CA1046217A

CA1046217A - Porous diaphragms

Info

Publication number: CA1046217A
Application number: CA230,328A
Authority: CA
Inventors: Christopher Vallance
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1974-06-28
Filing date: 1975-06-27
Publication date: 1979-01-16
Also published as: IT1039523B; GB1505077A; BE830739A; ZA754091B; DE2529153A1

Abstract

ABSTRACT OF THE DISCLOSURE

A method of manufacturing a porous diaphragm for an electrolytic cell from a plurality of sheets of filled polytetra-fluoroethylene which method comprises fusing a melt-processable fluorinated polymer into juxtaposed edges of said sheets at a temperature which will not substantially decompose the filler in said sheets, solidifying the melt-processable polymer so as to effect joining of the shoots and thereafter removing filler from the thus joined sheets.

Description

~0~ 17 This invention relates to improvements in the manu-facture of porous diaphragms~
More particularly, the invention relates to improve-ments in the manufacture of porous diaphragms suitable for use in electrochemical cells.
Porous diaphragms based on tetrafluoroethylene polymers are especially suitable for use in cells for the electrolysis of alkali metal chloride solutions. In our U.K.
Patent Specification No. 1,081,046 there is described a method of manufacturing such diaphragms which comprises forming a filled polytetrafluoroethylene sheet from an aqueous dispersion of polytetrafluoroethylene and a removable solid particulate additive, e.g. starch, by adding an organic coagulating agent such as acetone to said dispersion and then drying the co-agulated dispersion. An organic lubricant such as petroleum ether is then added to the dried coagulated material to serve as a processing aid when the material is being rolled into a sheet. on completion of the rolling operation the starch is removed from the sheet to give the desired porous diaphragm.
The lubricant can also be removed if required.
Another method of manufacturing such porous diaphragms in which the organLclubricant is replaced by water as lubricant is described in our Canadian Patent No. 1004819 issued on 8th February, 1977. This method comprises forming a filled poly-tetrafluoroethylene sheet from an aqueous dispersion comprising polytetrafluoroethylene and a removable solid particulate ad-ditive by thickening said aqueous dispersion to effect agglo-meration of the solid particles therein, forming from the thickened dispersion a sheet-formable material containing suf-ficient water to serve as lubricant in a subsequent sheet forming

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~i~;

lO~iZ17 operation, forming a sheet of desired thickness from said material, for example by passing the material through calender rolls and removing solid particulate additive from the sheet.
As indica-ted above, suitable removable solid particulate ~, ~ _ , _ _ _ . ,, , _ , , , , , , , . . , . . . , _ _ _ _ _ . _ _ .. _ . _ . .. _ . , . . , ... ... .. .. .. .., . , .., .. _ _, ... _ _ _ _ _ _ 104~217 additives include starch, e.g maize and/or potato starch~
Other removable additives are water-insoluble inorganic bases or carbonates, e.g calcium carbonate. Cellulose also is a suitable additive If desired, these solid particulate ad-ditives, which constitute the means of imparting porosity to the diaphragm, may be removed from the diaphragm prior to introducing the diaphragm into the electrolytic cell. Alter-natively, the solid particulate additives may be removed from the diaphragms in situ in the cell Hereafter in this specification and claims the term filled polytetrafluoroethylene sheet shall mean poly-tetrafluoroethylene sheet containing removable solid parti- -culate additive as described above Unfortunately, however, there are problems asso-ciated with the development of the use of such diaphragms in electrolytic cells For example, there is generally a limit on the dimensions of the diaphragm sheets that can be produced in practice Of necessity the width of the diaphragm sheet is governed by the size of the rolls employed in producing the sheet. The cost of increasing the size of the manufacturing equipment is exponential with the result that there is an optimum size of roll which is dependent upon purely commercial factors, Moreover, diaphragms of simple rectangular sheet form are extremely difficult to fit onto the complicated cathode designs of modern diaphragm cells because of the numerous recesses and protuberances presented by the cathode. The aforesaid 104~217 problems are accentuated in the case of diaphragms made of non-melt-processable materials such as PTFE. The main reason for this is that it is extremely difficult to join together small sheets of polytetrafluoroethylene in order to produce a diaphragm of the desired complex shape and size.
It is an object of the present invention to provide a method of manufacturing polytetrafluoroethylene diaphragms which obviates or mitigates the aforesaid disadvantages.
According to the present invention there is pro-vided a method of manufacturing a porous diaphragm for anelectrolytic cell from a plurality of sheets of filled poly-tetrafluoroethylene which method comprises fusing a melt-processable fluorine-containing polymer into said sheets at or near juxtaposed edges of said sheets at a temperature which will not substantially decompose the filler in said sheets, solidifying the melt-processable polymer 54 as to effect joining of the sheets and thereafter removing filler from the thus joined sheets.
In one embodiment of the invention two or more sheets of filled polytetrafluoroethylene are joined along juxtaposed edges by overlapping said edges with one or more strips of melt-processable fluorine-containing polymer and fusing said strip or strips into the areas of the sheets adjacent to said juxtaposed edges.
However, in a preferred embodiment of the invention, one or more strips of melt-processable fluorine-containing polymer can be made to partially overlap one or more edges of a sheet of filled polytetrafluoroethyethylene and protruding portions of said strip or strips can be utilised as desired to bond said polytetrafluoroethylene sheet to other polytetra-~,~, .
. ,. . -104~217 fluoroethylene sheets which have not had melt-processable strips of fluorine-containing polymer fused thereto. Con-veniently all four sides of a rectangular sheet of filled polytetrafluoroethylene can be pro~lided with overlapping strips of melt-processable fluorine-containing polymers to give a window-frame effect and such unit window-frames of melt-processable polymer can be joined to other filled polytetrafluoroethylene sheets by conventional plastics fabrication techniques.
The last mentioned procedure can be modified by replacing the strips of melt-processable fluorine-containing polymer with tabs of melt-processable polymer at intervals along one or more edges of a filled polytetrafluoroethylene sheet.
As aforesaid, the melt-processable fluorine-contain-ing polymer must be such that it fuses into the filled poly-tetrafluoroethylene sheet below the temperature at which the filler substantially decomposes. For example, in the case where the filler is starch, the upper temperature for the ~0 fusion process must not exceed 300C. Furthermore, it is important that the melt processable polymer be impermeable or of porosity not greater than that of the eventual poly-tetrafluoroethylene diaphragm. Consequently the temperature at which fusion takes place must not be so high as to allow decomposition products to blow holes in the melt-processable polymer.
When the diaphra-~m manufactured according to the present invention is intended for use in electrolytic cells then the melt-processable fluorine-containing polymer must be resistant to conditions in the cell.

