Diaphragm for an electrolytic cell

An improved diaphragm for an electrolytic cell is prepared by mixing a slurry of an additive, such as poly(ethylene chlorotrifluoroethylene), and asbestos fibers with a dispersion of titanium dioxide in isopropyl alcohol, depositing the treated asbestos fibers onto a cathode, heating the diaphragm to an elevated temperature of from about 100.degree. C. to about 400.degree. C., and allowing the diaphragm to cool. The diaphragm prepared according to this process exhibits improved mechanical strength and integrity as well as a decrease in electrical energy consumption in comparison to diaphragms prepared using conventional techniques.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for preparing an improved 
diaphragm for use in chlor-alkali electrolytic cells. The diaphragm of 
this invention has improved mechanical properties which result in superior 
electrical performance and increased energy savings. 
The chlor-alkali industry currently employs a large number of electrolytic 
diaphragm cells for the commercial production of chlorine and caustic 
soda. These electrolytic cells have an anode contained in an anolyte 
chamber and a cathode contained in a catholyte chamber separated by a 
porous diaphragm. The diaphragm is generally formed by depositing a slurry 
of asbestos fibers directly onto the foraminous cathode. The cells contain 
brine which is electrolyzed to produce chlorine gas in the anolyte chamber 
and sodium hydroxide (caustic) in the catholyte chamber. 
Technical advances in this field have generated various improvements in 
component service life and cell operating efficiency or energy savings. 
These technical developments include dimensionally stable anodes, polymer 
reinforced diaphragms, activated cathodes, and decreased anode/cathode 
gaps. The improved electrodes have lower overvoltages, while the polymer 
reinforced asbestos diaphragm has reduced swelling which enables the 
anode/cathode gap to be significantly decreased. 
Present technology for preparing reinforced asbestos diaphragms requires 
the use of various polymeric reinforcing agents which are added to a 
slurry of asbestos fibers prior to deposition onto a cathode. The polymers 
used in this application must be resistant to attack and degradation by 
the electrolytic solution and cell products. Typical polymers include the 
fluoropolymers such as polytetrafluoroethylene and 
polychlorotrifluoroethylene. 
After being deposite onto the cathode, the diaphragm/cathode structure is 
heated to the fusion point of the polymer and subsequently cooled to room 
temperature. The deposition of the slurry is effected by means of a 
vacuum. Polymer-reinforced diaphragms of this type are disclosed in U.S. 
Pat. No. 4,410,411, issued Oct. 18, 1983 to Fenn et al., U.S. Pat. No. 
4,142,951, issued Mar. 6, 1979 to Beaver et al., and Canadian Patent No. 
1,027,898 to Rucker. U.S. Pat. No. 4,142,951 also discloses that various 
surfactants, wetting agents, dispersing agents, modifiers or other 
processing aids can be added to the asbestos slurry in order to improve 
the dispersion of the asbestos fibers and fluorocarbon polymer and to 
impart increased porosity to the diaphragm. Titanium dioxide is listed in 
this patent as such a processing aid. 
Although the polymer-reinforced diaphragms of the prior art do possess 
improved mechanical stability as compared to unmodified asbestos 
diaphragms, there are still opportunities for further technical 
improvements. For example, polymer-reinforced diaphragms prepared from 
polymers which are less resistant to the cell environment swell after a 
few days exposure to the cell environment, and the polymer itself tends to 
be degraded over a period of time, losing its capacity to effectively bond 
the fibers. Alternatively, when using more environmentally resistant 
polymers, the diaphragm-deposited cathode must be heated to the fusion 
temperature of the polymer which is typically in the range of about 
350.degree. C. In addition to requiring more expensive heating furnaces, 
the use of such high temperature conditions can accelerate the mechanical 
degradation of the cathode and diaphragm. 
