ICI Amecicas) was blended with 120cc of "Isopar M" odocless solvent (pcoduced by Exxon Corpocation) pec pound of PTFE, compressed into a tubular billet, heated to 60C and extruded into a 6mm I. D. tube in a ram extruder having a reduction ratio of about 123:1 in cross-sectional area ~com billet to the extruded tube. The extruder tooling consisted of a tip with four helical grooves machined into it at a pitch angle of 45 degcees with cespect to the longitudinal axis of the tip with a le~t hand twist and a die with foue helical gcooves machined into it at an angle of 45 degcees with cespect to the longitudinal axis of the die with a right hand twist. The depth of gcooves in the die is a continuous 0.51mm. The grooves in the tip ace this same depth at the upstream end and then taper gcadually to zeco depth at the exit end of the tip. This pcoduced an extruded tube with ridges on the outer sur~ace and a smooth innec surface.
This extruded tubing was then processed into a cylindrical tube of porous, expanded PTFE by the process described in U. S.
Patent No. 3,953,566. The stcetch conditions were:
Tempecatuce: approximately 300C
Stretch Ratio: 5:1 (400~ increase in length) Stcetching Rate: approximately 400~/sec. (determined by dividing the percent change in length by the duration of the stretching opecation).

Trade mark - 23 -,~, 1~ 159~

The stretched tube was not amorphously locked, i.e., the PTFE was in an unsintered state.
Four such tubes with a mean inner diameter of 4.2mm and a mean outer diameter of 5.5mm were tested to determine longitudinal matrix tensile strength. The mean value was 13852 psi. The testing was done with whole tubes on an*Instron Model 1122 pull tester. The following settings were used: 1) cros&head speed of 200mm/min.: and 2) initial distance between the jaws of 150mm. Matrix tensile strenqth values a~e at 23C.

Expansion Of Tube: Tip ~nd Die With Four Helical Grooves And Subsequent Smoothing Of External Surfaces Of Tubes A porous, expanded, amorphously locked PTFE tube was made from PTFE resin similar to that disclosed in U.S. Patent Nos.
4,576,869: 4,016,345: and R31,341. The method employed is as described in Example 2, except that 135cc of ~Isopar M~ was blended with each pound of cesin. The extruder tooling had grooves pitched in opposite directions, 45 degrees on the die and 135 degcees on the tip. After extrusion and prior to expansion, pressure was applied to the external surface of the tube by placing the tube on a mandrel and using roller means to smooth the ridges on the external surface of the tubes.
Another tube was made in the same manner but no smoothing of the external ridges was performed.
A third tube was similarly produced using smooth-walled tip and die. Certain physical properties of these tubes are summarized in Table 2.
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~urst Water Entry Suture Retention Fibril Wall Pressure Pressure Long. Transverse Length Thickness Tube (psi) ~psi) ta) (g) (microns) ~mm) 5moothed External Surfaoe 74 7.1 2069 1243 16 .076 Ridged External Surface ~No Smoothing) 70 6.8 1678 1161 13 .071 Smcoth-Walled Tip 1 0 And Die 46 5.0 1009 1084 22 .081 Expansion Of Tube: Tip And Die With Four Helical Grooves A pocous, expanded, sintered PTF~ tube was made from PTFE
resin similar to that described for examples in U. S. Patent Nos. 4,576,869 4,016,345: and R31,341. The method employed was similac to the method described above in Example 4 except for the following conditions:
Drying of Extruded Tubing: approximately 295C for 24 hours Stretch Ratio: 11.2:1 (1020% increase in length) Stretching Rate: appcoximately 47%~sec. (determined by dividing the peccent change in length by the duration of the stretching operation) The extruder tooling were of the same configuration as stated in Example 4.
Certain physical properties of this tube are summa~ized in Table 3. These are mean data calculated from several tubes.

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1~1 1590 While the invention has been disclosed hscein in connection with certain embodiments and detailed descciptions, it will be cleac to one skilled in the art that modifications or variations of such details can be made without deviating f~om the gist of this invention, and such modifications or va~iations ace consideced to be within the scope of the claims hereinbelow.

- ~7 -

Claims

WE CLAIM:

1. An extruded and expanded tube of a porous material consisting essentially of polytetrafluoroethylene (PTFE) said tube having a longitudinal axis and tube wall having an inside radius and an outside radius, which PTFE material has a microstructure characterized by elongated nodes, each node having a longitudinal axis and a transverse axis, which nodes ace interconnected by fine fibrils, substantially all of said longitudinal axes of said nodes at at least one radial position within said tube wall being oriented at an angle between about 85 degrees and about 15 degrees with respect to said longitudinal axis of said tube.

2. The tube of Claim 1 which has not been heated to a temperature above its crystalline melt point and which material has a crystallinity greater than about 95%.

3. The tube of Claim 1 wherein substantially all of said longitudinal axes of said nodes which are located adjacent said inside radius of said tube are oriented at an angle between about 85 degrees and about 15 degrees with respect to said longitudinal axis of said tube, and substantially all of said longitudinal axes of said nodes which are located adjacent said outside radius of said tube are oriented at an angle between about 15 degrees and about 165 degrees with respect to said longitudinal axis of said tube.

4. The tube of Claim 2 wherein said material has a matrix tensile strength in at least one direction above about 7,300 psi.

5. The tube of Claim 1 wherein said tube wall is slit longitudinally thereby forming a sheet.

6. The tube of Claim 1 wherein said tube is rolled down without slitting thereby forming a sheet.

7. The tube of Claim 1 wherein substantially all of said longitudinal axes of said nodes at at least one radial position within said tube wall are oriented at an angle of about 30 degrees to about 60 degrees with respect to said longitudinal axis of said tube.

