A method of producing a crosslinked polyvinyl alcohol structure, such as a battery separator membrane or electrode envelope is described. An aqueous solution of a film-forming polyvinyl alcohol is admixed with an aldehyde crosslinking agent a basic pH to inhibit crosslinking. The crosslinking agent, perferably a dialdehyde such as glutaraldehyde, is used in an amount of from about 1/2 to about 20% of the theoretical amount required to crosslink all of the hydroxyl groups of the polymer. The aqueous admixture is formed into a desired physical shape, such as by casting a sheet of the solution. The sheet is then dried to form a self-supporting film. Crosslinking is then effected by immersing the film in aqueous acid solution. The resultant product has excellent properties for use as a battery separator

Hsu, L. C.

Philipp, W. H.

Sheibley, D. W.

English

NASA Technical Reports Server

United States Patent [191 
Philipp et al. 
L 
[111 4,262,067 
[45] Apr. 14, 1981 
[54] IN-SITU CROSS LINKING OF POLYVINYL 
ALCOHOL 
[75] Inventors: Warren H. Philipp, North Olmsted; 
Li-Chen Hsu, Westlake; Dean W. 
Sheibley, Sandusky, all of Ohio 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 113,014 
[22] Filed: Jan. 18,1980 
Related U.S. Application Data 
[63] Continuation of Ser. No. 971,475, Dec. 20, 1978, aban- 
doned. 
[51] Int. Cl .3  ......................... C08F 8/28; HOlM 2/00, 
HOlM 2/16; HOlM 2/18 
[52] U.S. Cl. ...................................... 429/139; 429/27; 
429/28; 429/249; 429/253; 264/104; 264/105; 
525/61 
[58] Field of Search .................. 525/61; 264/104, 105; 
429/27, 28, 253, 139, 249 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,169,250 8/1939 Izard ........................................ 526/7 
3,034,999 5/1962 Wilson ..................................... 526/7 
Primary Examiner-Stanford M. Levin 
Attorney, Agent, or Firm-Norman T. Musial; John R. 
Manning; James A. Mackin 
r571 ABSTRACT 
A method of producing a cross-linked polyvinyl alco- 
hol structure, such as a battery separator membrane or 
electrode envelope. An aqueous solution of a film-form- 
ing .polyvinyl alcohol is admixed with an aldehyde 
cross-linking agent at basic pH to inhibit cross-linking. 
The cross-linking agent, preferably a dialdehyde such as 
glutaraldehyde, is used in an amount of from about $ to 
about 20% of the theoretical amount required to cross- 
link all of the hydroxyl groups of the polymer. The 
aqueous admixture is formed into a desired physical 
shape, such as by casting a sheet of the solution. The 
sheet is then dried to form a self-supporting film. Cross- 
linking is then effected by immersing the film in aqueous 
acid solution. The resultant product has excellent prop- 
erties for use as a battery separator, including a low 
electrical resistivity, the value of which is principally 
determined by the amount of cross-linking which is 
easily and closely controlled by the amount of cross- 
linking agent added to the polymer. 
7 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19810015722 2020-03-21T09:52:41+00:00Z
4.262,067 
1 
IN-SITU CROSS LINKING OF POLYVINYL 
ALCOHOL 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
ees of the United States Government and may be manu- 
factured and used by or for the Government for govern- 
mental purposes without the payment of any royalties 
thereon or therefor. 
This is a continuation of application Ser. No. 971,475, 
filed Dec. 20, 1978, now abandoned. 
FIELD OF THE INVENTION 
The present invention relates to a polyvinyl alcohol 
structure and a method for producing such a structure 
and, more specifically, to a method for the in-situ self 
cross-linking of polyvinyl alcohol which is particularly 
useful in producing battery separator films. 
BACKGROUND OF THE INVENTION 
The use of polyvinyl alcohol as a separator material 
in alkaline batteries is well known. A standard electro- 
lyte for alkaline batteries is concentrated aqueous potas- 
sium hydroxide. Although polyvinyl alcohol is soluble 
in water, it is not readily dissolved in such a concen- 
trated potassium hydroxide solution, and thus, films of 
this material can be used in such electrolytes. In general, 
polyvinyl alcohol films have been utilized as battery 
separators because of their high conductivity in alkaline 
electrolytes and their ease of fabrication from aqueous 
solution. 
Efforts have been made to improve the mechanical 
properties and chemical stability of polyvinyl alcohol 
by synthesizing numerous derivatives of the polymer. 
However, the number of processes for the in-situ reac- 
tion of prefabricated polyvinyl.alcoho1 structures, such 
as battery separators, is limited. The in-situ acetalization 
of polyvinyl alcohol separators has been achieved 
