CA1165080A - Composite polytetrafluoroethylene article and a process for making the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A porous polytetrafluoroethylene (PTFE) article which is made up of a number of smaller articles with a microstructure of nodes interconnected by fibrils, these articles having been joined to one another such that their microstructure is virtually unaltered across the join. A process for producing such a PTFE article by closely abutting small-shaped PTFE segments and applying force perpendicular to the seam while heating to a temperature above the crystalline melt point of the segments.

Description

./ 11 1.
1¦¦ FIELD OF TIIE INVEMTIO~I

3 I This invention relates to a process for making a novet composite 4 Iarticle of expanded polytetrafluoroethylene 7 BACKGROUN~ ~F THE INVENTIO~

9 Polytetrafluoroethylene (hereinafter "PTFE"~ has excellent heat resistance, chemical resistance, insulation resistance, non-adhesiveness and 11 self-lubrication. This polymer has found wide use in medical, industrial and 12 recreational fields.
13 A recent invention (U. S. Patent 3,953,566) provides a process for 14 manufacturing highly porous, yet high strength, shaped, PTFE articles. This process involves blending highly crystalline, fine powder PTFE with a liquid 1~ lubricant, extruding this mixture through a die which may have desired 17 crosssectional configuratlon, and subsequently expanding the shaped article in 18 one or more di-rections at rates in excess o~ 10~ per second.
19 Products produced by thls process have found widespread acceptance in 20 the industrial, medical, electrical, and clothing arts. The process is some-21 what limited in that it is not readily adaptable to the production of large 22 articles with complex cross-sections. A need for such articles is found, for 23 example, in the industrial filtration arts and in large vessel vascular 24 surgery. Although large composite articles can be manufactured by joining 25 smaller articles together by such conventional methods as sewing, ~Jelding or ~6 ;gluing~ such articles have a discontinuity at the seam. IJhile in many applica- !
27 'tions this does not present any severe prohlems, in others such as filtration 28 land body part replacement, it is extreniely important that the s-tructure be as 3~ 2-I '' ;5~8~

uniform as possible over the entire article. If welding or gluing is used to produce large articles, a dense non-porous area is produced. On the other hand, sewing may produce areas which have a greater porosity than the rest of the article. It has been found that the microstructure of nodes and fibrils present in products produced by U.S. Patent 3,~53,566 is particularly desir-able as both a filter media and as surface for contacting blood and other body fluids. It would, therefore, be desirable to produce composite, complex shapes by joining articles of expanded PTFE wlth this microstructure in such a manner that the microstructure remains virtually uninterrupted across the join or seam.
BRIEF DESCRIPTION OF THE INVENTION
An objective of the present invention is a process for producing a composite, shaped, PT~E article. Such an article is produced by joining segments of smaller-shaped articles. These smaller segments are held close to-gether and their temperature raised to a temperature above the crystalline melt point of the segments. They are then allowed to cool to room temperature to facilitate handling of the article. A further objective of this invention is the production of a composite, shaped, PTFE article by the above process, with a virtually uninterrupted microstructure of nodes interconnected by fibrils across the join.
Thus, in accordance with the present teachings, a process is provided for joining a plurality of shrinkable expanded porous polytetra-fluoroethylene segments which have a microstructure of nodes interconnected by fibrils such that the microstructure is virtually uninterrupted at the seam joining the segments. The process comprises the steps of arranging the segments such that their edges are disposed in abutting relationship, causing a pressure to be applied perpendicular to the abuttlng edges, causing the segments to be restrained from shrinhing in any direction, heating the segments while they are so held to a temperature above the crystalline melt point of polytetrafluoroethylene for a predetermined t~le and allowlng the segments to cool while still being held restrained under ~` ~'`' ,5(~8~

pressure.
In accordance with a further embodiment of the present concept, a method is provided for manufacturing a porous composite tube from a plurality of segments of shrinkable expanded porous polytetrafluoroethylene which have a microstructure of nodes interconnected by fibrils such that the microstructure is virtually uninterrupted at the seam joining the segments.
A tube is formed of expanded porous polytetrafluoroethylene segments by disposing the segments around a suitable mandrel in abutting relationship, winding a film of high strength shrinkable expanded porous polytetrafluoro-ethylene with a microstructure of nodes interconnected by fibrils around thesegments, restraining the segments from shrinking longitudinally, bonding and shrinking the film to the segment thereby applying pressure perpendicular to the edge of the segments by heating to a temperature above the crystalline point of polytetrafluoroethylene for a predetermined time and allowing the the wrapped bonded segments to cool.
BRIEF DESCRIPTION OF THE DR~WINGS
Fig. 1 is a schematic illustration of one of the embodiments of the present invention, -3a-L