An attempt to overcome the shortcomings of polymer-modified asbestos 
diaphragms is disclosed in U.S. Pat. No. 4,180,449, issued Dec. 25, 1979, 
to Heikel. This patent utilizes an organic titanate, such as 
tetraisopropyl titanate, which is dissolved in a solvent capable of 
wetting the asbestos fibers, such as anhydrous isopropanol. The titanate 
solution is used to impregnate a diaphragm which has been previously 
deposited onto a cathode member by vacuum deposition. The diaphragm is 
dried prior to treatment with the titanate solution to prevent hydrolysis 
of the titanate compound. The titanate contained in the diaphragm must 
then be hydrolyzed prior to pyrolysis. Hydrolysis is carried out in the 
presence of a hydrolyzing agent, such as water vapor, while pyrolysis 
occurs at temperatures of about 400.degree. C. The diaphragm produced 
according to this process is stated to be more durable and stable than 
unmodified diaphragms. However, this multistep process is both cumbersome 
and expensive to run commercially. 
Patent application Ser. No. 941,459, filed Dec. 15, 1986, discloses and 
claims an asbestos diaphragm having the asbestos fibers bonded together 
with an oxide of titanium, zirconium, hafnium, niobium, tantalum or 
tungsten. 
It is therefore a principle objective of the present invention to provide 
an improved process for preparing an electrolytic chlor-alkali cell 
diaphragm which has superior physical and electrical properties in 
comparison to diaphragms disclosed in the prior art and those currently in 
commercial use. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process for preparing an 
improved electrolytic cell diaphragm comprises the steps of 
(A) forming an aqueous dispersion of at least one valve metal oxide 
selected from the group consisting of the oxides of titanium, zirconium, 
hafnium, niobium, tantalum, tungsten, and mixtures thereof, and at least 
one water-soluble solvent which is capable of wetting the valve metal 
oxide and the asbestos fibers, 
(B) mixing the dispersion with an additive and asbestos fibers to form a 
slurry, 
(C) immersing a cathode in the slurry and depositing a uniform mixture of 
slurry solids onto the cathode, 
(D) heating the diaphragm-deposited cathode at a temperature of at least 
about 100.degree. C. to cure the diaphragm, and 
(E) allowing the diaphragm to cool. 
Preferably, the cell is a chlor-alkali cell, the valve metal oxide of 
choice is titanium dioxide, the solvent of choice is isopropanol, and the 
additive of choice is poly(ethylene chlorotrifluoroethylene). A wetting 
agent can be incorporated in the slurry for improved wetting of the 
asbestos fibers and dispersion of the solids prior to deposition onto the 
cathode.

DETAILED DESCRIPTION OF THE INVENTION 
The diaphragm of the present invention is formed by depositing treated 
asbestos fibers onto a suitable cathode member. The cathode member, which 
generally traverses the width of the cell and is adapted to be interposed 
between adjacent anode members, is a foraminous structure, such as a 
perforated sheet or expanded or woven metal screen. The cathode is 
generally fabricated from steel and may also have an activated coating on 
its surface. 
Procedures for depositing the fibers onto the cathode are well known in the 
art and involve either one- or two-stage variations. In the one-stage 
process, a slurry containing a mixture of asbestos fiber and a 
fluoropolymer is deposited onto a cathode member, while in the two-stage 
process, asbestos fibers are first deposited and subsequently impregnated 
with a thermoplastic fluoropolymer. These techniques are disclosed in U.S. 
Pat. No. 4,410,411 and Canadian Patent No. 1,027,898, respectively, the 
disclosures of which are incorporated herein by reference. 
Irrespective of the particular deposition process employed, the first step 
is the preparation of a slurry of asbestos fibers. Suitable asbestos 
fibers are also well known in the art and include the crocidolite and 
chrysotile varieties. Particularly suitable are mixtures of the Hooker 1 
and Hooker 2 fibers, and preferably equal weight mixtures of these fibers. 
The asbestos fiber slurry is modified by the addition thereto of a valve 
metal oxide, an additive and a water soluble solvent. The valve metal 
oxide and solvent are first combined as a dispersion and subsequently 
added to the asbestos slurry. which also contains the additive. This 
insures complete dispersion of the components in the slurry. 
