CA1060286A

CA1060286A - Porous diaphragms

Info

Publication number: CA1060286A
Application number: CA230,163A
Authority: CA
Inventors: Christopher Vallance
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1974-06-25
Filing date: 1975-06-25
Publication date: 1979-08-14
Also published as: ZA753604B; GB1503514A; FR2276401A1; FR2276401B1; IT1039072B; BE830602A

Abstract

ABSTRACT OF THE DISCLOSURE
Porous diaphragms based on polytetrafluoroethylene are manufactured by forming a sheet of polytetrafluoroethylene in admixture with a sulid particulate additive to be removed therefrom, coating a surface of a polypropylene gauze and/or a surface the sheet of polytetrafluoroethylene with an aqueous emulsion of polytetrafluoroethylene, bonding the polytetra-fluoroethylene sheet to the polypropylene gauze by contacting the said surfaces under pressure, and subsequently romoving the solid particulate additive from the polytetrafluoroethylene sheet.

Description

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This invention relates to porous diaphragms for electrolytic cells.
More particularly, the invention relates to porous diaphragms based on tetrafluoroethylene polymers. Such diaphragms are especially suitable for use in cells electrolysing alkali metal chloride solutions.
In the specification of our UK Patent 1,081,046 there is described a method of manufacturing porous diaphragms

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'. . ' ' " lO~;S)Z86 which comprises forming an aqueous dispersion of polytetrafluoroethylene and a solid particulate additive such as starch, adding an organic coagulating agent such as acetone to said dispersion and then drying the coagulated 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 rollin~ operation the starch is removed to give the desired porous diaphragm. The lubricant can also be removed if required.
An improved method of manufacturing porous diaphragms in which the organic lubricant is replaced by water as the lubricant is described in the specification of our Canadian Patent No. 1,004,819 issued on 8th February, 1977. This method comprises preparing an aqueous dispersion comprising polytetrafluoroethylene and a removable solid particulate additive, thickening said aqueous dispersion to effect agglomeration of the solid particles therein, forming from the thickened dispersion a dough-like material containing sufficient water to serve as lubricant in a subsequent sheet forming operation, forming a sheet of desired thickness from said dough, for example by calendering and removing solid particulate additive from the sheet.
Suitable removable solid particulate additives include starch, for example maize starch and/or potato starch, or a water-soluble inorganic base or carbonate~ for example calcium carbonate. If desired, these solid particulate ;
additives may be removed from the diaphragm prior to .

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introducing the diaphragm into the cell, for example, by soaking the diaphragm in an acid, preferably a mineral acid e g. hydrochloric acid. The diaphragm is then washed with water to remove the acid and assembled, whilst wet, into a cell Alternatively, the solid particulate additives may be removed from the diaphragm in situ in the cell, for example as described in our copending Canadian application Serial No 205,247 filed on 9th August, 1974, in which there is described the removal of such additives by treating unextracted diaph-ragm with an acid, for example hydrochloric acid, containinga corrosion inhibitor, for example propargyl alcohol, or by filling the cell with a working electrolyte e.g. with an alkali metal chloride brine and electrolysing the said electro-lyte The aforesaid porous diaphragms based on polytetra-fluoroethylene may be used without any support, but it is preferred to strengthen the diaphragm by¦incorporating a sheet of a suitable strengthening material, for example a polymer gauze such as a polypropylene or fluorinated ethylene/
propylene copolymer gauæe The extra strength of the supported polytetrafluoroethylene increases the ease of handleability of the porous diaphragm during assembly into the cell, and the support substantially prevents any extrusion of polytetra-fluoroethylene through the coarse net of the metal cathode during cell operation as a result of the hydrostatic head applied. Polypropylene is especially suitable as the polymer gauze because it is flexible, relatively cheap as compared with fluorinated ethylene/ .