through treatment of the separators with acid solutions 
of aldehydes, such as an aqueous formaldehyde solution 
containing a small amount of sulfuric acid. The alde- 
hyde acetalates the 1,3-diol units present in the polymer 
thereby creating tough, water insoluble derivatives. In 
many of the aldehyde treatments, the extent of acetali- 
zation is difficult to control. Extensive acetalization of 
polyvinyl alcohol leads to poor conductivity in alkaline 
electrolyte, which is probably due to the decreased 
number of hydrophilic alcohol groups resulting from 
the using up of such groups by the acetalization reac- 
tion. 
It is an object of the present invention to provide a 
method of making cross-linked polyvinyl alcohol struc- 
tures in which the extent of cross-linking is easily con- 
trolled. It is a further object to provide cross-linked 
polyvinyl alcohol structures having low electrical resis- 
tivity suitable for use as a separator or electrode enve- 
lope in an alkaline battery. 
BRIEF SUMMARY OF THE INVENTION 
The foregoing and other objects which will be appar- 
ent to those having ordinary skill in the art are achieved 
in accordance with the present invention by providing a 
method of producing a cross-linked polyvinyl alcohol 
element which includes the steps of: providing an aque- 
ous admixture of a film-forming polyvinyl alcohol resin 
and an aldehyde cross-linking agent capable of cross- 
linking the polyvinyl alcohol resin, the aldehyde being 
present in an amount of from 0.5 to 20% of the theoreti- 
cal amount required to react with all of the hydroxyl 
groups of the polyvinyl alcohol resin, the aqueous ad- 
mixture having a basic pH to inhibit a cross-linking 
reaction between the aldehyde and the resin; forming 
the aqueous admixture into a desired physical shape; 
drying the shaped admixture to form a self-supporting 
polymeric article containing the cross-linking agent; 
1o and contacting the polymeric article with an aqueous 
acid catalyst solution to effect the cross-linking reaction 
between said resin and said aldehyde to produce a cross- 
linked polyvinyl alcohol element. 
Other features and advantages of the invention will 
15 be apparent from the description of preferred embodi- 
ments which follows. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
20 The polyvinyl alcohol resin useful in the invention is 
a conventional, film-forming, polymeric material 
readily available commercially and can be of the type 
used heretofore for making separators for alkaline bat- 
teries. Commercially available polyvinyl alcohol nor- 
25 mally contains about 0.5 to 6% of 1,Zdiol units in the 
predominantly 1,3-diol structure. The presence of 1,2- 
diol units in the chain results in reduced resistance to 
oxidation. Accordingly, it is preferred, in the practice of 
3o the present invention, to use a polyvinyl alcohol sub- 
stantially free (Le. containing less than about 5%) of 
1,Zdiol units. Polyvinyl alcohol having an essentially 
head-to-tail structure of this type is readily available. It 
is made by the alcoholysis of polyvinyl acetate having 
35 an essentially head-to-tail structure which is made by 
low temperature polymerization of vinyl acetate. In the 
event that a polyvinyl alcohol having a substantial 
amount of 13-diol units is employed, of if it is desired to 
reduce or eliminate the amount of 1,Zdiol units that 
40 may be present in the polyvinyl alcohol, then it is pre- 
ferred to contact the present polymer film with an oxi- 
dizing agent (e.g. periodic acid) prior to effecting cross- 
linking, as disclosed in our co-pending application Ser. 
No. 897,829, filed Apr. 19, 1978, now U.S. Pat. No. 
45 4,154,912, issued May 15, 1979, the disclosure of which 
is herein incorporated by reference. The polyvinyl alco- 
hol is a film-forming resin having, generally, a number 
average molecular weight of 10,000-200,000 and is con- 
veniently provided and used in aqueous solution such as 
10-20% resin by weight. 
The cross-linking agent is an aldehyde and preferably 
a dialdehyde. Suitable dialdehydes include glyoxal and 
other dialdehydes which may be represented by the 
55 formula 
0 0  
II II 
HC-R-C-H 
60 
wherein R is a divalent organic radical such as an ali- 
phatic, cycloaliphatic, aromatic, and heterocyclic radi- 
cal. Preferred dialdehydes include glutaraldehyde and 
other aliphatic ‘dialdehydes wherein R is a divalent 
65 aliphatic hydrocarbon radical having from 1 to 12 car- 
bon atoms, preferably 1 to 6 carbon atoms, and furan 
aldehydes, e.g. furan dialdehyde, represented by the 
formula: 
3 
4,262,067 
4 
where x and y are each 0 or 2. Other aldehydes, how- 
ever, may be used, for example, the polyaldehyde- 
polysaccharide compounds disclosed in our co-pending 
application Ser. No. 145,271 filed Apr. 30, 1980, entitled 