The valve metal oxide is in particulate or finely divided form, and is 
preferably a pigment grade material. For purposes of this invention, the 
term "valve metal" includes titanium, zirconium, hafnium, niobium, 
tantalum and tungsten, or mixtures of any of these materials. These metal 
oxides are electrical insulators and will not interfere with electrical 
processes occurring within the cell. The preferred valve metal oxide is 
titanium dioxide. 
Any alkanol such as methanol, ethanol and propanol, including both branch 
and straight chain varieties, both substituted and unsubstituted, can be 
used as the solvent in the practice in this invention, the only provision 
being that the alkanol must be soluble in water and should be capable of 
thoroughly wetting the valve metal oxide and the asbestos fibers. 
A particularly preferred alkanol is isopropanol. Isopropanol is capable of 
readily dispersing titanium dioxide and is also effective in thoroughly 
wetting the asbestos fibers to form a complete and uniform dispersion of 
the titanium dioxide within the fiber matrix. It has been found that the 
use of such a solvent is essential to the practice of this invention since 
its omission results in a lack of bonding of the titanium dioxide to the 
asbestos fibers as illustrated in Example 4 below. In the absence of such 
a solvent, the titanium dioxide is exceedingly difficult to disperse, and 
upon depositing the diaphragm onto the cathode, does not adhere to the 
asbestos. 
The additive, preferably present in powder or fibrous form, includes a 
variety of polymeric and inorganic materials such as silicon dioxide, 
polyvinyl chloride, polyethylene, polypropylene, polytetrafluoroethylene, 
poly(ethylene chlorotrifluoroethylene), chlorinated polyvinyl chloride, 
chlorinated propylene, calcium carbonate and sodium chloride. 
Polytetrafluoroethylene and poly(ethylene chlorotrifluoroethylene) are 
sold under the trademarks "Teflon" and "Halar", respectively. The 
additive, when used in powdered form, typically has an average particle 
size of 0.2 to 5.0 microns. 
A wetting agent can be suitably added to the dispersion for improved 
wetting of the asbestos fibers. Typical wetting agents include the Triton 
products, which are manufactured and sold by the Rohm & Haas Corp. A 
particularly suitable wetting agent is Triton X-100, which is a non-ionic 
octyl phenoxy polyethoxy ethanol compound. Although such wetting agents 
are generally effective for wetting the asbestos fibers, they are not 
effective in wetting the valve metal oxide particles, and therefore, must 
be employed in combination with a solvent such as isopropanol which 
possesses this capability. 
The amounts of the individual components required to achieve the beneficial 
results of this invention are not critical, and can vary within wide 
limits. Preferably, the amount of valve metal oxide employed is in the 
range of from about 0.5% to about 3%, based on the weight of asbestos. The 
amount of additive employed is in the range of from about 2% to about 5%, 
also based on the weight of asbestos. 
After the slurry has been prepared and thoroughly mixed, a cathode can be 
immersed threin and a vacuum applied through the cathode chamber to draw 
the fibers onto the cathode surface. The diaphragm-deposited cathode can 
then be removed from the slurry, dried and heated at a temperature of at 
least about 100.degree. C., and preferably in the range of from about 
100.degree. C. to about 400.degree. C., for a sufficient time to cure the 
diaphragm. Curing occurs when the asbestos fibers are firmly bound 
together to form an adherent and dimensionally stable structure, and is a 
function of the duration of the heat treatment and temperature employed. 
A particular advantage of this invention is that a lower baking temperature 
can be employed then has been generally found necessary in the prior art. 
This assists in preventing damaging warpage of the cathode. In this 
manner, a diaphragm typically having a thickness of from about 30-125 mils 
can be obtained. 
