A film-forming polyvinyl alcohol polymer is mixed with a polyaldehyde-polysaccharide cross-linking agent having at least two monosaccharide units and a plurality of aldehyde groups per molecule, perferably an average of at least one aldehyde group per monosaccharide units. The cross-linking  agent, such as a polydialdehyde starch, is used in an amount of about 2.5 to 20% of the theoretical amount required to cross-link all of the available hydroxyl groups of the polyvinyl alcohol polymer. Reaction between the polymer and cross-linking agent is effected in aqueous acidic solution to produce the cross-linked polymer. The polymer product has low electrical resistivity and other properties rendering it suitable for making separators for alkaline batteries

Hsu, L. C.

Philipp, W. H.

Sheibley, D. W.

English

NASA Technical Reports Server

United States Patent 4,272,470 
Hsu et al. WI Jun. 9,1981 
References Cited 
U.S. PATENT DOCUMENTS 
[561 [54] CROSS-LINKED POLYVINYL ALCOHOL AND METHOD OF MAKING SAME 
[75] Inventors: Li-Chen Hsu, Westlake; Dean W. 
Sheibley, Sandusky; Warren H. 
Philipp, North Olmsted, 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. 
(211 Appl. NO.: 145,271 
[22] Filed: Apr. 30, 1980 
Related U.S. Application Data 
[63] Continuation of Ser. NO. 971,473, Dec. 20, 1978, aban- 
doned. 
Int. CI.3  ......................... B29C 25/00; C08J 3/24; 
HOlM 2/16; HOlM 2/18 
U.S. CI. ............................ 264/lq 260/17.4 UC, 
429/27; 429/28; 429/1 39; 429/249: 429/253: 
525/56; s w 6 i  
[58] Field of Surch ................................... 525/56. 61: 
260A7.4 U C  264/104; 429/27, 28,253,249; 
139 
3,034,999 V I 9 6 2  Wilson ..................................... 526/7 
4,086,405 4/1978 Gadessaud ........................... 429/253 
Primary Examiner-Stanford M. Levin 
Attorney, Agent, or Firm-N. T. Musial; J. R. Manning; 
J. A. Mackin 
[571 ABSTRACT 
A film-forming polyvinyl alcohol polymer is mixed 
with a pol yaldehyde-polysaccharide cross-linking agent 
having at least two monosaccharide units and a plurality 
of aldehyde groups per molecule, preferably an average 
of at least one aldehyde group per monosaccharide 
units. The cross-linking agent, such as a polydialdehyde 
starch, is used in an amount of about 2.5 to 20% of the 
theoretical amount required to cross-link all of the 
available hydroxyl groups of the polyvinyl alcohol 
polymer. Reaction between the polymer and cross-link- 
ing agent is effected in aqueous acidic solution to pro- 
duce the cross-linked polymer. The polymer product 
has low electrical resistivity and other properties ren- 
dering it suitable for making separators for alkaline 
batteries. In that event, the mixture of polymer and 
cross-linking agent is formed into a sheet or film or the 
like and the film is cut to size and otherwise fabricated 
into a configuration suitable for a particular end use. 
The cross-linking reaction is then carried out to pro- 
duce the final product. 
6 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19810020622 2020-03-21T09:40:04+00:00Z
4,272,470 
1 2 
readily available commercially and can be of the type 
used heretofore for making separators for alkaline bat- 
teries. As noted in our earlier application Ser. No. 
897,829, filed Apr. 19, 1978, now U.S. Pat. No. 
4,154,912, issued May 15, 1980, commercially available 
CROSS-LINKED POLYVINYL ALCOHOL AND 
METHOD OF MAKING SAME 
ORIGIN OF T H E  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. 971,473 
filed Dec. 20, 1978. now abandoned. 
polyvinyl alcohol normally contains about 0.5 to 6% of 
1,2-diol 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 pre- 
ferred, in the practice of the present invention, to use a 
polyvinyl alcohol substantially free (iSe. containing less 
than about 0.5%) of 1,2-diol units. Polyvinyl alcohol 
having an essentially head-to-tail structure of this type is 
vinyl acetate having 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 1,Zdiol units is em- 
Polyvinyl alcohol is a commercially available poly- 20 ployed, or if it is desired to reduce or eliminate the 
amount of 1,tdiol units that may be present in the poly- 
vinyl alcohol, then it is preferred to contact the present 
polymer film with an oxidizing agent (e.g. periodic 
FIELD OF THE INVENTION 
This invention relates to cross-linked plyvinyl alto- 15 It is made by the Of Poly- 
hol polymers, to 
rators for alkaline batteries made therefrom. 
of making same, and to sepa- 
BACKGROUND OF T H E  INVENTION 
mer which is readily available and relatively inexpen- 
sive. It has been used for preparing battery separators. 