"Cross-Linked Polyvinyl Alcohol and Method of Mak- 
ing Same", the disclosure of which is herein incorpo- 
rated by reference. In general, the aldehyde is one 
which forms a homogeneous admixture with the polyvi- 
nyl alcohol polymer. 
The aldehyde cross-linking agent is preferably pro- 
vided as an aqueous solution or dispersion for ease of 
mixing with the polyvinyl alcohol which is also prefera- 
bly provided in aqueous solution. In any event, the 
polymer and cross-linking agent are admixed, prefera- 
bly in aqueous solution. The cross-linking agent is used 
in an amount of 0.5 to 20% of the theoretical amount 
required to cross-link all of the available hydroxyl 
groups of the polymer. As will be apparent from the 
Examples which follow, the electrical resistivity of the 
cross-linked polymer is dependent upon the amount of 
cross-linking agent admixed. Since low electrical resis- 
tivity is desired, a minimum amount of cross-linking 
agent is preferred provided, of course, that the desirable 
physical properties achieved by cross-linking are main- 
tained. Accordingly, it is preferred to employ about 
0.5% to 5% or 0.5% to 10% (of theoretical) of the 
cross-linking agent. Since the amount of agent added is 
closely controlled by the simple expedient of weighing 
or measuring the volume of the agent added, precise 
control of cross-linking in the product is readily 
achieved. 
The admixture of resin and cross-linking agent is 
preferably first formed into a self-supporting structure, 
such as a film having a thickness suitable for a particular 
intended use. For example, an aqueous solution of the 
admixture may be cast into a sheet having a dry thick- 
ness of 50 to 400 microns suitable for use as a separator 
for an alkaline battery. Cross-linking is then effected by 
contacting the sheet material with an acid catalyst such 
as a 1% aqueous sulfuric acid solution containing a 
dissolved salt, such as ammonium sulfate, present in an 
amount sufficient to substantially prevent dissolution of 
the non-cross-linked polymer in the acid solution. The 
reaction system is preferably heated to accelerate the 
reaction. A temperature in excess of 40" C., preferably 
50"-80" C. is suitable. A reaction time of several hours 
is generally sufficient. 
The acid catalyst can be any acid such as sulfuric 
acid, hydrochloric acid, etc., suitable to catalyze the 
cross-linking reaction between the aldehyde-containing 
cross-linking agent and the hydroxyl-containing poly- 
mer. An aqueous acid solution is preferred. Various 
salts, such as sodium sulfate, potassium sulfate, or the 
like, can be used to prevent dissolution of the non-cross- 
linked polymer. 
In making battery separators, the sheet or other self- 
supporting structure containing the admixed resin and 
cross-linking agent can be cut or otherwise treated, 
prior to effecting cross-linking, to provide a physical 
cated by sealing the edges of superposed sheets. Sealing 
is readily accomplished by heat sealing or by suitable 
adhesive. A heat sealing temperature of about 200" C. is 
appropriate. A preferred adhesive is a solution of ad- 
5 mixed resin and cross-linking agent similar to that from 
which the film was cast. 
While a sheet thickness of 50-400 microns, preferably 
100-200 microns, is customary for use as separators or 
electrode envelopes in conventional alkaline batteries, 
lo the sheet structures may be substantially thinner, e.g. 10 
microns, and substantially thicker, e.g. - inch, for more 
specialized applications. 
The invention is further illustrated in the Examples 
which follow. 
15 
EXAMPLE 1 
To four separate batches of 10% aqueous poly(viny1 
alcohol) solutions (Polyscience Inc. viscosity 55-65 cp. 
2o for 4% resin solids in water) is added the specified quan- 
tity of 50% aqueous glutaraldehyde to make solutions 
containing the following glutaraldehyde concentrations 
based on the amount of poly(viny1 alcohol) present: 
29%, 5%, lo%, and 20%. In order to prevent prema- 
25 ture acetalization, the casting solution is made alkaline 
to litmus paper by addition of several drops of concen- 
trated aqueous ammonia. To make the films, the well 
mixed viscous solution is cast on a smooth clean glass 
surface with the aid of a Gardner knife and the cast 
30 sheet is air dried at room temperature. The dried film 
having a thickness of about 150 to 180 microns thick is 
easily removed from the glass. To effect cross-linking, 
the film is immersed in the following solution at about 
70" C. for about 15 minutes: 
35 Water: 1 liter 
conc. H2SO4: 20 ml 
Na2S04: 200 grams 
Following this treatment, the film is immersed in about 
1% HCl at about 70" C. for about one half hour to 
40 complete the cross-linking reaction. The cross-linked 
