CA1316312C

CA1316312C - Fluoropolymer working

Info

Publication number: CA1316312C
Application number: CA000530184A
Authority: CA
Inventors: Heinz Eilentropp; Kurt Aeschbach; Hans Reinmann
Original assignee: Lara Reinmann & Cie
Current assignee: Lara Reinmann & Cie
Priority date: 1986-02-21
Filing date: 1987-02-20
Publication date: 1993-04-20
Anticipated expiration: 2010-04-20
Also published as: BE1001311A3; CH672094A5; GB2187460A; AU6910087A; GB2187460B; CN87100900A; SE8700684L; FR2594744B1; GB8704045D0; AU6659690A; FR2594744A1; IT8719195A0; NL8700379A; JPS62205136A; IT1202444B; BR8700796A; SE8700684D0

Abstract

FLUOROPOLYMER WORKING
ABSTRACT OF THE DISCLOSURE
A method of making porous, elongated objects such as ribbon, foil, solid or hollow sections, from a fluoropolymer powder or from a polymer blend which includes such fluoropolymer and a lubricant, is compressed or compacted, and paste extruded, is continued by passing the extruded object continuously through a furnace for drying the same, thereby removing the lubricant; and concurrently sintering the product at an elevated temperature above the crystallite melting temperature (342 degrees centigrade) of the fluoropolymer, while simultaneously and concurrently stretching the product longitudinally, laterally, or both, preferably in the range of 100 to 1000% stretch rate.

Description

6~506-219 BACKGROUND OF THE INVENTION
The present invention relates to the manufackure of porous elonga~ed sections, components, and other work pieces, particularly, however, ribbons, strips, hollow or solid profiles using fluoropolymers, such as polytetrafluoroethylene which is usually used as a powder and is not amenable to working in the molten state, but which material is to be used ei~her by itself or in a blend with a lubrican~. The blend is compressed or compacted and shaped into the desired shape. This procedure is known as paste extrusion. Subsequently, the lubricant is removed, the shape is stretched, and the polymer is sintered The invention relates specifically to the post extruslon processing.
A method of the kind to which the invention pertains particularly for the manufacture of porous tetrafluoropolymer, is, for example, disclosed in German Patent 2,417,90~. Herein, it is particularly known to make such a product through paste extrusion, and following the removal of the lubricant, it is stretched a~ a ra~e of more than 2,000%. The part or element is particularly stretched in a longitudinal directlon by more than the 50-fold value in length of the unstretched object. Stretching iB normally conducted at a temperature of 35 degrees centigrade (C~ and above, but not more than 327 degrees centigrade. The goal is to inarease drastlcally the tensile strength while reducing its density. In order to obtain this particular objective, the rate of stretching in combinatlon with a particular temperature range, is deemed critiaal.
Aside f rom the point that the known method poses : ", ,, -;

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: ~ , 1 3 1 6 -~ 1 2 problems of manufacture, particularly on account of the high rate of stretching it was ~ound that produc~s made in ac~ordance with this method are limited as to ~heir use. For example, ribbons or foils which were s~retched by more than 2,000~, are not suitable, for example, to be used in a layer of flexible electrical cables or conduc~ors, particularly their use is limited as wrapping for purposes of layer separation or to obtain internal sliding of layers above each other. Ribbons and foils made for that purpose are intimately adapted to overlying contours or to substrates underneath in order to follow any movement of flexing or the like of the cable or conductor. In other words, the flexibility of the cable and of the conductor must not be interfered with, at least not to a noticeable degree when such foils or ribbons are used as wrapping. Particularly so-called wrinkles or folds have to be avoided which may well occur whenever the cable or conductor is, ~o to speaX, bent in a more or less sharp curve.
Ribbons which adapt to the contour of an object rather easily have been developed, for example, for the special purpose of sealing screw connections or the like. See ~or example German Published Paten~ Application 2,028,393. These ribbons or foils are comprised of a polymer of tetrafluorethylene and they have been stretched in ~he un-sintered state or just tensioned, the purpose being to use as little polymeric material as possible without interfering with the capability of such ribbon to provide for sealing.
However, these kinds of ribbons are not well suitable for electrical purposes, for example, because their tensile .

