SPECIFICATIONAn Apparatus for the Formation, by Extrusion, of a Fibrillated PTFE StrandThe present invention relates to an apparatus for the formation by extrusion, of a strand of fibrillated PTFE material, the strand being a semiproduct which can itself be stretched to form a porous PTFE product which has filtering and sealing applications. The apparatus has a first section in which the PTFE-material is transformed into a rectangular shaped strand by a longitudinal stretching whereafter in a second section the bar shaped strand is given the form of a web shaped strand by a lateral stretching. Due to the shearing stresses produced thereby in the PTFE-material fibrils are formed, some of which commensurate with the deforming steps extend substantially in the longitudinal direction and others of which extend substantially in the lateral direction.If such an extruded strand, after the removal of the lubricant commonly present in an extrusion process, is then stretched and thereafter sintered or simultaneously sintered, a porous PFTEproduct is produced which due to its porosity is used in filtering applications and due to the corresponding easy deformability is also used in sealing applications.
It must be noted that in many apparatuses for the extrusion of a fibrillar stock material, such as disclosed e.g. in the DE-AS 1 5 04 291, a portion of the PTFE-particles are not transformed into fibrils after having been subjected to the transformation in both sections of the apparatus.
According to that published application such is advantageous during a subsequent working of the product into non-stretched and thus non-porous formed bodies. If, however, the object is to produce a material having as fine as possible pores such non-fibrillar particles are of a disadvantage because they influence the pore size of the stretched product specifically due to their size and mutual distance.
The invention provides an apparatus which produces tighter interlaced fibrils and accordingly finer pores at a comparatively same stretching rate which is specifically advantageous for a filtering duty. It has been now recognized that this is achieved in apparatuses of the kind mentioned above which comprise as a final section a converging constriction, preferably a frustoconical constriction converging at all sides, having an inclination of the constricting walls relative to the axis of 5-20#, preferably 10-1 50 whereby the reduction of the cross-sectional area of the exit compared with the cross-sectional area of the inlet amounts to at least 20%.The resistance component produced at such a reduction of the cross-sectional area of the strand extends due to the mentioned inclination acting on all sides of the converging walls of the constriction inclining and relative to the direction of material flow through the apparatus that no additional orientation in the longitudinal as well as lateral direction is produced and acts rather onto the existing fibrils in the form of an inclined force in the sense of a tighter interlacing.
The end section designed in accordance with the present invention as a conically converging constriction in order to reduce the crosssection area of the strand does not only produce the above described additional interlacing of existing lateral and longitudinal fibrils but forms from the possibly present PTFE-particles additional fibrils resulting from a given design of the conical constriction so that these PTFE-particles will be completely eliminated, too. Accordingly, after the stretching a uniformly porous final product, having tightly interlaced fibrils, is produced whereby its pore size is considerably reduced in comparison with the known porous products.
Dependent from the prevailing dimensions of the inventive extruder it may be possible in some instances that the stock material exiting out of the conical constriction comprises a fibrillar surface and/or a larger cross-sectional area than such of the constriction proper. In order to avoid such the final section can be shaped in accordance with a further preferred embodiment such that said conical constriction proceeds smoothly into a stabilizing section having a length which is about five to twenty times larger than the longest diagonal line of the cross-section of the exit of the conical constriction.
The present invention will be more fully understood by reference to the following detailed description thereof when read in conjunction with the attached drawing, wherein the single figure shows schematically a preferred embodiment of the invention.
Referring now to the single figure there is shown schematically a piston cylinder device 1, a first shaping section 2 having a cylindrical rectangular cross-sectional shape 21, a second shaping section 3 having flattened cross-sectional shape 31 at its exit, a third shaping section 4 having a cross-sectional shape 41 at its exit, a further shaping section 5 having a cross-sectional shape 51 at its exist described further below and a stabilizing section 6. The direction of the flow of the material is identified by arrow 10. Also shown is a first heating section 7 and a second heating section 8 and finally a stretching section 9.
A known apparatus is a so-called cordextruder. In such an apparatus a mixtureconsisting of PTFE (polytetrafluoroethylene) andlubricant are compressed in cylinder 1 having adiameter of 60 millimeters and thereafter shapedin the first section 2 of the apparatus bylongitudinal stretching to about 800% to a barshaped strand having the shape 21. In thesubsequent second shaping section 3 of theapparatus the strand is shaped by a lateralstretching to about 600% to a web having a shape 31 and a cross-sectional area of 4.5 squarecentimeters.Because the final product of thisknown apparatus is a circular shaped strand 51, the second shaping section 3 is followed by a third section of the apparatus 4 in which the width of the strand is reduced and its thickness continuously increased until a circular exit opening of 15 millimeters diameter is reached~ shape 41. This section (of the known apparatus forming the prior art) follows the second section 3 and its lateral cross section is not described in detail.
If a mixture is used for the extrusion process which has been pretreated, namely a mixture consisting of PTFE (Fluor CD 123) and 25 weight parts of a lubricant (Shellsol E) the strand of about15 millimeters leaving the apparatus is treated for 25 minutes at 1400C in order to expel the lubricant such as indicated by the first heating section 7. Thereafter, the strand is heated up to 2000C (second heating section 8) and thereafter stretched by 600% by means of two stretching rollers located at a distance of 3.25 meters from each other and a stretching speed of 8.66% per second. The subsequent sintering produces a porous strand; this strand comprises unchangedPTFE-particles which are interconnected by fibrils.
The void spaces defined by these fibrils and the unchanged PTFE-partic [ es form pores corresponding in length to the distance of the unchanged particles and in width to the distance of adjacent fibres. These pores have a length of about 35,um (1377.95x10-6 inches) and a width of 2-4 ym (78.74x 10-6 inches~157.48 10-6 inches).
The above description relates to a known apparatus. According to the embodiment of the present invention now being described there is provided a tapered, converging section 5 which is inclined relative to the direction of material flow by an angle of inclination of 100 and which has a length of 7 millimeters. Where the extruded strand is to take the form of a strip, the third section 4 may be dispensed with so that the second section 3 is followed by the tapered, converging section 5. Where the extruded strand is to take the form of a cord, the third section 4 is retained and this leads to the tapered converging section 5. Accordingly, an exiting strand is produced which has been reduced by the taper from 15 millimeters (i.e. the diameter on exiting from section 4) to a diameter of about 12.5 millimeters. Accordingly, the cross-section area of the strand has been reduced by about 30%.This handling of the material leads to a product which does not contain any unchanged PTFE-particles and comprises rather-viewed in a plane-a netlike structure in which the pores have a length of in the range of 0.1-5#um (3.937x 10 inches~ 196.85x10-6 inches) and a width in the range of 0.001 -0.05#um (0.003937 xl 10~6 inches~ 1.9685x 10~6 inches).
By means of the taper of the final section 5 a substantial decrease of the pore-size is achieved whereby the specific gravity of the product remains unchanged.
In some instances it may be possible that the strand leaving the final section 5 tends to swell and thus to comprise finally a cross-sectional area, which is larger than the final cross-sectional area of the taper of the final section 5. In order to avoid such the final section 5 is followed by a stabilizing section 6 having a length which is about five to twenty times larger than the longest diagonal line of the lateral cross-section of the exit of the final section 5. Where this final section 5 is of circular section, the stabilizing section has a length which is about five to twenty times the exit diameter of the final section 5, but neither final section 5 nor stabilizing section 6 need be circular. They may, for example be of elliptical, rectangular or square cross section.