10~217 The melt-processable fluorine-containing polymer used in the presen-t invention is preferably one which sub-stantially returns to its original form on the removal of heat and also retains its original properties. By comparison, polytetrafluoroethylene, which is considered as a non-melt-processable material in the context of the present invention, fuses when heat is applied but also decom~oses within a few degrees of its melting point. In other words the melt vis-cosity of polytetrafluoroethylene is too high for the ap-plication of conventional plastics fabrication techniques.
Preferably, the melt-processable fluorine-con-taining polymer is selected from polychlorotrifluoroethylene, polyvinylidene fluoride, FEP (a fluorinated ethylene/propylene copolymer) or a copolymer of ethylene and chlorotrifluoroethy-lene.
Conveniently the melt-processable fluorine-containing polymer is fused into the filled polytetrafluoroethylene sheet by the application of heat and pressure. The temperature at which the melt-processable polymer is fused into the polytetrafluoroethylene sheet preferably is lower than the melting point of polytetrafluoroethylene. The temperatures and pressures employed depend upon the specific melt-process-able polymer involved. We have found, however, that in most cases it is advantageous to operate at a constant pressure of around 10 psi and to apply heat over a varying period of time which does not result in deformation of the diaphragm.
The present invention is also a porous polytetra-fluoroethylene diaphragm whenever manufactured by a process as hereinbefore described.

104~217 The diaphragms of the invention may contain a non-removable filler such as titanium dioxide in order to render the diaphragm wettable when installed in an electrolytic cell.
Embodiments of the invention will now be described with reference to the following Examples in which all parts and percentages are by weight.

