Leak-proof alkaline cell and its manufacture

A leak-proof alkaline cell comprising a positive can and a negative electrode collector kept at the opening end of the positive can with intervention of a gasket so as to make the inside of the cell liquid-tight, characterized in that the gasket is made of an alkali resistant, elastic material and has a moisture content of not more than 0.3% by weight.

The present invention relates to a leak-proof alkaline cell and its 
manufacture. 
An alkaline cell is usually manufactured by admitting a positive 
depolarizing mix with a small amount of an electrolyte to a positive can, 
placing a separator on the positive depolarizing mix and fitting to the 
opening end of the positive can a negative electrode collector 
accommodating a negative electrode material mix and a large amount of an 
electrolyte therein with intervention of a gasket having an L-letter form 
section, followed by crimping the rim of the positive can inwardly so as 
to press the gasket to the negative electrode collector. Thus, 
irrespective of a button type or a cylinder type, a gasket has a role not 
only of separating a positive electrode and a negative electrode from each 
other but also of keeping the inside of an alkaline cell liguid-tight. 
However, the leakage of an electrolyte in alkaline cells is frequently 
observed at a gasket part. 
As the result of an extensive study to enhance the leak-proof property of 
an alkaline cell at the gasket part, it has found that a certain 
correlation is present between the moisture content of a gasket and the 
leak-proof property of an alkaline cell. Namely, a gasket having a lower 
moisture content has a higher elasticity and shows a higher sealing or 
packing effect. Thus, an alkaline cell incorporated with a gasket having a 
lowered moisture content shows a better leak-proof property. The present 
invention is based on the above finding. 
According to the present invention, there is provided a leak-proof alkaline 
cell comprising a positive can and a negative electrode collector kept at 
the opening end of the postive can with intervention of a gasket so as to 
make the inside of the cell liquid-tight, characterized in that the gasket 
is made of an elastic material and has a moisture content of not more than 
0.3% by weight. 
As the material for preparation of a gasket in this invention, there is 
used an alkali-resistant, elastic material, preferably having a saturated 
moisture content of not less than 0.1% by weight. Specific examples of 
such material are polyacetals, ABS resins (acrylonitrile-butadiene-styrene 
copolymers), polyamides (e.g. Nylon 6, Nylon 66, Nylon 610, Nylon 11, 
Nylon 12), synthetic rubbers (e.g. butyl rubber, silicone rubber, 
ethylene-propylene rubber, chloroprene rubber), etc. Polyethylene, which 
is one of the most popular materials for preparation of a gasket, has a 
saturated moisture content of less than 0.1% by weight and is not suitable 
in this invention. Among the said suitable materials, preferred are 
polyamides. Particularly, the use of Nylon 11 (polycondensation product of 
11-aminoundecanoic acid; molecular weight, 10,000 to 40,000; saturated 
moisture content, 2.5%; "RILSAN BMNO" manufactured by ATO Chimie) is 
recommended. 
The preparation of a gasket by the use of the said suitable materials may 
be carried out by a conventional procedure such as injection molding or 
punch molding. Characteristically, the moisture content of the gasket is 
lowered to 0.3% by weight or less prior to its assembly into an alkaline 
cell. In usual, the gasket as prepared above absorbs a considerable amount 
of moisture so as to show, for instance, a moisture content of several %, 
when allowed to cool in atmosphere after the molding. Such gasket may be 
dried, for instance, under a reduced pressure of about 10 mmHg for a 
period of 4 to 20 hours to make a moisture content of not more than 0.3% 
by weight. Elevation of the temperature is usually recommendable for 
shortening the drying time. However, such excessive heating as causing 
deformation to the gasket should be avoided. In general, heating at a 
temperature not exceeding 90.degree. C. is favorable. When desired, the 
drying may be effected in the presence of a drying agent in a closed 
system connected or not to a pressure reducing system. Alternatively, the 
gasket immediately after the molding and having a low moisture content 
(frequently nearly 0) may be allowed to cool under the condition of low 
humidity and kept as such avoiding moisture. 
In order to enhance the liquid-tight property at the contact face between 
the surface of the gasket and the negative electrode collector and also at 
the contact face between the surface of the gasket and the positive can, a 
sealing material is often applied to such contact faces. The use of the 
gasket according to this invention does not interfere with the application 
of such conventional liquid-tight procedure. Rather, the application of a 
sealing material is recommended since it is effective in enhancing the 
leak-proof property. 
As the sealing material, there may be used a liquid material or a solution 
of a solid material in a suitable solvent. Specific examples of the 
sealing material are polyamides (e.g. Nylon 6, Nylon 66, Nylon 610, Nylon 
11, Nylon 12), polyethylene, polypropylene, styrene resins, fluororesins, 
rubbery materials, pitch, fatty polyamides, rubbery adhesives (e.g. 
chloroprene rubber, nitrile rubber, silicone rubber butyl rubber), 
silicone oils, etc. Among them, preferred are pitch, fatty polyamides, 
rubber adhesives and silicone oils. 
The sealing material may be applied to either one or both of the surface of 
the gasket and the surface(s) of the negative electrode collector and/or 
the positive can, normally at room temperature. Usually, the application 
is made to both surfaces. In case of the application being made to the 
surface(s) of the negative electrode collector and/or the postive can, it 
should be effected at least at the part(s) which are brought into contact 
with the gasket. When the sealing material is applied to the surface of 
the gasket, the decrease of the moisture content of the gasket to 0.3% by 
weight or less may be effected prior to the application, but preferably 
after the application. 
For enhancing the leak-proof property more, the gasket applied the sealing 
material thereon may be fitted to the negative electrode collector applied 
the sealing material thereto to make an integral body. This integral body 
may be, after decreasing the moisture content of the gasket to 0.3% by 
weight or less by an appropriate procedure such as drying while heating 
(below 90.degree. C.), incorporated with the positive can so as to 
assemble an alkaline cell. 
In the present specification, the term "moisture content" is intended to 
mean the one measured by the following procedure: the total weight (Wa) of 
10 to 20 gaskets sampled at random is weighed; those gaskets are dried at 
45.degree. C. in vacuum (10 mmHg or less) for 20 hours and cooled to 
20.degree. C., and then the resulting weight (Wb) is measured; and 
calculation is made according to the following formula: 
##EQU1## 
The assembly of an alkaline cell provided with the gasket according to this 
invention may be carried out by a per se conventional procedure.