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64506--21g strength is inadequate, but a relatively high tensile strength i5 needed for wrapping and winding a cable, a conduc~or, a tube, or a tube bundle cable with a rlbbon without deforma~ion in any instance.
DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a new and improved method for the manufacture of porous elements, bodies, shapes, members, construction parts, or the like, with simplified manufacturing techniques, but using particularly fluoropolymer, so as to take advantage of its well known properties.
It is a particular object oi the present invention to improve me~hods of making porous elongated shapes, such as ribbons, foils, solid or hollow sections made of a fluoropolymer which are used by and in itself or in a blend under utilization of a lubricant, were compressed or compacted and paste extruded.
In accordance with the present invention, the body, element or a like as extruded, is dried in a first step through a furnace during a continuous passage ln order to remove therefrom ~he lubricant, and in a second subsequent step, the body or element having attained a temperature which is at least as hlgh as the melting temperature of the material in crystalline state, is simultaneously sintered and stretched in at least one direction.
The invention is based on the discovery that even in the case o~ low rates of s~retching, high tensile strength in a fluoropolymer can be obtained if the stretching concurs, at least to a significant extent, wikh the sintering of the polymer : - 3 ~
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material. Broadly speakiny, lt is thus feasible to impart upon that part, e.g. a ribbon or a foil, different properties hy means of a particular mechanical process to occur or overlap with the sintering and as the particular foil, ribbon, or other part runs in a particular direction. The mechanical process may be tied to that direction of passage. In conjunction therewith it is of par~icular advan~age, if the particular body, part, or element (ribbonsr foil, etc.) is stretched during sintering in the direction of, for example, unwinding from a supply spool or the like.
Foils or ribbons made in such a manner were found to be partiaularly suitable as wrapping of flexible elongated stock such as electrical cable or conduits, because suah foils made in that manner, will adapt smoothly and snugly to the respective substrate, owing to the diminishing of the transverse stability so that the flexibility of the final product is not, or hardly, impeded. Any damage owing to edges, folds, wrinkles, or the like, are avoided. On the other hand and in other cases, the transverse stability of the ribbon may be more important while the tensile strength in axial (longitudinally) direation is ~ lesser importance. In this case, it may be of advantage, for example, to stretch the fluoropolymer ribbon transversely to the direction of pay-out during heating to a temperature above the crystallite melting tempqrature.
In acaordance with a further concept of the invention, stretching may occur in both dlreations, i.e. in axial direction of tape or foil withdrawal from ~he supply, as well as .
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1 3 1 6 ~ l ~ 64506-219 transversely thereto, of course, within ~h,e plane of the foil or ribbon. This way one obtains in a rather simple manner, and even for low stretch rates, a particular shape whlch has high mechaniGal as well as eleGtrical qualities, and can, in fact, be used in a large variety of ways.
Contrary to the known fea~ures of a tension relief annealing as shown in German Published Patent Applicakion

2,028,393, at temperature below 342 degrees centigrade, which means below the melting poin~ of the un-sintered polymer, the invention requires sin~ering. In furtherance of the invention, therefore, the sintering temperature is above 342 degrees centigrade. This higher temperature is necessary for sintering.
Broadly speaking, the sintering temperature should be at least 380 degrees centiyrade but not more than 1,000 degrees centigrade.
Drying of the ribbon, foil, or the like, made through paste extrusion, is carried out also on a continuous basis, and is, as such separate from the subsequent stretching and sintering.
Drying is carried out at a temperature between 150 degrees centigrade and 320 de0rees centigrade, preferably between 200 and 300 degrees centigrade; definitely below the sintering temperature. This ensures that the temperature range is sufficiently well below the self-igniting point of the usual lubricants. The self-igniting point is for the usual lubricants between 350 and 400 degrees centigrade.
If the body to be made is a foil or ribbon or tape-like product, then in accordance with a further feature of the invention, the stretching should be up to 2,000%, preferably _ ~ _ ~ - p , . . .