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To 100 parts of an aqueous dispersion of polytetra-fluoroethylene containing 60% of the polymer in the form of particles approximately all in the size range 0.15 to 0.2 micron were added 101 parts of water, 60 parts of titanium dioxide of particle size approximately 0.2 micron, 60 parts of maize starch of particle size approximately 13 microns and 120 parts of potato starch of particle size less than 75 microns. The mixture was then stirred with a paddle-mixer for 30 minutes to form a substantially uniform paste. This paste was spread on trays and dried at 24 for 48 ho-irs to a water content of 5.7%. 100 parts of the resultant crumb we~e mixed with 52 parts of water to form a sheet-formable material akin to a dough having a viscosity of 4 x 106 poise. The sheet-farmable material was then spread along the shortest edge of a rectangular piece of card, and calendered on the card between dual, even-speed, calender rolls, set 3 mm apart, into an oblong sheet.
After calendering, the oblong sheet was cut, in the direction of calendering, into four equal pieces. These were laid congruently over each other to obtain a four-layered lami-nate. The card was picked up, rotated 90 in the horizontal plane, and calendered (directed 90 to the original direction 1~4~;Z17 of calendering) again through the 3 mm roll separation.
This process, the successive cutting into four, stacking, rotating and calendering was repeated until the composition had been rolled a total of five times. The resultant lami-nate was cut into four, in the direction of calendering, stacked, removed from the card, and calendered, without rotation through 90, the inter-roll space being reduced by the thickness of the card~ After calendering, the laminate was cut, at xight angles to the direction of calendering, into four equal pieces, stacked, rotated through 90 and calendered again. This process, cutting at right angles to the direction of calendering, stacking, rotating and calendering was repeated until the composition had been rGlled a total of nine times. The resultant essentially rectangular laminate was then passed through the rolls with its largest side directed at 90 to the direction of calendering, and with the inter-roll space slightly reduced, no cutting, stacking or rotating through 90 being involved. This process was repeated through a gradually reduced inter-roll space, the same edge of the laminate being fed to the rolls on each occasion, until the thickness of the laminate was 1.5 mm.
A square of 22 x 26 mesh gauze woven of 0.011 inch diameter monofilament polypropylene yarn was placed on top of the laminate, and rolled into the laminate by calendering through a slightly reduced inter-roll space.
The edges of two sheets of filled polytetrafluoroethylene prepared as aboJe were brought together in a butt-joint and a strip of FEP of thickness 0.02 inch was laid along the length of the butt-joint. This composite join was then heated by means of heated plattens under a pressure of 10 psi _ g v~'~

for a period of 10 minutes until a temperature of 275 C
was attained. By this means an adequate joining of the two PTFE sheets was effected, charring of the starch being minimised.
The aforesaid sample of joined PTFE sheets was then immersed in 5N HCl in order to remove the starch filler.
A porous PTFE diaphragm suitable for use in diaphragm cells was produced. After removal of the starch, the PTFE/FEP
joint was still effective.

Two starch filled PTFE sheets prepared according to Example 1 but with an FEP backing sheet instead of poly-propylene were joined together at their edges by means of a strip of FEP of thickness 0.01 inch, The welding took place between parallel flat heating elements 5/16 inch wide and under a pressure of 24 psi. The elements were lightly greased with silicone grease to prevent sticking. Pressure was maintained on the weld for 30 seconds after turning off the welding cur~ent. The maximum temperature attained was 280 C and was measured between the interfaces of the materials with a dwell time of between one and two seconds.
The sample of joined PTFE sheets was immersed in 5N HCl in order to remove the starch filler. A porous PTFE
diaphragm suitable for use in diaphragm cells was produced.

Two PTFE sheets prepared according to Example 1 but with cellulose as filler and with FEP backing sheet were joined together by the technique of Example 2 with the exception that current was only supplied to the element in contact with the 0.01 inch thick FEP strip to prevent ~,';`

104~Z17 undue degradation of the cellulose. An adequate join was obtained and the cellulose filler was removea as before to give a porous PTFE diaphragm.

The juxtaposed edges of two PTFE sheets prepared according to Example 1 with starch as filler and each having an FEP backing sheet were joined together by the technique of Example 2 except that a strip of Halar (a copolymer of ethylene and chlorotrifluoro ethylene) (Halar , is a Registered Trade Mark) of thickness 0.005 inch was placed over the butt joint in place of the FEP, and a maxi-mum temperature of 260C was used. An adequate joining of the two PTFE sheets was effected and the starch was removed as before to give a porous diaphragm.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of manufacturing a porous diaphragm for an electrolytic cell from a plurality of sheets of filled polytetrafluoroethylene which method comprises fusing a melt-processable fluorine-containing polymer into said sheets at or near juxtaposed edges of said sheets at a temperature which will not substantially decompose the filler in said sheets, solidifying the melt-processable polymer so as to effect join-ing of the sheets and thereafter removing filler from the thus joined sheets, said melt-processable fluorine-containing polymer being impermeable or of porosity not greater than that of the eventual polytetrafluoroethylene diaphragm.

2. A method as claimed in Claim 1 wherein the melt-processable fluorine-containing polymer is resistant to conditions in an electrolytic cell.

3. A method as claimed in Claim 1 or 2 wherein the melt-processable fluorine-containing polymer is one selected from the group of polychlorotrifluoroethylene, polyvinylidene fluoride, a fluorinated ethylene/propylene copolymer and an ethylene/chlorotrifluoroethylene copolymer.

4. A method as claimed in Claim 1 or 2 wherein the melt-processable fluorine-containine polymer is fused into the polytetrafluoroethylene sheet by the application of heat and pressure.

5. A method as claimed in Claim 1 or 2 wherein the temperature at which the melt-processable fluorine-containing polymer is fused into the polytetrafluoroethylene sheet is lower than the melting point of polytetrafluoroethylene.