GB2187460A

GB2187460A - Production of porous elongate moulded bodies from fluorine-containing polymers

Info

Publication number: GB2187460A
Application number: GB08704045A
Authority: GB
Inventors: Heinz Eilentropp; Kurt Aeschbach; Hans Reinmann
Original assignee: Reinmann & Cie Lara
Current assignee: Reinmann & Cie Lara
Priority date: 1986-02-21
Filing date: 1987-02-20
Publication date: 1987-09-09
Anticipated expiration: 2007-02-20
Also published as: IT8719195A0; SE8700684D0; GB2187460B; AU6659690A; GB8704045D0; FR2594744B1; FR2594744A1; CA1316312C; SE8700684L; CN87100900A; CH672094A5; BE1001311A3; BR8700796A; NL8700379A; IT1202444B; JPS62205136A; AU6910087A

Abstract

In the production of such bodies, moulded bodies produced by paste extrusion are freed by drying, in a first step of a continuous process, from the lubricating or parting agent used in their production and, in a second step, are stretched and the polymer material simultaneously sintered at a temperature at least equal to the crystallite melting temperature of the polymer. Stretching may be effected in the output direction and/or transversely thereto. The moulded bodies, e.g. tapes or foils, have properties making them suitable for use in the construction of elongate products, such as electric cables and leads, tube assembly cables and pipelines.

Description

SPECIFICATION Production of porous elongate moulded bodiesfrom fluorine-containing polymers The present invention relates to a method of producing porous, elongate moulded bodies, in particular, tapes, foils and solid or hollow profiled sections, from fluorine-containing polymers, such as poly tetrafluoroethylene, in which the powdery polymer, which cannot be processed in molten form, is compressed, alone or as a mixed polymer, with the aid of a lubricating or parting agent and is extruded to form the moulded body (paste extrusion), whereupon the lubricating or parting agent is removed, the moulded body is stretched, and the polymer is sintered.

In orderto produce porous moulded bodies from a tetrafluoropolymer, it is already known from German Patent Specification No. 2417 901 to stretch or draw the moulded body produced by paste extrusion after removal ofthe lubricating agent, ata speed in excess of 2000% per second, the moulded body being extended in the direction of stretching by more than 50 times the length of the unstretched body. The extension or stretching of the moulded body should be effected in a temperature range of between 35 and 327"C, with the object of substantially increasing the tensile strength whilst simultaneously reducing the density. In order to achieve this object, the stretching speed combined with a specifictemperature range have been considered to represent decisive factors.

Apart from the fact that this known method gives rise to production problems owing to the high stretching values per second, the range of use of the moulded bodies produced by this known process is limited. Thus, for example, tapes orfoils exposed to a stretching speed of more than 2000% per second are lesssuitablefor use aswound separating orsiiding foils in the layer construction of flexible electrical cables or leads.This is because tapes or foils suitable forthis purpose must be capable ofclinging in order tofollowthe movements ofthecable or lead, but equally should not in the wound state impairtheflex- ibility of the cable or lead to any appreciable extent, or give rise to so-called crease4olds when the cable or lead has a small radius of bending. Tapeswhich will cling are already known forthe special purpose of sealing screw connections and the like (see German Patent Specification OS 2028393). These tapes, and also foils, consist of polymers of tetrafluoroethylene, are unsintered, and are stretched or extended in the longitudinal direction in orderthat small amounts of polymer material can be used without impairing the sealing ability of the known tape.Such tapes are less suitableforelectrical purposes; interalla they do not exhibit the tensile strength necessary for winding the tape onto a cable or lead, a tube bundle cable, ora pipeline,without deformation ofthetape.

Starting from this prior art, it is an object of the present invention to provide a method with which simple production technology can be used to produce porous moulded bodies and thus to open up additional, advantageous fields for use for fluorinecontaining polymers.

According to the invention, there is provided a method of producing a porous, elongate moulded body from a fluorine-containing polymer which cannot be processed in the molten state, in which the powdered polymer, alone or as a mixed polymer, is compressed and extruded to form the mou Ided body with the aid of a lubricating or parting agent, wherein after its formation, the moulded body is subjected in a acontinuous process in which, in a first step, the moulded body is freed from said lubricating or par- ting agentbydrying and, in a subsequent second step, the mou Ided bodyisstretchedatatemperature which is at least equal to the crystallite melting temperature of the polymer and the polymer is sintered substantially simultaneously with the stretching of the moulded body.

The invention is based on the recognition that even at low stretching or extension rates, high tensile strength values can be achieved in moulded bodies having a basis of fluorine-containing polymers, provided that the stretching ofthe moulded body and the sintering of the polymer material of which it is made are carried out virtually simultaneously. Moreover, this also makes it possible for the moulded body, for example, a tape or foil, to be provided with different characteristics by means of mechanical treatment during the sintering proce dure, either inthe direction of throughput ofthe moulded body, ortransversely thereto.