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1 Fig. 2 - 4(b) are electromicroscop;c photographs of various surfaces

2 1f a tube produced in accordance with the invention, DETAILED DESCRIPTION OF THE INVENTION
6 _ 7 Bonding of PTFE to PTFE by mechanically holdin~ the two parts in con-8 tact and heating them above the crystalline melt point of PTFE is known in the 10 ¦art. Generally, however, this has resulted in a solid non-porous seam Thepresent invention utilizes a modiFication of the process to produce a product 11 in which the node-fibril microstructure present in both parts is maintained 12 virtually uninterrupted across the seam. The result is that a seam-free product is produced. In the present context, the term "edges" is used to refer 14 to that portion of the expanded PTFE article which is to be bonded together and 16 "seam" refers to the area so bonded. Articles refer to any shaped cross-section, e.g., tube, rod, sheet or segment.
17 PTFE material is available in a variety of shapes, including sheets, 18 rods and tubes from W. L. Gore & Associates, Inc. The articles to be bonded are cut to the required size. Care must be taken to ensure that the edges to 221 ¦be joined are clean, that is neither ragged or dirty. The two edges are then 22 ¦placed in close proximity, i.e., touching one another.
23 If expanded PTFE is heated above its crystalline melt point while Iunrestrained, the material will tend to shrink and coalesce into a solid mass.
24 ¦In order, therefore, to ensure that the t~o articles to be bonded together 226 ¦remain in contact while being heated, mechanical means must be employed to so 27 Ihold them.
28 ~ For example, as illustrated in Fig. 1, a lar~e tube can be made from Isheets or sections from a number of smaller tubes. The sheets 4 are trimmed a-t 32~I -4-. 1.

¦ g ~ ~;r~ 3 ~ their edges 6(a) and 6(b) to ensure that these edges are clean, that is, not 21ll ragged or dirty. The sheets ~ are then laid around a mandrel 2. Th2 edges311l6(a) and 6(b) of each sheet 4 are closely butted to the edges 6(a) and 6(b) of 4llthe adjoining sheet. The ends of the sheets, A and B, are fixed at these 51lpoints to the mandrel. This can be achieved in a variety of ways, such as hose¦
6¦1~clamps or tying the sheets to the mandrel by wire. The reason for so fixing71 the tube, is to prevent longitudinal retraction of the PTFE on heating.
8 A strip of expanded PTFE film about .75 inches wide and having a 9 longitudional Matrix Tensile Strength of about 70,000 p.s.i. is spirally wound 10 ¦around the sheets on the mandrel and fastened at the end of the mandrel so it 11 Icannot unwrap. Upon heating, this restraining Film shrinks, applying pressure 12 lon the sheets and keeping the edges 6(a) and 6(b) in close contact. A
13 ¦satisfactory film is commercially available as GORE-TEX expanded filament from 14 ¦W. L. Gore ~ ~ssociates, Inc., P. O. Box 1220, Elkton, Maryland 291~1.
15 ¦Although wrapping with an expanded PT~E fi'lm is a preferred means of 16 Imechanically restraining and holding the edges of the sheets in contact during 17 heating, other means may be used. -18 The important factor is that there must be some force perpendicular19 to the seam during sintering. When the film ~rapped around the tube retracts, it supplies the'necessary force.
21 The heating of the wrapped tube can be achieved in a salt bath, an22 air oven, a radiant oven or other heating means. A suitable salt bath can be a 23 molten mixture of sodium nitrites and nitrates and is main~ained at a tempera-24 ture above the crystalline melt point of the s29ments. The tube is then 25 removed and allowed to cool ~hile still being held restrained. The time above 26 the crystalline melt point will vary depending on the mass of material involved 27 and resin properties. The exact time to produce an optimum bond will depend on 28 a number of factors such as mass of material and the configuration of the shape 29 being produced. Such a time, ho~ever, is easily deterlllined with a minimurn of 30 experimentation. The following example is intended to illustrate and not limit 31 the present invention. The technique can be used in any of a variety of sh2pes ~2 ;and sizes ~Jhere it is important to maintain a virtual'ly uninterrupted ~_ ~ 5 Il . . . . . ..

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1,lmicrostruc~ure across any joining line or seam.

