GB1570686A

GB1570686A - Joining of polytetrafluoroethylene bodies

Info

Publication number: GB1570686A
Application number: GB45932/76A
Authority: GB
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1975-11-05
Filing date: 1976-11-04
Publication date: 1980-07-09
Also published as: FR2330518A1; FR2330518B1; DE2549475B2; DE2549475C3; IT1063650B; JPS5263275A; DE2549475A1

Description

(54) IMPROVEMENTS IN OR RELATING TO THE JOINING OF POLYTETRAFLUOROETHYLENE BODIES (71) We, HOECHST AKTIENGES ELLSCHAFT, a body corporate organised according to the laws of the Federal Republic of Germany, of 6230 Frankfurt/Main 80, Postfach 80 03 20, Federal Republic of Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to joining bodies of polytetrafluoroethylene by a butt joint.

Shaped bodies which consist of polytetrafluoroethylene (PTFE) or which contain predominantly PTFE (hereinafter designated polytetrafluoroethylene material) are increasingly used as corrosion-resistant linings, for example for chemical apparatus such as heat exchangers, columns and vessels, because of their excellent thermal and chemical stability.

Since PTFE tubes of large diameter cannot be made by ram or paste extrusion, it is necessary, particularly for equipment with a large inside diameter, to join together shaped parts, e.g. sheets or film, to make them gasand liquid-tight.

Further difficulties are due to the fact that a separate press orifice is necessary for every diameter, and the ratio of wall thickness to diameter can be varied only within certain limits.

It has also not been possible to date to make cemented joints with the quality required, for example, for chemical apparatus, particularly with high thermal and chemical stability, because of the poor wettability of polytetrafiuoro- ethylene and the comparatively low thermal and chemical stability of adhesives. Even if the PTFE surfaces to be joined together are first etched, the quality of the joint is insufficient. The welding methods commonly used for joining conventional thermo plastic materials, in which the parts pressed together are heated above the melting or softening point, are not usable for polyteurafluorthy- lene. However, overlapping joints between thin PTFE films have been made by this method, but are not well suited for tank linings because of the extra material thickness at the overlap. The sealing of such seams, for example, at flanges, is particularly difficult and expensive if thicker film or sheets are used. Attempts to avoid such overlap points have been made by a method for making a joint between PTFE parts by a butt joint, in which the V-shaped seam gap between two PTFE plates fixed on a support is filled with a PTFE powder and the latter is heated by heating elements which are pressed on the joint. Sufficient and reproducible welding quality which depends, in a complicated manner, on the quality and grain of the powder as well as on the packing density and on other factors, can be achieved by this process only if all the factors influencing the operation are favourable, which is hardly possible under normal operating conditions. The asymetrical temperature distribution in the material to be welded, caused by the arrangement of the support underneath the weld results in unfavourable welding quality.

There is therefore a need for a method for making a butt joint of high welding quality suitable for shaped bodies of PTFE of any thickness and size.

Our copending application No. 45931/76 (Serial No. 1 570 685) describes one method of joining polytetrafluoroethylene bodies.

This invention now provides a method of joining bodies of polytetrafluoroethylene having edge surfaces, at those surfaces,which comprises positioning the bodies with the edge surfaces in abutting relationship, clamping the bodies in this position by holding them between gripping jaws provided with cooling means, leaving free a strip including the abutting edges between those jaws, pressing together the abutting edges, and heating the said strip by heating means maintained out of direct contact with the strip to a temperature above the melting point of polytetrafluoroethylene.

Heating may be by convection and/or radia tion. It may be by blowing hot gases on to the abutting surfaces.

A thin film of fluorine-containing thermo plastics material inserted between the abutting surfaces facilitates joining of the surfaces.

One or both sets of gripping jaws i.e.

clamps, may be moveable, for example one or both clamps may be mechanically loaded so that the pressure exerted on the butt joint may be increased by displacing at least one clamp substantially perpendicular to the abutting surfaces, i.e. in a direction towards the butt joint. This additional pressure may be exerted at any stage of the process, e.g. during clamping and/or heating and/or cooling.

For producing a high-quality joint, it is necessary to heat the abutting surfaces under pressure to a temperature of the melting point of PTFE crystalslites and to let the temperature and the pressure act on the joint surfaces until a thickening of the seam has been produced.

