A membrane that is insoluble in an aqueous alkali metal hydroxide medium is described. The membrane is a resin which is a water-soluble C2-C4 hydroxyalkyl cellulose ether polymer and an insolubilizing agent for controlled water sorption, a dialytic and electrodialytic membrane. It is particularly useful as a separator between electrodes or plates in an alkaline storage battery

Hoyt, H. E.

Pfluger, H. L.

English

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(NASA-Case-XGS-05584-1) AQUEOUS ALKALI
METAL HYDROXIDE INSOLUBLE CELLULOSE ETHER
MEMBRANE Patent (NASA) 3 p CSCL 07D
00/25
N82-29370
Unclas
22767
https://ntrs.nasa.gov/search.jsp?R=19820021494 2020-03-21T17:10:32+00:00Z
United States Patent Office Patenled
Y
3,442,674
AQUEOUS ALKALI METAL HYDROXIDE INSOLU-
BLE CELLULOSE ETHER MEMBRANE
Howard E. Hoyt and Helmuth L. Pfluger, Huntingdon
Valley, Pa., assignors to The Bordea Company, New
York, N.Y., a corporation of New Jersey
No Drawing. Filed Apr. 6, 1965, Ser. No. 446,071
Int. Cl. C08b 21/26; HOlm 3/02
U.S. Cl. 106—197 6 Claims
ABSTRACT OF THE DISCLOSURE
This invention relates to a membrane that is insoluble
in an aqueous alkali metal hydroxide medium, said mem-
brane comprising a resin which is a water-soluble C2-Q
hydroxyalkyl cellulose ether polymer and an insolubiliz-
ing agent for controlled water sorption, a dialytic and
electrodialytic membrane and is particularly useful as a
separator between electrodes or plates in an alkaline stor-
age battery and will be described in connection therewith.
The invention described herein was made in the per-
formance of work under a NASA contract and is subject
to the provisions of section 305 of the National Aeronau-
tics and Space Act of 1958, Public Law 85-568 (72 Stat.
435;42U.S.C.2457).
Generally, in electric storage batteries of this type, one
electrode is silver oxide, the other is zinc or cadmium, and
the medium providing the bath between the electrodes is
an aqueous solution of potassium hydroxide. There may
be used for some purposes other alkali metal hydroxides,
e.g., sodium or lithium hydroxide. The zinc or cadmium
may be replaced by other metals that have a valence of
at least 2 and are above cadmium in the electromotive
series of the chemical elements, examples being aluminum,
manganese and chromium. In such batteries it is neces-
sary to have low electrical resistance of the membrane.
It is a problem in batteries of this type, particularly when
used for space exploration, to obtain membranes of ade-
quate strength and stability when in contact with the alkali
and silver oxide without the membrane being excessive in
weight.
In our co-pending application, Ser. No. 396,716, filed
Sept. 15, 1964, now abandoned, we have therein disclosed
a battery separator membrane comprising a film which
is a combination or solid solution of a cellulose ether such
as methyl cellulose together with a resin which is either
a polymer with dependent carboxyl groups such as sodium
carboxy methyl cellulose or a polymer or copolymsr of
an alpha-unsaturated aliphatic acid. We have found that,
although the membrane of the co-pending application has
excellent conductivity and its tensile strength is superior
to that of cellophane membranes which have previously
been used in alkaline batteries, a separator membrane is
required having still further improved tensile strengths.
This property of higher tensile strength is especially needed
in the course of assembling battery cells. The dry mem-
brane should be capable of being wrapped around the
electrodes without suffering physical damage.
The present invention provides a membrane that ab-
sorbs sufficient alkali solution to make sufficiently low the
electrical resistance but is of very high tensile strength,
even after immersion in and long contact with 25%—45%
potassium hydroxide solution, said solution having silver
oxide dispersed and dissolved (herein. This is the corro-
10
15
20
35
•ID
50
GO
65
sive environment to which the membrane is exposed when
the battery is in operation.
Briefly stated, the invention provides a membrane which
may also be referred to as a film or sheet, wherein said
membrane comprises a resin that is a cellulose ether poly-
mer with dependent hydroxyalkyl groups. The resin may
be further combined with insolubilizing agents in order
to prevent solutions of membranes made with said resin
when the alkaline concentration of the battery cell is
lowered to and below the threshold concentration at
which the resin used to make the membrane is soluble.