However. known battery separators made of polyvinyl 
alcohol are subject to oxidative degradation similar to acid) prior to effecting cross-linking, as disclosed in our 
that suffered by separators made Of Cellophane* In an 25 co-pending application Ser. No. 897,829, filed Apr. 19, 
alkaline battery, a separator is contacted with, typically, 
a 45 wt*% Of aqueous potassium hydroxide 
1978, now U.S. Pat. No. 4,154,912 issued May 15, 1980, 
the disclosure of which is herein incorporated by refer- containing an oxidizing agent such as silver oxide. This 
environment leads to the oxidative degradation of the 
separators. 
ence. The polyvinyl alcohol is a film forming resin 
3o having, generally, a number average molecular weight 
It is an object of the present invention to provide a 
polyvinyl alcohol material that is resistant to oxidative 
Of 10~ooo-200~ooo and is 
used in aqueous solution 
provided and 
as 10-20% resin by 
degradation and otherwise has properties rendering it 
suitable for use as a separator for an alkaline battery. It The cross-linking agent is a PolYaldehYde-PolYsac- 
is a further object of the invention of providing a 35 charide having at least two monosaccharide units and a 
method of making such polyvinyl alcohol, to provide a plurality of aldehyde units per molecule, preferably an 
method of making battery separators, and to provide average of at least one aldehyde group per monosaccha- 
battery separators. ride unit. Suitable cross-linking #agents, such as dialde- 
hyde starches, are inexpensive and readily available 
40 commercially. Excellent battery separators can be pro- 
vided in accordance with the present invention when 
using these inexpensive materials. 
The cross-linking agents are made by the partial oxi- 
45 charide units. The term  polysaccharide^^ as used herein 
weight. 
’IJMMARY 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 p r m n t  invention by mixing a 
hyde-polysaccharide cross-linking agent having at least 
film-forming polyvinyl alcohol resin with a wlyalde- 
two monosaccharide units and a plurality Of 
grouP% preferably an average Of at least One 
dation of a polysaccharide having at least two monosac- 
thus includes what are commonly referred to as “oligo- 
saccharides** such as disaccharides, trisaccharides, and group per monosaccharide unit, in an amount of about 
2.5 to 20% of the theoretical amount required to cross- 
,,I alcohol, and effecting reaction between the 
and the cross-linking agent in the presence of an acid 
uce a cross-linked polyvinyl alcohol 
e polymer thus produced has excellent 
a polyvinyl alcohol battery separator and such separa- 
tors are preferably prepared in accordance with the 
present invention by cutting or otherwise forming the 
polymer film into suitable configuration prior to carry- 
ing out the cross-linking reaction. 
Other features and advantages of the invention will 
be apparent from the description of preferred embodi- 
ments which follows. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
The polyvinyl alcohol resin useful in the invention is 
a conventional, film-forming, polymeric material 
the like. The saccharides are oxidized to produce up to 
term “dialdehyde” (e.g. as in “dialdehyde-starch”) is 
employed to denote that essentially all of the monosac- 
charide units have two aldehyde groups formed by the 
oxidation reaction. This dialdehyde structure is illus- 
]ect&al properties eminently suitable fo; 55 trated by the reaction of sucrose to form dialdehyde- 
sucrose which may be illustrated as follows: 
link all of the available hydroxyl groups of the polyvi- 50 two aldehyde groups per monosaccharide unit, and the 
H OH 
CHzOH 
65 
3 
-continued 
OH 
I 
I I CH2 I 
4,272,470 
It is preferred that the cross-linking agent has a large 
number of aldehyde units, up to the theoretical maxi- 
mum of two, per monosaccharide unit. In general, it is 
preferred that there are an average of at least 1.8 alde- 
hyde units per monosaccharide units, less preferably at 
least 1.5, and generally, at least 1.0. The polyaldehyde- 
polysaccharide cross-linking agent is preferably inex- 
pensive and water soluble. With these preferences in 
mind, preferred polysaccharides include starch and 
cane sugar (sucrose). Other oligosaccharides and poly- 
saccharides, however, are useful such as cellulose, al- 
ginic acid, and pectic acid. Cross-linking agents derived 
by partial oxidation of the latter two polysaccharides 
include carboxylic acid groups in their molecular struc- 
ture and, when used in the present invention, result in 
the production of battery separators which have a very 
low electrical resistivity. 