8. The tube of Claim 3 wherein substantially all of said longitudinal axes of said nodes which are located adjacent said inside radius of said tube are oriented at an angle of about 30 degrees to about 60 degrees with respect to said longitudinal axis of said tube, and substantially all of said longitudinal axes of said nodes which are located adjacent said outside radius of said tube are oriented at an angle of about 30 degrees to about 150 degrees with respect to said longitudinal axis of said tube.

9. An extruded and expanded tube of a porous material consisting essentially of polytetrafluoroethylene (PTFE) that has been heated to a temperature above the crystalline melt point, said tube having a longitudinal axis and tube wall having an inside radius and an outside radius, which PTFE
material has a microstructure characterized by elongated nodes, each node having a longitudinal axis and a transverse axis, which nodes ace interconnected by fine fibrils, substantially all of said longitudinal axes of said nodes at at least one radial position within said tube wall being oriented at an angle between about 85 degrees and about 15 degrees with respect to said longitudinal axis of said tube.

10. The tube of Claim 9 wherein substantially all of said longitudinal axes of said nodes which are located adjacent said inside radius of said tube ace oriented at an angle between about 85 degrees and about 15 degrees with respect to said longitudinal axis of said tube, and substantially all of said longitudinal axes of said nodes which are located adjacent said outside radius of said tube are oriented at an angle between about 15 degrees and about 165 degrees with respect to said longitudinal axis of said tube.

11. The tube of Claim 9 wherein said material has a matrix tensile strength in at least one direction above 7300 psi.

12. The tube of Claim 9 wherein said material has a matrix tensile strength in at least one direction above about 9,290 psi.

13. The tube of Claim 9 wherein said tube wall is slit longitudinally thereby forming a sheet.

14. The tube of Claim 9 wherein said tube is rolled down without slitting thereby forming a sheet.

15. The tube of Claim 9 wherein substantially all of said longitudinal axes of said nodes at at least one radial position within said tube wall are oriented at an angle of about 30 degrees to about 60 degrees with respect to said longitudinal axis of said tube.

16. The tube of Claim 9 wherein substantially all of said longitudinal axes of said nodes which are located adjacent said inside radius of said tube are oriented at an angle of about 30 degrees to about 60 degrees with respect to said longitudinal axis of said tube. and substantially all of said longitudinal axes of said nodes which are located adjacent said outside radius of said tube are oriented at an angle of about 30 degrees to about 150 degrees with respect to said longitudinal axis of said tube.

17. A process for extrusion of PTFE tubing comprising extruding a preformed billet of a mixture of coagulated dispersion of PTFE and liquid lubricant through an extruder comprising a hollow, cylindrical barrel containing a mandrel centrally positioned within said barrel, said barrel having a resin supply region, a tapered region leading from said supply region to an annular outflow region comprising an external, hollow, circular cylindrical die having positioned centrally therein the circularly cylindrical tip of said mandrel, the tip and die forming an annulus, at least one of said tip and die having at least one groove in the surface thereof, said groove being oriented helically with respect to the longitudinal axis of said tip and die, the apparatus including a ram capable of reciprocation within said barrel and surrounding said mandrel, forcing said PTFE through said annulus, thereby producing extruded, unsintered tubing having at least one helically oriented ridge on at least one surface thereof.

18. The process of Claim 17 wherein said die has at least one groove therein and including the step of smoothing the external surface of said extruded tubing thereby smoothing its surface and substantially eliminating any ridges thereon.

19. The process of Claim 18 wherein said smoothing is applied by roller means rolling over the external surface of said extruded tubing while said tubing is held on a mandrel extending through the bore of said tubing.

20. The process of Claim 17 including removing said lubricant from said extruded tubing.

21. The process of Claim 20 wherein said lubricant is removed by heating said tubing to volitalize said lubricant.

22. The process of Claim 21 including expanding said extruded tubing, after removal of liquid lubricant, by stretching said unsintered tubing longitudinally and maintaining said tubing at a temperature between about 35°C and the crystalline melt point of said PTFE during said stretching.

23. The process of Claim 22 in which the rate of stretch exceeds about 10% per second.

24. The process of Claim 22 in which the rate of stretch exceeds about 100% per second.

25. The process of Claim 22 in which the tubing is expanded such that its final length in the direction of expansion is greater than about four times its original length.

26. The process of Claim 25 in which said final length is greater than about seven times the original length.

27. The process of Claim 22 which includes the subsequent step of heating the stretched tubing to a temperature above the crystalline melting temperature of PTFE.

28. The process of Claim 17 including the subsequent step of slitting said tubing along its length thereby producing a sheet of PTFE.

29. The process of Claim 17 including the subsequent step of rolling down the tubing without slitting thereby producing a sheet of PTFE.

30. The process of Claim 29 wherein said sheet is further rolled down.

31. The process of Claim 22 including the subsequent step of slitting said stretched tubing along its length thereby producing a sheet of expanded PTFE.

32. The process of Claim 22 including the subsequent step of rolling down the tubing without slitting thereby producing a sheet of expanded PTFE.

33. The tube of Claim 1 having a hoop stress exceeding 250 psi.

34. The tube of Claim 33 having a hoop stress exceeding 450 psi.

35. The tube of Claim 33 in use as an artificial vascular graft.

36. The tube of Claim 9 having a hoop stress exceeding 250 psi.

37. The tube of Claim 36 having a hoop stress exceeding 450 psi.

38. The tube of Claim 36 in use as an artificial vascular graft.