An example of a resin and insolubilizing agent that in
combination illustrates one embodiment of the present
invention is hydroxyethyl cellulose and methyl cellulose.
Using this combination of insolubilizing agent and
resin, for instance, we have obtained membrane strengths
from about 120% to 780% of the strength given by the
control membrane consisting of regenerated cellulose
sheeting (cellophane). Strength tests were conducted on
membranes after immersion for 72 hours in a 30% potas-
sium hydroxide solution which had been saturated with
silver oxide (Ag2O). Our membranes also are satisfactori-
ly low in electrical resistance, all provided that the critical
proportions hereinafter described are used in the mem-
brane.
As to the resin, the lower hydroxyalkyl cellulose poly-
mers are best suited and €2.4 hydroxyalkyl cellulose ethers
are preferred. Examples that illustrate the class to be used
are hydroxy- ethyl, iso- and n-propyl and butyl cellulose.
The degree of replacement of hydrogen atoms of the
hydroxyl groups by hydroxyalkyl groups is between about
.8 and 1.5 per glucose unit in ths cellulose molecule.
As to the insolubilizing agent, lower alkyl cellulose
ethers are preferred. Methyl and ethyl cellulose are
satisfactory, particularly the methyl ether having a degree
of substitution by CH3 groups or if a higher alkyl ether
is used, the corresponding alkyl radical, corresponding
to replacement of 1.0-2 H's of the OH groups, per glucose
unit in the cellulose.
The silver oxide used as one electrode is of grade and
in form that is conventional in the commercial alkali bat-
teries, there being some uncertainty as to whether the
compound that is active in the battery function is silver
oxide (Ag2O) or silver peroxide (AgO). Ag2O is used
throughout as representative of the oxide.
As to the proportions, when an insolubilizing agent is
used, the ratio of the agent to the resin is dependent upon
the concentration of alkali in the battery cell. Insolubilizing
agents of the lower alkyl cellulose ether type and resins of
the lower alkyl hydroxy cellulose type are separately
soluble in water. Further, lower alkyl hydroxy cellulose
resins are soluble in alkali solutions in concentrations up
to about 40%. It has therefore been found necessary
to add increasing proportions of insolubilizing agents to
the resin composition as the alkali concentration of the cell
is reduced below 40%. If insufficient quantities of said
agent were to be used in the membrane composition for a
specific alkali solution strength, the membrane would dis-
solve and therefore be useless for its intended purpose.
Generally, up to 100% resin can be used at alkali concen-
trations of. 40% and greater, and proportionately less
resin as the concentration is reduced.
The following table shows proportions of ingredients
at various alkali solution concentrations. The proportions
are shown as illustrative of the proper range for best
results. It is understood that the ranges given in the
3,442,674
Table are by way of example and not by way of limitation,
the only limitation upon the proportions being the mini-
mum quantity of insolubilizing agent that will keep the
lower alkyl hydroxy cellulose membrane 'from dissolving
into the alkaline battery solution at the particular alkali
concentration. Proportions here and elsewhere herein are
expressed as parts by weight on a dry 'basis unless specifical-
ly stated to the contrary.
TABLE i
Percent alkali
For 100 parts total membrane 10
composition
Resin Insolubilizing agent
Over 40...
35-40..:..
30-35
Up to 30..
100 .
80-100
45-80
1-45
20-0
55-20 15
99-55
By way of example of the deterioration that occurs if
insufficient insolubilizing agent is used, a membrane of
proportion of 50 parts hydroxyethyl cellulose and 50 parts
of methyl cellulose was prepared and immersed in 30%
potassium hydroxide. After 72 hours there was a signi-
ficant falling off in strength of the membrane, indicating
partial solubility in the alkaline solution.
If rib fesih is used in the membrane composition, the
conductance as well as the strength falls off appreciably.
In one example of such a system the ohms of resistance of
a methyl cellulose membrane, after the 72 hours in an
alkali solution, was objectionably high, actually 21 ohms
in 30% KOH and 507 ohms in 40% KOH.
Alkaline storage batteries in which the membranes of
this invention have utility generally operate with an alkali
solution concentration in the range of between about
25%-45%. Concentrations outside this range are per-
missible, however, the efficiency of the battery falls off.
The usually preferred alkali concentration is between about
28%-42%.
• Important to note (see Table 2) is the excellent resist-
20
25
30
35
solution and then drying to form the cast. Alkaline solu-
tions should not be so concentrated as to precipitate the
components before drying.