In this connection, it is particularly advantageous if, in carrying out the invention,the moulded body is stretched or extended in the output direction during the sintering process. Foils or tapes produced in this way are particularly suitable, for example, for wind- ing on flexible elongate products, such as cable and leads, since they readily cling to the base in question and, because ofthe reduced transverse stability, scarcely, if at all, impairthe flexibility of thefinal product. Damage due to warped tape edges or creasefolds is avoided.

If, in contrast to the embodiment described above, the transverse stability of the strip is of greater sig nificance and the tensile strength in the axial direction ofthe moulded body is not of primary importance, it may be advantageous to stretch, for example, a tape made of a fluorine-containing polymer transversely to the output direction during heating to above the crystallite melting temperature of the material, and to effect axial stretching or extension, i.e. for stretching to be carried out both in the axial direction and transversely thereto. Thus, even at low rates of stretching or extension the invention enables moulded bodies of high mechanical and electrical quality to be provided in a simple manner which bodies fully satisfy requirements imposed in various fields of use.

In contrast to known methods of relaxation heating (see German Patent Specification OS 2028393) at temperatures below 34200, thus in any case below the melting point ofthe unsintered polymer material, a sintering process is essential to the present invention. In accordance with the invention, therefore, the temperature range used forthis material will exceed 342"C, and will preferably be in the range of between i and 10000C.

In the method ofthe invention,the drying ofthe moulded body which is produced by paste extrusion may be carried out in a continuous operation and is, at any rate, carried out separately from the following step of stretching and simultaneous sintering. The drying may be carried out at temperatures of between 1500 and 320 C, preferably between 2000 and 30000. This provides an adequate interval belowthe self-ignition point of the lubricating or parting agents in current use, which ranges between 35000 and 40000.

Particularly when the moulded body is a tapeshaped orfoil-shaped product, the moulded body may be stretched by up to 2000%, preferably between 100 and 1000%, in the output direction. This results, for example, in a tape quality which is particularly suitable for use in flexible, elongate products. In the stretched and sintered state, such tapes orfoils have a specific gravity of 0.2 to 1.3 g/cm3. Corresponding stretching conditions naturally also apply to the mechanical treatment ofthe moulded body during the sintering procedure transverselyto the axial orthroughput direction, if the required properties of the product make this necessary.

As already described,for specific products, such as flexible elongate products, it is important that when these are wound with a tape-shaped material, for whatever purpose, the tape orfoil should not exhibitthe same tensile strength values in the axial direction and in the direction transversely thereto.

Since the ratio ofthetensile strength values in the stretching direction ofthe tape orfoil and in the direction transversely thereto, may be, for example, 10 -50: 1, for this special application andforsimilar uses, these tapes or foils are superiorto all known constructions. In this connection, the term "known constructions" is to be understood as meaning not only known stretched foils having a basis offluorinecontaining polymers, but also other tapes and foils having a polymer basis which are in use.Thus, for example, in the electrical industry, foils in strip form having a basis of linear polyester, widely known underthe Trade Name "Hostaphan", for example, have long been used for many different purposes, for example, as a dielectric. However, these foils are also used in the cable or lead technology to separate individual layers, eitherto stop the migration of volatile components (plasticisers), brto permit a relative mechanical displacement of individual layers ofthe cable or lead structure when the tensile and com pressiveforces acting during the winding and unwinding ofthe cable or lead, suggest that this would be beneficial.Adisadvantage of these known foils consists in the low level of ability to cling to the base during the winding process and the impairment of the flexibility of a cable, for example, wound with such a foil, because ofthe relative rigidity of the material ofthefoii.

In particularfor use as a sliding foil in a particularly flexible, elongate product, in accordance with a furtherfeatureofthe invention, it has been found to be advantageousforthe tape or foil to be provided with a perforation. Thus, for example, a cutting roller can be used to introduce slots extending in the longitudinal direction and arranged at spaced intervals into the tape orfoil; a plurality of such rows of slots can also be provided, one beside another. In the event of a strong alternating bending load, the tape orfoil wound in one or more layers, can then break up into individual elements, so that the flexibility of the product is retained and the tape or foil continues to function as a sliding means.