While the process of the present invention is primarily useful for 
preparing diaphragms for electrolytic cells, and particularly chlor-alkali 
cells, a variety of other useful articles can also be prepared following 
the procedure described herein, as will be readily understood by those 
skilled in the relevant art. These other articles include filters, mats 
and cords, as well as other porous structures formed from asbestos fibers 
by heating fibers which have been at least partially coated with a 
dispersion of a valve metal oxide in at least one solvent capable of 
wetting the valve metal oxide and asbestos fibers. The process of this 
invention is particularly useful for preparing such articles which are 
subject to high temperature conditions of use, since the inorganic binder 
does not decompose or degrade under such conditions. 
The following examples are intended to further illustrate various 
embodiments of the present invention without limiting it thereby. 
EXAMPLE 1 
An asbestos slurry was prepared by mixing equal parts of Hooker #1 asbestos 
fiber and Hooker #2 asbestos fiber in a mixing tank containing cell liquor 
(average concentration about 150 gpl NaOH). Halar powder was added to the 
slurry and adjusted to a concentration in the range of from about 3.75% to 
about 4.20% by weight of asbestos. Approximately 10%, by weight of 
asbestos, of a 0.5% by weight solution of Triton X-100 wetting agent 
(trademark of Rohm & Haas Corp. for a non-ionic octyl phenoxy polyethoxy 
ethanol surfactant) was added to the slurry. 
This slurry was thoroughly mixed, deposited onto a series of cathodes under 
vacuum and dried for about 2 hours under vacuum. The cathodes were then 
placed in an oven and heated to 100.degree. C.-120.degree. C. for 2 hours. 
The oven temperature was then raised to 240.degree. C. and held for one 
hour at this temperature to cure the cathodes. The oven was then allowed 
to cool to ambient temperature. 
The cathodes were then installed in a series of electrolytic chlor-alkali 
H-4/75 diaphragm cells, each cell containing 75 pairs of anodes and 
cathodes. After 50 days of operation at an average current of 150 KA and 
160 gpl caustic, an average cell voltage of 3.90 volts and a caustic 
current efficiency of 95.5% were recorded. After 187 days of operation 
under these conditions, a caustic current efficiency of 88.4% was 
recorded. 
EXAMPLE 2 
An asbestos slurry was prepared following the procedure of Example 1. To 
this slurry was added a dispersion of approximately equal parts by weight 
of titanium dioxide powder and isopropyl alcohol at a concentration level 
for each component of from about 0.5% to about 1.5% by weight of asbestos. 
Approximately 10%, by weight of asbestos, of a 0.5% by weight solution of 
Triton X-100 was added to the slurry. 
This slurry was thoroughly mixed, deposited onto a series of cathodes under 
vacuum and dried for about 2 hours under vacuum. The cathodes were then 
placed in an oven and heated to 100.degree. C.-120.degree. C. for 2 hours. 
The oven temperature was then raised to 240.degree. C. and held for one 
hour at this temperature to cure the cathode. The oven was then allowed to 
cool to ambient temperature. 
The cathodes were then installed in a series of electrolytic chlor-alkali 
H-4/75 diaphragm ells, each cell containing 75 pairs of anodes and 
cathodes. After 50 days of operation at an average current of 150 KA and 
160 gpl caustic, an average cell voltage of 3.57 volts and a caustic 
current efficiency of 96% were recorded. After 187 days of operation under 
these conditions, a caustic current efficiency of 91.4% was recorded. 
EXAMPLE 3 
A series of electrolytic chlor-alkali H-4/75 diaphragm cells were operated 
as described in Examples 1 and 2, with one series of cells containing 
cathodes prepared as in Example 1 (without TiO.sub.2) and the other series 
containing cathodes prepared as in Example 2 (with TiO.sub.2). After 200 
days of operation at an average current of 165 KA and 160 gpl caustic, the 
cathodes were removed from the cells and visually examined. The cathodes 
prepared according to Example 1 were swollen, while the cathodes prepared 
according to Example 2 did not swell. 
Although various embodiments of this invention have been shown and 
described in the specification, this invention is intended to be construed 
liberally and not limited by any specific embodiments as will be readily 
appreciated by those skilled in the art. It is to be understood, 
therefore, that the appended claims are intended to cover all 
modifications and variations which are within the spirit and scope of the 
present invention.