The invention will be further illustrated by descrip-
tion in connection with the following specific examples of
the practice of it, the proportions here and elsewhere here-
in are expressed as parts by weight unless specifically stated
to the contrary.
Example 1
700 grams of a 10% methyl cellulose solution (Metho-
cel 15) and 300 grams of 10% hydroxyethyl cellulose
(Cellosize QPO9) were completely mixed at 5° C. The
ratio of methyl cellulose to hydroxyethyl cellulose was
7:3. Films were cast on levelled plate'glass by the draw-
down procedure using a 20 mil clearance on the doctor
blade. Clear, homogeneous films of about 1.5 mil thick-
ness were obtained.
By similar process films were made in which the ratios
of hydoxyethyl cellulose to methyl cellulose were re-
spectively, 1:9, 2.8, 3.5:6.5 and 5:5. The 5:5 and great-
er ratios are considered unsatisfactory for use in 30%
KOH as the film disintegrated or dissolved in KOH solu-
tion of this concentration.
Samples of films were tested for tensile strength after,
(1) conditioning at 50% relative humidity (R.H.) and
73° F. for one week, (2) immersion for 72 hours, in 30%
KOH solution and, (3) immersion for 72 hours in 30%
KOH solution saturated with silver oxide. There was
continuous stirring in the last mentioned solution: In
each series, the conditioned membranes were blotted, to
remove surface solution, and immediately tested on the
tensile strength machine.
Table 2 tabulates the results obtained with films of
this invention, in comparison with each other, with a
cellophane (control) membrane of the prior art and with
membranes made separately from each constituent
component.
TABLE 2
Parts of hydroxyethyl cellulose for 100 parts membrane
Proportions
Tensile strength at 50% R.H. and
73° F., p.s.i
Flex test MIT ' at 50% R.H. and
73° F cycles
Tensile strength after:(1) 72hrs. In 30% KOH, p.s.l...(2) 72 hrs. In AgjO-30% KOH
Absorption, percent of orig. wt.:(1) In 30% KOH(2) In 40% KOH
Resistance * of film, ±2 ohms:(1) Soaked 30% KOIT(2) Soaked 40% KOH
100
5,250
720
Sol.
Sol.
Sol.
65
Sol.
0
0
8,340
3.0C5
6,400
5,810
12
12
21
507
10
10, 330
10, 000
39
31
11
41
20
9,720
5,420
6,940
47
35
8
38
30
11, 120
8,073
5,860
5,320
51
55
5
26
35
8,680
3,131
1,090
976
93
57
H
0
Cellophane
control
17, 320
10,000
3,130
457
220
300
0-2
0-2
11 kg. tension on film. .1
 In conductivity cell with .2 Inch diameter opening (resistance of cell plus membrane minus resistance of cell.
ance to the action of silver oxide by the membrane of
the present invention. Notice is taken of the lack of
significant reduction :in tensile strength of the membrane
after exposure 'for 72 hours in 30% KOH solution and
exposure for 72 hours in 30% KOH solution saturated GO
with silver oxide. By way of contrast, an exposure test was
performed with cellophane. There resulted a reduction in
tensile strength of about 85% under the same exposure
conditions.
The membrane may be prepared by combining aqueous Co
solutions Of the hydrOxyalkyl cellulose compound and
when required and in proportion sufficient to impart in-
solubility in the alkali solution, rnethyl cellulose or other
lower alkyl cellulose ether. The resulting composition may
be cast in sheet form and evaporated to dryness. Other 70
forms may be made as for example plates, slabs, "but-
tons," films and the like. When the dried membrane is
introduced into alkaline solution alkali is absorbed. Varia-
tions in the procedure 'include combining alkaline solu-
tions of the components or adding alkali to said aqueous 75
The membranes of columns "0" and "Cellophane Con-
trol" are not a part of the present invention.
The results show generally, the superiority of our
membrane over the control membrane and methyl cellu-
lose membrane, particularly in tensile strength and re-
sistance measurements. Not tabulated is the fact that the
silver oxide-KOH solution, having substantial oxidizing
power, rendered the cellophane membrane opaque by a
heavy coating of .silver oxide, whereas the films of this
invention, while discolored, retained their transparency.
Also is the fact that, surprisingly, our membranes are
generally as strong as the methyl cellulose membranes.
A reinforcement action is observed when note is taken of
the weighted tensile strength average of hydroxyethyl
cellulose membranes and methyl cellulose membranes.
That average is less than that of our membranes.