Elongate flexible products made of a plurality of layers which are separated from one another but must be easily displaced relative to one another, for example, during bending, may be in the form ofthe previously-mentioned cables and leads which serve to transmit energy or data. They can also be in the form of so-called tube bundle cables which serve to transmit measured or control values pneumatically or hydraulically and consist of individual tubes assembled to form a group. Other fields of use ofthe invention lie in pipelines or hoses used, for example, to transport liquid or gaseous media, in which case not only are tapes orfoils used, but also one or more ofthe pipes may be in the form of a porous moulded body. Porous pipes of this kind, which can also be installed individually, are used, for example, as sam~ pling pipelines, to monitor volatile gases.

When used in electrical cables or leads, the tapes or foils of the invention can form the sliding orseparating layer; the insulation can also consist of these tapes orfoils, and, in addition, the dielectric, for example of a coaxial high-frequency cable, can be produced from the stretched and simultaneouslysintered porous foils or tapes of the invention. The high proportion of air in the porous material in combination with the high level of longitudinal strength and the comparatively low level of transverse strength, fulfill the requirements existing in this field in a particularly satisfactory manner.

To carry out the method ofthe invention, it has been found to be advantageous to use apparatus comprising a drying device, the temperature of which can be adjusted, arranged in the throughput direction of the moulded body which has been moulded by paste extrusion, and an adjacent but separate continuous heating furnace with a roller device which is arranged at the output end and rotates at a speed which is greater than that of guide and/or driving rollers arranged at the input. The separation of the drying of the moulded body in a first step, and the combination of stretching and virtually simultaneous sintering thereof in a second step, but both steps being effected during the passage of the moulded body leads to an economical, operationallyreli- able production run.

The invention will now be further described with reference to the drawing, which is a schematic side view of apparatus for carrying out the invention.

Referring to the drawing, a moulded body in the form of a tape 1 of polytetrafluoroethylene which has been produced by paste extrusion, is unwound from a supply reel 2 and fed through a deflecting roller3to a drying furnace 4. When passing through this drying furnace, the temperature of which is set at about 28000, the tape 1 is freed from the lubricating or parting agent required for the paste extrusion. The now dried but unstretched and unsintered tape 1 now enters a sintering furnace 5, the furnace temperature of which is,430'C.Aguide roller6arranged atthe output of the sintering furnace 5 is driven, in such a way that its speed of rotation is higher than the speed of rotation ofthe deflecting roller 3.The tape 1, which has been heated to the sintering temperature when passing through the sintering furnace 5, is thus axially stretched. The tape 1 which has been virtually simultaneously stretched and sintered, is then wound by way of a guide roller 7 on to a supply drum or reel 8.

The basic material for the tape 1 wound on the supply reel 2 is a polymer powder which, mixed with a known lubricating agent or agents, is compressed under pressure to form a preform. In the example under consideration, this preform is then extruded in known manner in a plunger-type extrusion press to form a round-section string which is then rolled out to form the tape.

If the preform :string reduction ratio is, for ex- ample, 40 : 1 and if the stretching ratio after the rolling out of the tape is 1: 5, values forthe tensile strength of 360 kp/cm3 in the longitudinal direction and 30 kp/cm3 in the transverse direction can be achieved forthe porous, sintered and stretched tape produced in accordance with the invention.

If, under otherwise identical operating conditions, a preform : string reduction ratio of 50 : 1 is selected, tensile strength values can be achieved of 280 kp/cm3 in the longitudinal direction and 20 kp/cm3 in the transverse direction. In either case, it is essential that the tensile strength values in the longitudinal direo tion of the tape (and the same applies to any other profiled section) should be a multiple ofthetensile strength values in the transverse direction. Apart from its high temperature resistance and insensitivityto aggressive media, a moulded body produced in this way is characterised in particular by good bending characteristics.

In contrast, it is of course also possible forthe moulded body, such as a tape or foil, to be subjected to stretching or extension transversely to the output direction simultaneouslywith the sintering ofthe polymer material. For this purpose, apparatus equipped with drawing elements which travel in the direction ofthroughput and which grip the edges of thetapeorfoil and by moving apart into the desired end positions within the sintering furnace 5, effect the required degree of stretching or extension whilst the tape or foil temperature is simultaneously raised above the crystallite melting temperature of the material ofthetape orfoil.

In other cases, however, such as in the field of industrial filters, or in the field of surgery, for example, in contrast to cable technology, it is often not im portantto achieve different tensile strengths in the longitudinal and transverse directions of the moulded body.