10602~6 propylene copolymers for example, and is chemically resistant to the catholyte liquors in the cell The polypropylene support may be attached to the polytetrafluoroethylene diaphragm by calendering the two together through a narrow setting on the calender rolls, but in practice, this has not proved entirely satisfactory because of the poor adhesion of polypropylene to polytetrafluoroethylene After intensive calendering, the support can separate easily from the diaphragm with consequent difficulties in handleability of the diaphragm.
We have now found that the aforesaid disadvantages associated with the preparation of porous polytetrafluoro-ethylene diaphragm with polypropylene supports may be obviated or mitigated by the use of a particular adhesive in the course of the preparation According to one aspect of the present invention we provide a method of manufacturing a porous diaphragm based on polytetrafluoroethylene which comprises the step of coating with an aqueous dispersion of polytetrafluoroethylene a surface of a polypropylene gauze and/or a surface of a sheet of polytetrafluoroethylene which is already porous or which contains a removable solid particulate material, and contacting the said surfaces under pressure thereby providing :
a strengthening support for the diaphragm.
According to another aspect of the present invention :
we provide a method of manufacturing a porous diaphragm based on polytetrafluoroethylene which comprises forming a ~
sheet of polytetrafluoroethylene in admixture with a solid :~ -particulate additive to be removed therefrom, coating a '. .

`~ 106~ 6 surface of a polypropylene gauze and/or a surface of the sheet of polytetrafluoroethylene with an aqueous emulsion of polytetrafluoroethylene, bonding the polytetrafluoroethylene sheet to the polypropylene gauze by contacting the said surfaces under pressure, and subsequently removing the solid particulate additive from the polytetrafluoroethylene sheet. A.
It is preferred to coat the polypropylene gauze rather than the polytetrafluoroethylene sheet, with the aqueous dispersion of polytetrafluoroethylene since uniform covering of the gauze can be ensured by using excess of the aqueous dispersion and by allowing the excess to pass through the pores in the gauze.
The aqueous dispersion of polytetrafluoroethylene is conveniently the same aqueous dispersion used in the preparation of the unextracted polytetrafluoroethylene diaphragms as described below. Suitably, the aqueous dispersion contains 20% to 8Q% by weight of polytetrafluoroethylene, for example 60%. The preferred particle size of the polytetrafluoroethylene in the aqueous dispersion, whether used for coating the polypropylene gauze or for preparing the polytetrafluoroethylene diaphragm, is in the range of 0.05 to 1 micron, for example 0.2 micron.
The polypropylene gauze may be coated with an aqueous dispersion of polytetrafluoroethylene using any of the conventional methods of spraying, brushing or dipping.
The coated polypropylene gauze is then placed on top of the porous or the unextracted polytetrafluoroethylene diaphragm `~ 106~ 6 and the contacting surfaces are bonded by applying pressure, most suitably b~lcalendering.
The une~tracted diaphragms may conveniently be prepared from aqueous dispersions of polytetrafluoroethylene and the solid particulate additive by the methods described in our UK Patent No 1, 081, 046 and in our Canadian Patent -No. 1,004,819, both of said methods being referred to a~ove.
The solid particulate additives may be removed from the supported polytetrafluoroethylene diaphragms, either before or after assembly in the cell, by the methods described hereinbefore.
Generally, the solid particulate additive has a particle size substantially all of which are within the range of 5 to 100 microns. The amount of additive will depend on the permeability desired in the final diaphragm. Thus, the weight ratio of additive to polytetrafluoroethylene may be, for example, from 10:1 to 1:10 preferably from 5:1 to 1:1.
In the preparation of the unextracted polytetrafluoro-ethylene diaphragms, in many cases it is desirable to incorporate other components in the aqueous dispersion which are not removed when the sheet is subjected to the treatment to remove the particulate additive. Examples of such components include particulate filler generally inorganic fillers, for example, titanium dioxide which is particularly preferred, barium sulphate, asbestos, (for example amphibole or serpentine asbestos), graphi~e and alumina~ Suitably the non-removable filler has a particle size of, for example, .. ..