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1 3 ~ 63 l ~ 64506 219 between 100 and 1,000~. This ~ay, the quali~y of the ribbon makes i~ particularly suitable as a wrapping or the like for flexible elongated stock. Such foils or ribbons, when stretched and sintered, have specific weight which is between 0.2 and 1.3 g/cm3.
The situation is analogous when the stretching is performed during the sintering transversely to the direction of ribbon and foil propagation.
As already mentioned, certain products such as flexible elongated stock (conductors, tubes, cable, etc.) have to be wrapped in foil or ribbon material for a variety of purposes, and here the ribbon or foil does not have to have ~he same s~rength values in longitudinal as well as in the direction transversely thereto. The ratio of the tensile strength may be between 10:1 and 50~1, as far as longitudinal tensile strength and transverse ; tensile strength are concerned. Since these values are readily adjustable through an appropria~e stretch procedure in accordance with the invention, these ribbons and foils can be adapted very alosely to the desired purposes being now quite superior to prior art ribbons. These aspects refer not only to foils made of fluoropolymers and stretched as per any known procedure, but apply ; also to other polymer tapes, foils, or ribbons. For example, ~or a variety of purposes, including particulary as a dielectric material, the electric industry uses certain ribbon and foils made of llnear polyester. The product is, for example, traded under the trade-mark "Hostaphan". These foils are used among others in cable and conduator engineering, for separatlng several different layers from each other, ~or example, in order to prevent migration -,6 -,~
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--" 1 3 1 6 3 1 2 of certain volatile components from/to these okher layers.
Typical here are plastici~ers or the like which should not di~fuse. Also, a separating foil made of Hostaphan polyester ensures that the various layers in a conductor or cable, or the like, can shift relakive to each other, for example, during winding or unwinding, or whenever certain tensile or upsetting forces act on that cable or the like. These known foils, however, are disadvantageous for the reason that they do not snugly fit onto their respective substrate or above, particularly during winding or unwinding, and they also interfere to some extent with the flexibility of the product of which they become a part. These difficulties are avoided when a ribbon produced by the method in accordance with the invention is used. This is particularly so if the foil or ribbon is used as a slide element separating layers in elongated stock. Herein it was found to be of advantage to provide the ribbon or foil with perfora~ions. For example, a drum with cutters may roll over the ribbon to cut space-apart slots;
~he slots extending basically in the longitudinal direction. The slo~s may be organized or arranyed in several parallel running rows. In case it is expected that the ribbon will, when finally used, experience back and forth bendlng, the lo~s may cause the ribbon to tear after lt has been so wrapped and while serving as an intermediate layer. Thus, if the ribbon tear~ and turns into a plurality of individual wrapping elements, the function of the ribbon as a gliding or sliding promoting object is retained, and any otherwise present stiffness is no longer noticeable, the flexibility o~ the stock as a whole remalns.