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3,,EXAM~LE

4¦1 Three 6.5cm long, 120 segments ~ere cut from 2nmm inside diameter

5~¦tubes ~Ihich had been produced according to the teachings of U. S~ Patent 61¦3,953,566. The resin used was Fluon 123 which is a fine po~der, PTFE resin 7 commercially available from ICI America. These segments were carfully tri~med 81 to ensure that the edges to be butted ~lere clean. The segments ~leré then care- ¦
9 l Ifully laid around a smooth, 20mm O.D.g stainless steel tube. The segments ~Jere lO larranged so that they butted closely together. They were then spirally wrapped 11 Illwith a .75 inch wide expanded PTFE film having a Matrix Tensile Strength of 12 labout 70,000 p.s.i. which was manufactured according to U. S. Patent 13 3~962,153.
14 ¦ The mandrel was then placed in an air oven at 380C for 12 minutes.
15 luPon removal from the air oven, the mandrel was allowed to cool to room tem-161!perature and the formed PTFE tube was carefully slid off the mandrel. For the 17~lpurpose of this experiment, the wrapping film was carefully removed frcm the 18 tube in order to photomicrograph the structure at the seam.
19 Fig. 2 is an angled electromicroscopic photograph of one of the seams 20 made in Example I. The top portion 10, is a topographical view of the inside 21 surface of the tube. The bottom portion 12, is a cross-section view of the 22 tube~ In Fig. 2, the seam runs from X tb Y. The magnification is 1~6 times.
23 Fig. 3(a) is an electromicroscopic photograph of the inside surface 2 of the tube made in Example 1. The seam runs from X to Y. The magnification 2 lis 122 timesO Fig. 3(b) is an electro~icroscopic photograph of the inserted 2 larea in Fig. 3(a). The seam runs from X to Y and the magnification is 610 271ltimes.
231j 32 l I
.. l i 1 Fig. 4(a) is an electromicroscopic photo~raph of the outside surfac2 after removal oF the film, of the tube made in Example I. The seam rUns from X
3 to Y and the ma~nification is 90 times. Fi9. 4(h) is an electromicroscopic 4 photograph of the inserted area sho~n in Fig. 4(a). The seam runs from X to Y5 land tile ma9nification is ~50 times,

6 I ~rom these electromicroscopic photographs, it is surprising to oh-

7 jserve that the node-fibril microstruCture is virtually uninterrupted across the

8 Iseam. Althou9h a small scale example ~as used to illustrate thiS inYention~
Ithis technique can be readily extended to cover large tubes up to several 10 inches in diameter. Equally~ the technique can be used with any of a variety11 of shapes and is not limited to tuhular cross-sections. For example, sheets of 12 uniaxially expanded PTFE ~llth thicknesses ranging from about .nn5" to more than 13 .l~0" could be joined to9ether in the follo~ing manner. Restrain the PTFE
14 sheets in the direction of their expansion, butt them in place between 1/16"
15¦llthjCk sheets of 60 durometer silicon rubber (tWo sheets of rubber on each side 16 IOT the PTFE that are not quite butted together with a gap of about .010"
17 !between them ~hich corresponds to the sea~ in the PTFE sheets), and place the 1~ PTFE and rubber sheets in a press ~Jith platens heated to about 380C. The 19 press could be closed to apply a very small pressure to the sheets sittin9 on20 the rubber. This would supply tlle necessary perpendicular force to the seam, 21 this time in a planar confi9uration. After an appropriate time, approximately 22 15 minutes, the electrical heaters on the press could be turned off and the 23 platens cooled by a stream of compressed air. When tile platens had cooled to 24 room temperature, the pressure could be released and the sheets removed.

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Claims (4)

CLAIMS:

1. A process for joining a plurality of shrinkable expanded porous polytetrafluoroethylene segments each having a microstructure of nodes interconnected by fibrils such that the microstructure is virtually uninterrupted at the seam joining said segments comprising the steps of:
(a) arranging said segments such that their edges are disposed in abutting relationship;
(b) causing a pressure to be applied perpendicular to said abutting edges;
(c) causing said segments to be restrained from shrinking in any direction;
(d) heating said segments while they are so held to a temperature above the crystalline melt point of polytetrafluoroethylene for predetermined time, and (e) allowing said segments to cool while still being held restrained and under pressure.

2. A composite article made in accordance with claim 1.

3. A method for manufacturing a porous composite tube from a plurality of segments of shrinkable expanded porous polytetrafluoroethylene each having a microstructure of nodes interconnected by fibrils such that the microstructure is virtually uninterrupted at the seam joining said segments comprising the steps of:
(a) forming a tube of expanded porous polytetrafluoroethylene segments by disposing said segments around a suitable mandrel in abutting relationship;
(b) winding a film of high strength shinkable expanded porous polytetrafluoroethylene with a microstructure of nodes interconnected by fibrils around said segments;
(c) restraining said segments from shrinking longitudinally;
(d) bonding and shrinking said film to said segment thereby applying pressure perpendicular to the edge of the segment by heating to a temperature above the crystalline melt point of polytetrafluoroethylene for a predetermined time; and (e) allowing said wrapped bonded segment to cool.

4. A composite article made in accordance with claim 3. 9