film, which is no longer water soluble, is washed with 
water, thenimmersed in dilute sodium bicarbonate solu- 
tion to neutralize absorbed acid, and finally rinsed with 
water again to remove salts. The treated film is air dried 
45 at room temperature. The resistivities of these cross- 
linked films at several glutaraldehyde concentrations is 
"shown in the table below. 
50 RESISTIVITY OF FILMS IN 45% KOH 
Resistivity 
Film Glutaraldehyde Microns ohm-cm2 
Percent Film Thickness 
A 20 
B 10 
170 greater than 100 
160 2.12 
5 160 0.89 
21 160 0.49 
For high conductivity in alkaline electrolyte, a low 
degree of cross-linking is more desirable. Accordingly, 
EXAMPLE 2 
60 film D is the best in this respect. 
Use of low molecular weight instead of high molecu- 
lar weight Dolv(vinv1 alcohol). - - - .  
configuration suitable for an intended use such as a 65 
battery separator. For example, a film dried to a thick- 
ness of 50-400 microns can be cut into sheets of appro- 
priate size (e.g. Y ' X 4 " )  and envelope bags can be fabri- 
To two separate batches of 20% aqueous poly(viny1 
alcohol) solution having a lower molecular weight than 
that of Example 1, (Polysciences Inc. viscosity 4-6 (ps., 
4% in water) is added the specified quantity of 50% 
4,262,067 
5 
aqueous glutaraldehyde to make solutions containing 
the following glutaraldehyde concentrations based on 
the amount of poly(viny1 alcohol) present: 2$% and 
the polyvinyl alcohol resin, said aldehyde being a dial- 
dehyde having the formula 
10%. The casting and treatment is as described in Ex- 
ample 1. The resistivities of the cross-linked poly(viny1 5 II II 
0 0  
H-C-R-C-H alcohol) films is shown in the following table. 
wherein R is a divalent organic radical having from 1 to 
12 carbon atoms, said aldehyde being present in an 
amount required to react with from 0.5 to 20% of all the 
Film Glutaraldehyde Microns ohm-cm2 hydroxyl groups of the polyvinyl alcohol resin, said 
A 10 170 1.33 aqueous admixture having a basic pH to inhibit a cross- 
linking reaction between the aldehyde and the resin; 
forming said aqueous admixture into a sheet drying the 
As is the case in Example 1, high conductivity in alka- 15 shaped admixture to form a self-supporting polymeric 
line electrolyte is favored by a low degree of cross-link- sheet containing said cross-linking agent; and contact- 
ing. For both the high and the low molecular weight ing said polymeric sheet with an aqueous acid catalyst 
poly(viny1 alcohol) films of Examples 1 and 2, good solution to effect the cross-linking reaction between 
mechanical properties in concentrated alkaline electro- said resin and said aldehyde to produce a cross-linked 
lyte were found down to 4% by weight glutaraldehyde 20 polyvinyl alcohol separator. 
in the poly(viny1 alcohol) films, and the conductivity of 2. A battery according to claim 1 wherein said poly- 
such films is high. vinyl alcohol has a number average molecular weight of 
from 10,000 to 200,000 and is provided in an aqueous 
solution. EXAMPLE 3 
3. A battery according to claim 1 wherein R is a 
lyscience Inc. viscosity 55-65 cps., 4% in water) was divalent aliphatic hydrocarbon radical. 
added about 10% by weight (based on the amount of 4. A battery according to claim 3 wherein said dialde- 
polyvinyl alcohol) of furan dialdehyde (Quaker Oats). hyde is glutaraldehyde. 
Dried film having a thickness of about 150p is prepared 5. A battery according to claim 3 wherein said dial&- 
and processed as described in Example 1. Electrical 30 hyde has the formula 
resistivity of the cross-linked film is 2.997 ohms-cm2. 
RESISTIVITY OF FILM IN AQUEOUS 45% KOH 
Percent Film Thickness Resistivity 
B 21 170 0.46 
To a 10% aqueous solution of polyvinyl alcohol (PO- 25 
6 
(CH2)Y-CH0 
EXAMPLE 4 
The same procedure as in Example 3 is followed OHC-(CH2), 
except that the amount of furan dialdehyde added is 7% 35 0 
based on the weight of the polyvinyl alcohol, and the 
polyvinyl alcohol used has a viscosity of 4 cps. (4% in 
water). The film is 150 microns thick and has an electri- 
cal resistivity of 0.170 ohms-cm2. 
wherein x and y are each 0 or 2. 
6. A battery according to claim 5 wherein x and y are 
each 0. 
7. A battery according to claim 1 wherein two of the 
sheets are superposed and joined at their edges to form 
an envelope bag for an electrode of an alkaline battery 
prior to effecting said cross-linking reaction. 
What is claimed is: 
1. An alkaline battery having a cross linked polyvinyl 
alcohol separator made by providing an aqueous admix- 
ture of a film-forming polyvinyl alcohol resin and an 
40 
aldehyde cross-linking agent capable of cross-linking * * * * *  
45 
50 
55 
60 
65 


In-situ cross linking of polyvinyl alcohol

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