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, ' ' As stated, the foll or ribbon may be used as one layer within the layer configuration of elonga~ed flexible stock, and wherein these layers are to slide relative to each other, the ribbon, as also stated, is to serve as a sliding and glidlng promoting agency. These cables are used, for example, for the transmission of electric power or for communication purposes.
Alternatively, so-called tube-bundle-cables may be assembled and wrapped. They are used for the transmission of measuring and control the signals on a pneumatic or hydraulic basis. Individual tubes for a pneumatic or hydraulic medium are bundled together, and within the bundling the foil or ribbon is used for the stated purpose. A foil or ribbon can also be included in conduits or hoses for the transport of liquidous or gaseous material, and in each case it is not only a foil or ribbon that can be used but the tube or tubes themselves may be made as a porous element as outlined above. Such porous tubes can be used also as individual conduit, they are used, for example, for monitoring volatile gases.
Upon using the invention, particularly ribbons and foils made n accordance with the inven~ion withln cable or conductors, the foils and ribbons can be used as a layer separating agency, as outlined above. But the insulation in case of an electrical cable can itself be provided fro~ such ribbons or foils. They can serve alsQ in the alternative, as a dielectric component to be used, for example, in coaxial high frequency cable. The requisite dielectric layer can then be made from a porous foil or ribbon, made in accordance wikh the invention. The relatively high air .
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645~6-219 content within the cells of the porous material ln conjunction with the high longitudinal streng~h further combined with a relatively low transverse strength, meets all the requisite requirements for such a high frequency cable, par~icularly a flexible one.
DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims, particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention, and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:
Figure 1 is a somewhat schematic view of equipmen~ for practicing the invention in accordance with a preferred embodiment of the present invention.
Proceeding now to the detailed description of the drawings, lt is assumed that a ribbon was made through pa~te extrusion of polytetrafluoroethylene powder. The extruded ribbon was then wound on a supply spool 2 for further use. One of the further uses is explained now with reference to the drawing. For this the ribbon is unwound from the spool 2, passes a deflection pulley 3 to be run through a furnace 4 for drying on a contlnuous basis. As the ribbon passes through the furnace 4, it assumes a ~` temperature of ahout 280 degrees centigrade. The lubricant and other volatile auxiliary material are removed in the furnace from the ribbon. The ribbon 1 as it leaves the furnace 4 is now dry : _ g _ ,~, .,.-., .~ .. , .. , . . .. . ~ .

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1 3 1 6 3 1 ~ 6~506-2~9 but relatively un-s~retched and positively not sintered.
The ribbon is now passed into and through a slnter furnace 5, the temperature thereof may be abou~ 430 degrees centigrade. Number 6 refers to a guide roll which is driven such that the revolution is higher than its revolution of the deflection pulley 3. This way ribbon 1, particularly as it passes through the sintering furnace 5, is significantly s~retched.
Owing to the higher temperature, the stre~ching is primarily effective in the ribbon as it passes through the sinter furnace 5.
This means that sintering and stretching occurs; at least the processes overlap in that there is no ~intering without stretching. Following deflection by another pulley 7, the porous and stretchad ribbon is wound upon another supply drum or spool 8.
Aiter having described the basic process~ cer~ain details should be discussed. As stated, the ribbon 1 was extruded from a polymer powder which had been blended and mixed with a lubricant, and was, in fact, compressed or compaated. This compressed or compacted powder was then extruded in a piston extruder, in order to obtain a round product which then was rolled flat to obtain the ribbon 1. The reduction may have been ~0:1, which means the powder was compressed in the extrusion process.
Following the rolling, the ribbon 1 is stretched as per the invention at a ratio of 15 5, which i5 a 500% length extension.
Under such conditions, a porous, sintered, and stretched ribbon acquires a tensile streng~h whlch, in longitudinal direction, is 360 kp~cm2. The transverse strength is 30 kp/am2. If under otherwise similar conditions a reduction is ahosen from 50~1, then 1 31 631 ~

the strength ln longitudinal direction may still be 280 kp/cm2 and in transverse direction 20 kp/cm2. In each case it is important that the tensile s~rength in longitudinal direction of the ribbon, but also in any other longitudinally oriented product, is a multiple of the tensile strength ln transverse direction. The ribbon or other shape ~hus made is highly stable and resistive to elevated temperatures, and owing to the material chosen, is insensitive against aggressive materials, and most importantly, it bends easily.
10In deviation from the foregoing, the particular element as made, such as a ribbon or foil, may be stretched during sintering transversely to the direction of movement to the equipmen~. For this one will use, for example, elements which for a certain distance run with the ribbon, and engage or grip the ribbon or foil edges, and are oriented in a diverging fashion so as to stretch the ribbon transversely in ~he longitudinal direction durlng slntering, l.e. at the elevated temperature, which is, as stated above the crystallite melting temperature of the m~terial.
20 ~ ~ ~ There are other fields of employment; for example, ~oils can be made to be used as filters and in surgery. In these cases, - and unlike in the cable engineering outlined above, it is of ~nterest to provide an isotropic tensile strength, i~e. a tensile ~trength~that differs not or only little in longitudinal and ; ~ transverse~directlon. In ~his case, one will still, for reasons :~ G~ manufacturing, use basically the equipment sh~wn in Figure 1, :: : :
~ but the dried and yet unstretched and un-sintered ribbon 1 i3 run :