The temperature distribution in the heated P1FE strip which protrudes unsupported between the clamping jaws should avoid buckling of the PTFE strip.Melting zones that are too wide or too narrow are not desired. Thus re gardless of the respective geometric shapes or dimensions of the bodies to be joined together, a melting zone of adequate width to be selfsustaining is produced and maintained for the duration of the welding process. The thickened seam may be machined off, for example, by planing or dressing to remove unwanted surface notches. We have found surprisingly that the.PTFE melted in the vicinity of the abutting surfaces does not drop off or flow out in spite of the relatively high pressure and long reaction times that are applied for producing a high-quality joint. This mechanical stability of the melt is unexpected.

Preferably, the abutting surfaces are heated to a temperature in the range of from 340 to 430"C. Below this temperature range, the welding times required are relatively long, while above the range, very short welding times can be obtained but the possibility of partial decomposition of the polytetrafluoroetyhlene increases. The contact pressure is preferably in the range of from 60 to 300 N/cm2, but may be from 30 to 150 N/cm2.

The welding time is preferably in the range of from 10 to 120 min. The parameters temperature, pressure and time have a similar effect up to a certain degree and a low value of one parameter may be compensated by increasing the other parameters in this substtution range.

The method of the invention is suitable for joining together PTFE bodies of the various shapes and dimensions, and especially, bodies having a thickness in the range of from 1 to 10 mm.

Advantageously, a thin film of a fluorinecontaining thermo-plastics material is inserted between the abutting surfaces, for example, a film of a tetrafluoroethylene-hexafluoro- propylene copolymer (FEP) or of a copolymer of tetrafluoroethylene and perfiuoroalkyl-per.

fluorovinyl ether (PFA). Such thin intermediary layers make a reduction of the welding time possible (preferably 1 to 10 minutes). They also allow a lowering of the pressure and/or temperature without thereby appreciably degrading the chemical and thermal stability of the joint. PFA film used as the intermediary layer gives especially advantageous results. The pressure acting on the abutting surfaces under these conditions is preferably in the range of from 2 to 100 N/cm2, but may be 1 to 50 N/cm2.

For obtaining a uniform temperature distribution in the butt joint, it is advantageous to let the width of the PTFE strip heated to a temperature above the melting temperature be in a definite ratio to the thickness of the bodies to be joined together. Favourable results are obtained with a ratio in the range of from 0.2 :1 to 3 : 1, especially 0.2 : 1 to 2 : 1, more especially 0.8 : 1 to 1.2 : 1, and very especially, 1 : 1.

The present invention also provides apparatus for joining together edge surfaces of polytetrafluoroethylene material by a butt joint, which comprises clamping jaws for clamping the polytetrafluoroethylene material on each side of the abutting surfaces to be joined, cooling means for controlled removal of heat from the gripped area of polytetrafluoroethylene, and heating means spaced from, and out of direct contact with, a suip of polytetrafluoroethylene between the clamping jaws on each side of the abutting surfaces to be joined for heating the strip by radiation and/ or convection. Preferably, the clamping jaws on at least one side of the abutting surfaces is moveable in a direction towards the butt joint for controlling the contact pressure of the abutting surfaces during joining of the surfaces.

For example, apparatus for carrying out the method according to the invention comprises gripping jaws which hold and press together the polytetrafluoroethylene bodies and which are provided with cooling arrangements for the controlled removal of the excess heat from the heated polytetrafluoroethylene strip and for controlling the contact pressure during the welding process, the clamping jaws may be moveable in at least one direction substantially perpendicular to the abutting surfaces.

Heat radiators, e.g. in the form of heating ribbons or heating wires, may be arranged berween the pairs of clamping jaws along the weld, for the contactless heating of the exposed PTFE strip; or hot gases, especially air heated in a heater placed a distance from the PTFE surface, are blown on to the PTFE surface into the gap formed by the pairs of clamping jaws, for heating the exposed strip. The PTFE bodies are advantageously clamped in such a manner that the pressedtogether abutting surfaces are approximately the same distance away from each of the clamping jaw pairs. The shape of the abutting surfaces and the layout of the welded seam may vary appreciably and the abutting surfaces may intersect the surface of the body at any angle or may be of concave or convex shape or may have any desired profile. The welded seams in their lengthwise direction may be straight or curved. Also the abutting surfaces may be serrated over the entire length of the welded seam or only in certain sections. The pressure required to weld the PTFE parts together is supplied from the outside. This pressure, taking into consideration the thermal expansion of the PTFE strip, is suitably held at a definite magnitude and changed during the welding time in accordance with a predetermined programme.