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- 1~6~Z86 less than 10 microns and preferably less than 1 micron. The weight ratio of filler to the polytetrafluoroethylene may be for example from 10:1 to 1:10, preferably from 2:1 to 1:2 The diaphragms thus produced are particularly suitable for use in electrolytic cells for the electrolysis of alkali metal halides, for the production of chlorine and caustic alkalies The invention is illustrated but notlimited in the following Example in which all parts and percentages are by weight EXAMæLE
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 hours to a water content 5,7% by weight. 100 parts of the resultant crumb were mixed with 52 parts of water to form a dough having a viscosity of 4 x 10 poise The dough 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 ~06~ 86 ~fter 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 laminate The card was picked up~ rotated 90 in the horizontal plane, and calendered (directed 90 to the original direction 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 lQ resultant laminate was cut into four, in the direction of calendering, stacked, removed from the card, and calendered, without rotation through 90, theinter-roll space being .
reduced by the thickness of the card After calendering, the laminate was cut, at right angles to the direction of calendering, into four equal pieces, stacked, rotated through 90 and calendered again. This process, cutting right angles to the direction of calendering, stacking, rotating and calendering was repeated until the composition had been rolled 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 was 1.5 mm A square of 22 x 26 mesh gauze woven of 0.011 inch diameter monofilament polypropylene yarn was sprayed with an aqueous dispersion of polytetrafluoroethylene containing -g ', , ,-' '' ~ - . ~:

~060Z86 60% of the polymer in the form of particles approximately all in the size range 0 15 to 0.2 micron The coated polypropylene was gauze was allowed to dry for about 5 minutes, placed on top of the laminate, and rolled into the laminate by calendering through a slightly reduced inter-roll space The resultant reinforced sheet was removed from the rolls and subsequently assembled into an electrolytic cell.
It was observed during storage and before assembly into the cell that the polypropylene gauze remained firmly attached to the unextracted polytetrafluoroethylene sheet The cell was filled with sodium chloride brine at 60C
and allowed to stand for 1 hour After 1 hour the current was switched on to commence electrolysis of the brine. Initial voltage was 4.1 volts at 2 kA/m . At this stage there was no flow through the sheet. After 2 hours on load, cell voltage had dropped to its usual value of 3.0 volts at 2 kA/m2.
Flow through the diaphragm commenced after 10 hours, and after 18 hours had reached its design valua. Removal of starch from the sheet could be followed by analysis of carbon dioxide in the gaseous chlorine From an initial level of 7% carbon dioxide concentration decreased steadily until after 18 hours it was constant at 0.5%, the levelattributable to excess carbonate is feed brine, this indicating that ;~
oxidation of starch was complete. After 24 hours, satisfactory cell operation at a current efficiency of 96 5% at 50% ~;
conversion was achieved ~

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 based on polytetrafluoroethylene which method comprises the steps of coating a surface of a polypropylene gauze and/or a surface of a sheet of polytetrafluoroethylene with an aqueous dispersion of polytetrafluoroethylene and contacting the said surfaces under pressure thereby providing a strengthening support for the diaph-ragm, the sheet of polytetrafluoroethylene either being porous or containing a removable solid particulate material.

2. A method of manufacturing a porous diaphragm based on polytetrafluoroethylene which comprises forming a sheet of polytetrafluoroethylene in admixture with a solid particulate additive to be removed therefrom, coating a surface of a poly-propylene gauze and/or a surface of the sheet of polytetra-fluoroethylene with an aqueous emulsion of polytetrafluoro-ethylene, bonding the polytetrafluoroethylene sheet to the polypropylene gauze by contacting the said surfaces under pressure, and subsequently removing the solid particulate additive from the polytetrafluoroethylene sheet.

3. A method as claimed in Claim 1 or 2 wherein only the polypropylene gauze is coated with the aqueous dis-persion of polytetrafluoroethylene.

4. A method as claimed in Claim 1 wherein the aqueous dispersion contains 20% to 80% by weight of poly-tetrafluoroethylene.

5. A method as claimed in Claim 4 wherein the aqueous dispersions contain 60% by weight of polytetrafluoro-ethylene.

6. A method as claimed in Claim 1 wherein the particle size of the polytetrafluoroethylene in the aqueous dispersion is in the range of 0.05 to 1 micron.

7. A method as claimed in Claim 6 wherein the particle size of the polytetrafluoroethylene in the aqueous dispersion is in the range 0.1 to 0.2 micron.

8. A porous diaphragm comprising a porous sheet of polytetrafluoroethylene and a gauze of polypropylene bonded to a surface of the said sheet by means of an aqueous dis-persion of polytetrafluoroethylene.