In this case, the procedure is that, as shown in the drawing, the dried, as yet unstretched and unsintered tape 1 enters the sintering furnace where it is brought to the sintering temperature, the furnace temperature being, for example, 500 C. During the sintering, because of their different speeds the guide roller 6 and the deflecting roller 3 effect a stretching or extension in the axial direction, for example with a stretching ratio of between 200 and 600%. At the same time, however, drawing elements (not shown) gripthetwo strip edges and are moved apart during the passage of the tape, for example, by means of suitable guide bars, at right angles to the direction of passage ofthe tape, and thus also stretch the tape in this direction. This leads to a biaxially stretched tape which is sintered during the stretching process and which exhibits high tensile strength in both directions. The tensile strength values which can be attained by the use ofthe invention in moulded bodies which have been extended or stretched in one direction, or in both directions, correspond in fact to those which can be attained in known processes by the use of extremely high degrees of extension or stretching. The drawing elements are not shown in the drawing, but are described, for example, in European Patent Specification No.

0075306.

Claims

1. A method of producing a porous, elongate moulded body from a fluorine-containing polymer which cannot be processed in the molten state, in which the powdered polymer, alone or as a mixed polymer, is compressed and extruded to form the moulded bodywiththeaidofalubricatingorparting agent, wherein after its formation, the moulded body is subjected in a continuous process in which, in a first step, the moulded body is freed from said lubricating or parting agent by drying and, in a subse quentsecond step, the moulded body is stretched at a temperature which is at least equal to the crystallite melting temperature of the polymer and the polymer is sintered substantially simultaneously with the stretching ofthe moulded body.

2. A method as claimed in Claim 1, wherein said fluorine-containing polymer is poly tetrafluoroethylene.

3. A method as claimed in Claim 1 or Claim 2, wherein the moulded body is stretched in the output direction.

4. A method as claimed in Claim 1 or Claim 2, wherein the moulded body is stretched transversely to the output direction.

5. A method as claimed in Claim 1 or Claim 2, wherein the moulded body is stretched in the axial direction andtransverselythereto, eithercon- secutively or simultaneously, the stretching process in each case being carried out at a temperature in a range which exceeds the crystallite melting temperature of the polymer used to form the moulded body.

6. A method as claimed in any one of the preceding Claims, wherein said temperature is in a range which exceeds 342 C.

7. A method as claimed in Claim 6, wherein said temperature is in the range of between 380C and 100000.

8. A method as claimed in any one of the preceding Claims, wherein the drying ofthe moulded body in the first step is carried out at a temperature of between 1500 and 32000.

9. A method as claimed in Claim 8, wherein the drying of the moulded body is carried out at a temperature in the range of between 200 and 30000.

10. A method as claimed in Claim 3 or Claim 5, or anyone of Claims 6 to 9 as dependentthereon, wherein the moulded body is stretched by upto 2000% in the output direction.

11. Amethod as claimed in Claim 10, wherein the moulded body is stretched by between 100 and 1000% in the output direction .

12. A method of producing a porous, elongate moulded body substantially as herein before described with reference to the drawing.

13. A porous, elongate moulded body produced by a method as claimed in any one of Claims 1 to 12.

14. Atape, foil, or solid or hollow profiled section produced by a method as claimed in any one of Claims 1 to 12.

15. Atapeorfoil asclaimed in Claim 14, having a specific gravity in the stretched and sintered state of 0.2 to 1.3 g/cm3.

16. Atape orfoil as claimed in Claim l4orClaim 15, wherein the ratio of the values of the tensile strength in the stretching direction ofthetape orfoil and in the direction transverse thereto is 10-50:1.

17. Atape orfoil as claimed in any one of Claims 14to 16, provided with at least one perforation.

18. An elongate product of layered construction, wherein a tape orfoil as claimed in any one of Claims 14to 16 is used.

19. An elongate product as claimed in Claim 18 in the form of an electric cable or lead, a tube assembly cable, or a pipeline.

20. An elongate product as claimed in Claim 19 in the form of an electric cable or lead, wherein the insulation of the cable or lead consists of said stretched and simultaneously sintered tape orfoil.

21. An elongate product as claimed in Claim 19 or Claim 20 in the form of an electric cable or lead having a layered construction, wherein one or more of said tapes orfoils is or are used as a sliding orseparating foil in said layered construction.

22. An elongate product as claimed in Claim 18, substantially as hereinbefore described.

23. Apparatus for carrying out a method as claimed in Claim 1, comprising a drying device, the temperature ofwhich is adjustably arranged downstream of an extrusion means forforming said molten body and by a separate continuous heating furnace arranged immediately downstream of said drying device, said furnace being equipped with a roller means arranged at the output and which rotates at a higher speed than one or more guide and/or drive rollers arranged at the inputofthe furnace,and/orwith stretching or drawing elements arranged to grip opposite sides of a moulded body travelling through the furnace and to travel with the moulded body in the throughput direction, and which elements can be moved awayfrom one another with in the continuous heating furnace.

24. Apparatus for carrying out a method as claimed in Claim 1, substantially as hereinbefore describedwith reference to and as illustrated in the drawing.