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6~506-219 into the sinter furnace and sintered at a temperature of 500 degrees centigrade. During sin~ering, again one provides longitudinal stretching as outlined ahove. But in addition, one will use laterally and transversely acting s~retching elements, as for example shown and disclosed in European patent 75,306. In each case, a stretching between 200 and 600% is provided. The rela~ive stretching elements engage the ribbon edges, and through appropriate guides the transverse stretch elements move in diverting direction to obtain stretching transversely to the longitudinal stre~ching. This means that the ribbon is stretched longitudinally as well as transversely, the ribbon is, so to speak, bi-axially stretched, since sintering concurs, the tensile strength in both directions is increased. The stretching whlch obtains is the same as in prior art methods, but the concurring sintering and s~re~ching improves the qualities o~ the stock significantly.
The invention is not limited to the embodiments described above, but all changes and modifications thereof, not aonstituting departures from ~he spirit and scope of the invention are intended to be included.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method of making a porous elongated object, which comprises:
mixing a fluoropolymer powder or a polymer blend which includes such fluoropolymer, with a lubricant, compressing or compacting, and a paste extruded, into an object, passing the extruded object continuously through a furnace for drying the same, thereby removing the lubricant; and sintering the product at an elevated temperature above the crystallite melting temperature of the polymer, while simultaneously and concurrently stretching the product in at least one direction.

2. The method as in claim 1, wherein the dried object is stretched in a direction of movement of the object.

3. The method as in claim 1, wherein the dried object is stretched transversely to the direction of movement.

4. The method as in claim 1, wherein the drive object is longitudinally as well as transversely stretched during the sintering .

5. The method as in claim 1, wherein the sintering temperature is well above 342 degrees centigrade

6. The method as in claim 5, wherein the sintering is performed at a temperature between 380 and 1000 degrees centigrade.

7. The method as in claim 1, wherein the drying is performed at a temperature between 150 and 300 degrees centigrade.

8. The method as in claim 2, wherein the dried object is stretched up to 2,000%.

9. The method as in claim 2, wherein the dried object is stretched within the range from 100 to 1,000%.

10. The method as in claim 1, wherein the stretching and sintering is carried out to produce the elongated object having a specific weight of between 0.2 and 1.3 g/cm3.

11. The method as in claim 4, wherein the transverse and longitudinal stretching is proportioned so that a resulting tensile strength has a ratio between 10:1 and 50:1.

12. The method as in claim 1, including the step of perforating the object, the object being a ribbon or a foil.

13. The method as in claim 1, wherein the object is a ribbon, and including the subsequent step of interposing the ribbon as a layer in a multi-layer configuration for elongated stock.

14. A method of making a porous elongated object in a ribbon, a foil or a solid or hollow section form, which comprises:
mixing polytetrafluoroethylene powder with a lubricant to form a mixture;
compressing or compacting the mixture and past-extruding the mixture in a molded object form;
passing the extruded object continuously through a furnace for drying the same, thereby removing the lubricant; and sintering the dried object at an elevated temperature above the melting point of polytetrafluoroethylene while simultaneously and concurrently stretching the dried object in at least one direction.

15. The method as in claim 14, wherein:
the drying is performed at a temperature between 150 and 300°C;
the sintering is performed at a temperature between 380 and 1,000 °C, and the dried object is stretched at a stretching degree of 100 to 1,000%.

16. The method as in claim 15, wherein the elongated object is a ribbon or a foil and which further comprises wrapping a flexible elongated stock with the ribbon or foil.

17. The method as in claim 16, wherein the flexible elongated stock is an electric cable or conduit.