The polyaldehyde polysaccharide 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 2.5 to 20% of the theoretical amount 
required to cross-link all of the available hydroxyl 
groups of the polymer. The admixture of resin and 
cross-linking agent is preferably first formed into a self- 
supporting structure, such as a sheet having a thickness 
suitable for a particular intended use. For example, an 
aqueous solution of the admixture may be cast into a 
film having a dry thickness of 5 to 10 mils 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 substantial 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 
configuration suitable for an intended use such as a 
battery separator. For example, a film dried to a thick- 
ness of 50 to 400 microns can be cut into sheets of ap- 
propriate size (e.g. 3 " X 4 " )  and envelope bags can be 
fabricated 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 admixed resin and cross-linking agent similar to that 
from which the film was cast. 
The invention is illustrated in the examples which 
5 follow. 
EXAMPLE 1 
To 800 ml water is added 176 grams of polyvinyl 
alcohol having an essentially head-to-tail structure. To 
lo this solution is added 5 grams of a commercially avail- 
able polydialdehyde starch powder cross-linking agent 
containing recurring units having the formula: 
-CH-CH-0-CH-0- 
I I  I 15 
CH CH2 k H  
6 b H  0 I I  
20 and the solution is well mixed. A film of this solution is 
cast in a thickness of about 170 microns (dry thickness) 
and dried over night. The dried film is cut into pieces 34 
in. X 32 in. in size. Envelope bags are prepared by seal- 
ing the edges of the cut film pieces at a temperature of 
25 about 200" C. The polymer is then cross-linked by im- 
mersion in an aqueous acid catalyst solution containing 
100 ml water, 7 ml concentrated sulfuric acid and 66 
grams of ammonium sulfate to substantially prevent 
dissolution of the film in the acid solution. The tempera- 
30 ture of the solution is maintained at 60"-70" C. for about 
three hours and the cross-linking reaction is thus ef- 
fected. After cross-linking, the envelope is washed with 
water, neutralized with dilute sodium carbonate solu- 
tion, water washed, and dried. The dried material is 
35 very tough, of light yellow color, and provides a bat- 
tery separator having excellent properties. The material 
has good tensile strength, excellent dimensional stabil- 
ity, and adequate flexibility. The electrical resistivity is 
0.909 ohm-cm2 in 45 w/o KOH at room temperature. 
The zincate ion diffusivity is 2.48X 10-7 moles/cm2- 
min. The efficacy of the material in an alkaline battery 
is illustrated by subjecting the material to the alkaline 
and oxidizing characteristics prevailing in a battery. 
Such conditions are simulated by immersing the envel- 
45 opes, prepared as above, in a 45% by weight aqueous 
solution of potassium hydroxide saturated with silver 
oxide (oxidizing agent) for more than three days at 80" 
C. The envelope bags are then rinsed with dilute KOH, 
5o water washed, and air dried. The battery separators, 
after being treated in this manner, retain the advanta- 
geous properties mentioned above. 
The membrane is evaluated as a battery separator in 
an 8 ampere hour Ni/Zn cell. The cell is cycled three 
55 times per day at 50% depth of discharge. The cycle 
consists of six hours charge and two hours discharge at 
C/12 and C/4 rates respectively. The cell displays at 
midpoint voltage of 1.61 volts of testing at the C/2 rate 
on discharge. Current (ampere-hour) efficiency is 
EXAMPLE 2 
A solution is made by admixing 52 grams of a 25% 
solids content poly(dia1dehyde-starch) commercially 
65 available as Sumstar-I90 (Hexcel Inc.) and 1760 grams 
of a 10% aqueous polyvinyl alcohol solution. Film and 
bags are made in the same manner as in Example 1 
except that the bags are sealed by using the same solu- 
60 greater than 90% for 200 cycles. 
4,272,470 
5 6 
tion of Sumstar-190 and polymer as an adhesive for 
sealing the bag edges, and the acid solution is made up 
of loo0 ml water, 50 ml concentrated sulfuric acid, 200 
grams of sodium sulfate, and 40 grams of potassium 
ples and electrical resistivity of the product is 0.190 
ohm-cm2. 