Although life tests on test storage cells are incomplete
at this date, available results indicate that in one test
the cells employing the membranes of column "30" and
"35" in Table 2 underwent 32 and 30 cycles respectively,
3,442,674
of charging and discharging before termination whereas
control cells using battery grade cellophane failed after
15 to 21 cycles.
Example 2
The procedure and composition of Example 1 are
used but with replacement of the hydroxyethyl cellulose
by hydroxyiso- and n-propyl, and hydroxybutyl cellulose
used separately and in turn in the various proportions of
hydroxyalkyl cellulose to the total membrane combination
as shown in Table 1. The films cast and dried are likewise
clear and homogeneous. The test results are similar to
those tabulated in Table 2.
Example 3
The procedure and compositions of Example 1 are
followed in making membranes except that ethyl cellu-
lose replaces methyl cellulose as the insolubilizing agent.
The properties of the resulting membranes, with propor-
tions varying as in Table 1, are similar to those tabulated
in Table 2. The ethyl cellulose is water soluble.
The films of this invention may also be prepared from
an organic solvent in place of the aqueous solvent. The
organic solvent is a solvent for the resin and agent, such
as butyl and higher alkyl alcohols.
10
20
4. A separator membrane of high tensile strength be-
ing insoluble in aqueous alkali metal hydroxide medium,
wherein said medium comprises alkali metal hydroxide in
proportion up to about 45% by weight, comprising up
to 100% by weight of resin which is a water-soluble C2.4
hydroxyalkyl cellulose ether polymer having between
about 0.8 and 1.5 hydroxyalkyl groups per glucose unit
in said cellulose polymer, the difference from said 100%
of said resin being an insolubilizing agent which is a
water-soluble lower alkyl cellulose ether having from
about 1 to about 2 alkyl groups per glucose unit in said
lower alkyl cellulose ether, the proportion of said agent
being increased as the concentration of said metal hy-
droxide is decreased in said medium.
5. The membrane of claim 4 wherein said alkyl group
of said resin is selected from the group consisting of ethyl,
iso- and n-propyl and butyl, and said agent is selected
from the group consisting of methyl and ethyl cellulose.
6. A membrane of high tensile strength being insoluble
in aqueous alkali metal hydroxide medium, said medium
comprising between about 28% and 42% by weight of
said alkali metal hydroxide, consisting essentially of
hydroxyethyl cellulose having between about 0.8 and 1.5
hydroxyethyl groups per glucose unit in said hydroxyethyl
about 2 methyl groups per glucose unit in said
cellulose, in proportion between about 1 and 10 parts of
hydroxyethyl cellulose for 10 parts of total membrane,
the proportion of said hydroxyethyl cellulose being de-
creased as the concentration of said metal hydroxide is
decreased in said medium.
It will be understood that it is intended to cover all 25 ^Uutose ^"i.met!LyL?e!lulOSl.h.aV_ingJ5°m ab?ut l,l°
changes and modifications of the examples of the invention
herein chosen for the purpose of illustration which do not
constitute departures from the spirit and scope of the
invention.
We claim:
1. A membrane of high tensile strength being insoluble
in aqueous alkali metal hydroxide medium, comprising
a resin which is a water-soluble C^ hydroxyalkyl cellu-
lose ether polymer having between about 0.8 and 1.5
hydroxyalkyl groups per glucose unit in said cellulose
polymer, and a water-soluble lower alkyl cellulose ether
having from about 1 to about 2 alkyl groups per glucose
unit in said lower alkyl cellulose ether, said water-soluble
lower alkyl cellulose ether being present in an amount
sufficient to keep the membrane from dissolving into the
aqueous alkali metal hydroxide medium.
2. The membrane of claim 1, wherein said alkyl groups
of said resin is selected from the groups consisting of
ethyl, iso- and n-propyl and butyl.
3. The membrane of claim 1 wherein said resin is
hydroxyethyl cellulose.
4Q
2,763,030
2,810,659
2,988,455
3,057,942
3,091,542
References Cited
UNITED STATES PATENTS
9/1956 Erickson ___________ 106—197
10/1957 Greminger et al ______ 106—197
Rosenberg et al. _____ 106—197
Smith et al __________ 136—179
Anderson __________ 106 — 197
6/1961
10/1962
5/1963
ALLAN LIEBERMAN, Primary Examiner.
45 U.S. Cl. X.R.136—146


Aqueous alkali metal hydroxide insoluble cellulose ether membrane

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