Compartmentalisation of the free PTFE strip is not necessary even though the temperature is above the melting point and the pressure is maintained over an extended period of time, as, especially because of the conuolled tem- perature distribution in the strip, only very narrow zones on both sides of the abutting surfaces are softened. The absence of com partmentalisation permits heating of the polytetrafluoroethylene by simple heat radiators or heated gases and thus simplifies the joining method.

Thus, shaped bodies of PTFE of various different geometric shapes such, for example, as sheets, rods, film, hollow cylinders and hollow cones, may be joined in a simple manner by butt joints with a tight material bond.

The welding factor (quotient of the tensile strength of the joint determined at room temperature and the tensile strength of the solid starting material) is almost 1. The welded seams generally do not adversely affect the permeability of a hollow shape fabricated by joining several PTFE parts and nor do they substantially change the resistance of PTFE to corrosive substances. Further advantages of the method are the excellent reproducibility, uniform quality of the seam and, also, the simple handling.

The present invention will now be described in detail, by way of example only, with reference to the accompanying drawing which shows schematically a sectional view of welding apparatus of the present invention.

Two PTFE bodies 1 and 2 which are to be joined together are pressed together at a butt joint 3, and are held by clamping jaw pairs 4.

The clamping forces are represented by the arrows A. To supply the necessary welding pressure, the jaws 4 are moveable in a direction towards the butt joint 3 and press the abutting surfaces together by means of a force B generated in any suitable manner as for example by springs, hydraulically or by weights.

Between the clamping jaws, heating elements 5 are arranged for heating the polytetrafluoroethylene in the vicinity of the butt joint 3. To control the temperature distribution in the welding and clamping zone, the clamping jaws 4 contain cooling arrangements 6 which may be passageways through which a cooling media with a high heat capacity flows or the heat capacity of the clamping jaws may be suffliciently large to maintain a given temperature distribution between the clamping jaws and the butt joint.

Thus, the polytetrafluoroethylene bodies 1 and 2 to be joined together are clamped, except for a narrow strip which includes the abutting surfaces 3, between clamping jaws 4 provided with cooling arrangements 6. In operation, the strip is heated, without making direct contact with the heating element 5 which is the source of heat, by radiation and/or convection, to a temperature above the melting point of polytetrafluoroethylene.

Heat is removed, at least in part, from the clamping zone via the clamping jaws 4. By pressing the abutting surfaces together, a thickening of the seam is produced, which is subsequently machined off to remove surface notches.

The following Examples illustrate the invention: EXAMPLE 1 For lining a funnel with internal diameters of 1400 mm and 500 mm, respectively, and a height of 1000 mm, a PTFE film 4 mm thick, which had been cut to size and planed off at the edge surfaces was clamped into the apparatus as shown in the drawing in such a manner that the butt joint was about equally far away from the pairs of gripping jaws and extended parallel to the jaws. The distance between the jaws was 10 mm. The zone on both sides of the butt joint was heated by the heat radiators to about 400"C, where equilibrium existed between the energy supplied into the about 5 mm wide welding zone and the energy re moved by conduction via the gripping jaws.

The temperature and a pressure of 120 N/cm2 were maintained for 40 min. and the welding zone was then cooled down under pressure to room temperature. The thickening of the seam, which had developed on both sides of the butt joint was then planed off and a surface free of notches created. As compared with unwelded PTFE, the welded seam had the following properties: Breaking strength 0.8 Breaking elongation 0.9 EXAMPLE 2 A pared PTFE film 3 mm thick, intended for lining a hollow cylinder, the diameter of which was 1600 mm and the length of which was 1200 mm, was cut to size and, after the edge surfaces were planed off, was clamped into the apparatus shown in the drawing as described in Example 1. A thin PFA film was inserted between the abutting surfaces. The welding zone was then heated under a pressure of 20 N/cm2 to a temperature of about 350"C and the pressure and the temperature were maintained for 5 min. After cooling down, the bead-shaped enlargement in the vicinity of the butt surfaces was dressed. The properties of the cold welded seam were as follows, as compared with unwelded PTFE: Breaking strength 0.9 Breaking elongation 0.95 WHAT WE CLAIM IS:- 1. A method of joining bodies of polytetrafluoroethylene having edge surfaces, at those surfaces, which comprises positioning the bodies with the edge surfaces in abutting relationship, clamping the bodies in this position by holding them between gripping jaws provided with cooling means, leaving free a strip including the abutting edges between those jaws, pressing-together the abutting edges, and heating the said strip by heating means maintained out of direct contact with the strip to a temperature above the melting point of polytetrafluoroethylene.