EXAMPLE 6 
peridate. The potassium periodate is added to effect 5 
cleavage of ],2-dio] units present in  the polyvinyl alto- 
About l.7 grams Of pectic acid (General Foods c0.1 
is dispersed into about 176 grams Of lo% aqueous Poly- 
vinyl alcohol solution. Film is cast and processed as in 
Example 2. Results are similar to the foregoing exam- hol in accordance with our co-pending application re- 
similar to those of the film of Example 1 except that the ,o cm2. 
film is slightly hazy. The electrical resistivity of the film What is claimed is: 
iS 0.679 ohm-cm2 and Zincate ion diffusivity iS 1. A method of making a cross-linked polyvinyl alto- 
2.68 X 10 - s moles/cm*-min. The membrane is evaluated hol polymer alkaline battery separator comprising the 
as a battery separator in the same manner as in Example steps of mixing an aqueous solution containing a film 
1. The cell displays at midpoint value of 1.65 volts of 15 forming polyvinyl alcohol polymer which comprises 10 
testing at the C/2 rate on discharge. Current efficiency to 20% by weight of said aqueous solution with a po- 
is about 90% for 200 cycles. lyaldehyde-polysaccharide cross-linking agent having 
at least two monosaccharide units, and a plurality of 
aldehyde groups amounting to between one and two 
~~~~~l~ 2 is followed except that the polydialde- 20 aldehyde groups per monosaccharide unit, said cross- 
linking agent being present in an amount of about 2.5 to 
20% of the theoretical amount required to cross-link all 
of the available hydroxyl groups of the polyvinyl alco- 
hol, casting a film from said solution, drying said film, 
25 and effecting a reaction between the polymer and the 
cross-linking agent in the presence of an acid catalyst by 
contacting the dried film with am aqueous acid solution 
consisting of sodium sulfate and potassium sulfate and in 
30 quantity sufficient to prevent substantial dissolution of 
said film in said aqueous acid solution to produce the 
2. A method according to claim 1 wherein the polyvi- 
nyl alcohol has a number average molecular weight of 
I II ! AH 3. A method according to claim 1 wherein said poly- OH 0 
vinyl alcohol is substantially free of 1,Zdiol units. 
4. A method according to claim 1 wherein the aque- 
ous acid solution comprises sulfuric acid and sodium 
ferred above. The physical properties Of the f i lm  are ples and electrical resistivity is very low: 0.076 ohm- 
EXAMPLE 3 
hyde-starch is replaced by 17 grams of ~ ~ ~ i ~ ~ ~ l  
Starch's polydialdehyde starch #78-1865. Results are 
similar to the previous examples. The electrical resistiv- 
ity of the film is very low: 0.076 ohm-cm2. 
EXAMPLE 4 
cane sugar with periodic acid. The tetra aldehyde is 
believed to have the following formula: 
A tetra is prepared by selective oxidation of containing a dissolved salt selected from the group 
H H CHzOH cross-linked polyvinyl alcohol polymer separator. 
I l l  
HC-C-0-C-0-C-0-CH-CH 
0 CHz CH CH CHI 0 35 10,000 to 200,000. 
I l l  I I I l l  
Seventeen grams of the tetra aldehyde is added to 1760 
grams of 10% aqueous polyvinyl alcohol solution. Film 4o sulphate. 
Results are similar to the foregoing examples and elec- 
trical resistivity of the product is 0.570 ohm-cm2. 
is prepared and processed as described in Example 1. 5. A method according to claim 2 wherein, prior to 
effecting the cross-linking reaction, the dried film is 
assembled into an envelope bag suitable for use in an - 
EXAMPLE 5 alkaline battery. 
45 6. A method according to claim 1 wherein said poly- 
aldehyde Dolvsaccharide comprises the partially oxi- About 0.35 grams of alginic acid (Kelco Inc) is dis- 
persed into 176 grams of 10% aqueous polyvinyl alco- 
hol solution. Film is cast and processed as described in 
dized-readtioh product of a 'polysacchakde seiected 
from the group consisting, pectic acid, and alginic acid. 
Example 2. Results are similar to the foregoing exam- * * * * *  
50 


Cross-linked polyvinyl alcohol and method of making same

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