2. A method as claimed in claim 1, wherein the abutting edges are heated by radiation.

3. A method as claimed in claim 1, wherein the abutting edges are heated by convection.

4. A method as claimed in any one of claims 1 to 3, wherein the-pressing-together of the abutting edges produces a thickening of the seam and the thickening is subsequently machined off to remove surface notches.

5. A method as claimed in claim 5, wherein the abutting edges are heated to a temperature in the range of from 340 to 430"C.

6. A method as claimed in any one of claims 1 to 6, wherein a film of fluorine-containing thermoplastics material is inserted into the gap between the abutting surfaces.

7. A method as claimed in claim 7, wherein the film is a film of a tetrafluoroethylene hexafluoropropylene copolymerisate.

8. A method as claimed in claim 7, wherein the film is a film of a copolymer of teurafiluoro- ethylene and a perfluoroalkyl-perfluorovinyl ether.

9. A method as claimed in any one of claims 1 to 8, wherein the abutting edges are pressed against each other with a pressure in the range of from 60 to 300 N/cm2.

10. A method as claimed in any one of claims 1 to 9, wherein the temperature and the pressure in the butt joint are maintained for a period of from 10 to 120 minutes.

11. A method as claimed in any one of claims 6 to 8, wherein the abutting surfaces are pressed against one another with a pressure in the range of from 2 to 100 N/cm2.

12. A method as claimed in any of claims 6 to 8 and 11, wherein the temperature and pressure in the butt joint are maintained for a period of from 1 to 10 minutes.

13. A method as claimed in any one of claims 1 to 12, wherein the thermoplastics material to be joined has a thickness in the range of from 1 to 10 mm.

14. A method as claimed in any one of claims 1 to 13, wherein the width of the heated thermoplastics material is 0.2 to 3 times the thickness of the thermoplastics material to be joined.

15. A method as claimed in claim 14, where in the width of the heated thermoplastics material is from 0.8 to 1.2 times the thickness of the thermoplastics material to be joined.

16. A method as claimed in claim 15, wherein the width of the heated thermoplastics material is substantially the same as the thickness of the thermoplastics material to be joined.

17. A method as claimed in any one of claims 1 to 16, wherein the heat is conducted away from the gripping jaws via passageways therein through which a fluid coolant passes.

18. A method as claimed in any one of claims 1 to 17, wherein additional pressure is exerted on the butt joint by moving at least one set of gripping jaws in a direction towards the butt joint.

19. A method as claimed in claim 1, carried out substantially as described herein, with reference to, and as illustrated by, the ac comDanving drawing.

20. A method as claimed in claim 1, carried out substantially as described in Example 1 or Example 2 herein.

21. Apparatus for joining bodies having edge surfaces of polytetrafluoroethylene or other thermoplastics material by a butt joint, which comprises gripping jaws for clamping the poly tetrafluoroethylene or other thermoplastics material on each side of the abutting surfaces to be joined, cooling means for controlled re moval of heat from the gripped area and heat ing means spaced from, and out of direct con tact with, a strip of polytetrafluoroethylene between the clamps on each side of the abut ting surfaces to be joined for heating the strip by radiation and/or convection.

22. Apparatus as claimed in claim 21, where in at least one clamp acted upon by a force is displaceable in a direction perpendicular to the abutting surfaces.

23. Apparatus as claimed in claim 21 or claim 22, wherein the cooling means are passageways in the gripping jaws through which a fluid coolant is passed.

24. Apparatus as claimed in any one of claims 21 to 23, wherein the heating means is a radiator disposed adjacent to, but spaced from, the strip and wherein the radiator radiates energy to heat the strip.

25. Apparatus as claimed in any one of claims 21 to 23, wherein the heating means is a gas heater disposed away from the strip

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

welding zone was then heated under a pressure of 20 N/cm2 to a temperature of about 350"C and the pressure and the temperature were maintained for 5 min. After cooling down, the bead-shaped enlargement in the vicinity of the butt surfaces was dressed. The properties of the cold welded seam were as follows, as compared with unwelded PTFE: Breaking strength 0.9 Breaking elongation 0.95 WHAT WE CLAIM IS:- 1. A method of joining bodies of polytetrafluoroethylene having edge surfaces, at those surfaces, which comprises positioning the bodies with the edge surfaces in abutting relationship, clamping the bodies in this position by holding them between gripping jaws provided with cooling means, leaving free a strip including the abutting edges between those jaws, pressing-together the abutting edges, and heating the said strip by heating means maintained out of direct contact with the strip to a temperature above the melting point of polytetrafluoroethylene.
2. A method as claimed in claim 1, wherein the abutting edges are heated by radiation.
3. A method as claimed in claim 1, wherein the abutting edges are heated by convection.
4. A method as claimed in any one of claims 1 to 3, wherein the-pressing-together of the abutting edges produces a thickening of the seam and the thickening is subsequently machined off to remove surface notches.
5. A method as claimed in claim 5, wherein the abutting edges are heated to a temperature in the range of from 340 to 430"C.
6. A method as claimed in any one of claims 1 to 6, wherein a film of fluorine-containing thermoplastics material is inserted into the gap between the abutting surfaces.
7. A method as claimed in claim 7, wherein the film is a film of a tetrafluoroethylene hexafluoropropylene copolymerisate.
8. A method as claimed in claim 7, wherein the film is a film of a copolymer of teurafiluoro- ethylene and a perfluoroalkyl-perfluorovinyl ether.
9. A method as claimed in any one of claims 1 to 8, wherein the abutting edges are pressed against each other with a pressure in the range of from 60 to 300 N/cm2.
10. A method as claimed in any one of claims 1 to 9, wherein the temperature and the pressure in the butt joint are maintained for a period of from 10 to 120 minutes.
11. A method as claimed in any one of claims 6 to 8, wherein the abutting surfaces are pressed against one another with a pressure in the range of from 2 to 100 N/cm2.
12. A method as claimed in any of claims 6 to 8 and 11, wherein the temperature and pressure in the butt joint are maintained for a period of from 1 to 10 minutes.
13. A method as claimed in any one of claims 1 to 12, wherein the thermoplastics material to be joined has a thickness in the range of from 1 to 10 mm.
14. A method as claimed in any one of claims 1 to 13, wherein the width of the heated thermoplastics material is 0.2 to 3 times the thickness of the thermoplastics material to be joined.
15. A method as claimed in claim 14, where in the width of the heated thermoplastics material is from 0.8 to 1.2 times the thickness of the thermoplastics material to be joined.
16. A method as claimed in claim 15, wherein the width of the heated thermoplastics material is substantially the same as the thickness of the thermoplastics material to be joined.
17. A method as claimed in any one of claims 1 to 16, wherein the heat is conducted away from the gripping jaws via passageways therein through which a fluid coolant passes.
18. A method as claimed in any one of claims 1 to 17, wherein additional pressure is exerted on the butt joint by moving at least one set of gripping jaws in a direction towards the butt joint.
19. A method as claimed in claim 1, carried out substantially as described herein, with reference to, and as illustrated by, the ac comDanving drawing.
20. A method as claimed in claim 1, carried out substantially as described in Example 1 or Example 2 herein.
21. Apparatus for joining bodies having edge surfaces of polytetrafluoroethylene or other thermoplastics material by a butt joint, which comprises gripping jaws for clamping the poly tetrafluoroethylene or other thermoplastics material on each side of the abutting surfaces to be joined, cooling means for controlled re moval of heat from the gripped area and heat ing means spaced from, and out of direct con tact with, a strip of polytetrafluoroethylene between the clamps on each side of the abut ting surfaces to be joined for heating the strip by radiation and/or convection.
22. Apparatus as claimed in claim 21, where in at least one clamp acted upon by a force is displaceable in a direction perpendicular to the abutting surfaces.
23. Apparatus as claimed in claim 21 or claim 22, wherein the cooling means are passageways in the gripping jaws through which a fluid coolant is passed.
24. Apparatus as claimed in any one of claims 21 to 23, wherein the heating means is a radiator disposed adjacent to, but spaced from, the strip and wherein the radiator radiates energy to heat the strip.
25. Apparatus as claimed in any one of claims 21 to 23, wherein the heating means is a gas heater disposed away from the strip

and the hot gas from the heater blown on to the strip, heating it by convection.
26. Apparatus as claimed in claim 21, substantially as described herein with reference to, and as illustrated by, the accompanying drawing.