At the bottom of a positive can 1, a positive depolarizing mix 2 comprising 
a positive active material (e.g. Ag.sub.2 O, HgO, MnO.sub.2) and an 
electroconductive material (e.g. graphite) and a small amount of an 
alkaline electrolyte (e.g. aqueous KOH) are admitted. On the positive 
depolarizing mix 2, a separator layer 3 comprising a protective film (e.g. 
polypropylene film), a separator sheet (e.g. cellophane sheet) and an 
electrolyte retaining sheet (e.g. non-woven fabric sheet) is placed. At 
the opening end of the positive can 1, a negative electrode collector 5 
accommodating a negative electrode material mix 4 comprising zinc powder 
and a large amount of an alkaline electrolyte is fitted with an 
intervention of a gasket 7 having an L-letter form section. The rim 8 of 
the positive can 1 is inwardly crimped so as to press the gasket 7 to the 
negative electrode collector 5, whereby the inside of the thus assembled 
alkaline cell is made liquid-tight. The said gasket 7 is made of an 
alkali-resistant, elastic material having a saturated moisture content of 
not less than 0.1% by weight and has a moisture content of not more than 
0.3% by weight. At the surface of the gasket 7, there is provided a 
sealing material layer 6 so as to enhance the sealing or packing effect. 
Practical and presently preferred embodiments of this invention are 
illustratively shown in the following Examples wherein % and part(s) are 
by weight unless otherwise indicated. 
EXAMPLE 1 
Nylon 610 having a saturated moisture content of 0.1% or higher was 
injection molded to make a gasket having an L-letter form section. The 
gasket was kept at 30.degree. C. under 10 mmHg for 20 days, and then a 10% 
pitch solution in toluene was applied to the surface of the gasket to make 
a coating film of 200 to 300 microns in thickness. The moisture content of 
the gasket immediately before assembly into a dry cell was 0.02%. By the 
use of this gasket, a button type alkaline dry cell was assembled 
according to a conventional manner. 
EXAMPLE 2 
In the same manner as in Example 1 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 3 hours and having a moisture content 
of 0.1%, a button type alkaline dry cell was assembled. 
EXAMPLE 3 
In the same manner as in Example 1 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 8 hours and having a moisture 
content of 0.3%, a button type alkaline dry cell was assembled. 
COMATIVE EXAMPLE 1 
In the same manner as in Example 1 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 20 hours and having a moisture 
content of 0.5%, a button type alkaline dry cell was assembled. 
COMATIVE EXAMPLE 2 
In the same manner as in Example 1 but using a gasket kept at 20.degree. C. 
under a relative humidity of 90% for 25 hours and having a moisture 
content of 1.0%, a button type alkaline dry cell was assembled. 
Each of the dry cell as prepared in Examples 1 to 3 and Comparative 
Examples 1 and 2 was cleaned at the surface and allowed to stand at 
45.degree. C. under a relative humidity of 90% for 50 days, 100 days or 
150 days. Thereafter, an indicator prepared by dissolving cresol red (0.1 
part) in a mixture of water (80 parts) and ethanol (20 parts) was dropped 
at the gasket part on the outer surface of the dry cell for detection of 
the leakage of the electrolyte; the presence of leakage gave the color 
change to red. 
The results are shown in Table 1 wherein the numeral indicates the number 
of the dry cells having leakage in the tested 100 dry cells. 
Table 1 
______________________________________ 
Number of dry cells having leakage 
After 50 After 100 After 150 
Example days days days 
______________________________________ 
1 1 12 35 
2 2 15 43 
3 3 21 60 
Comparative 
1 5 42 90 
2 12 70 100 
______________________________________ 
From the above results, it is understood that a button type alkaline dry 
cell assembled by the use of a gasket having a moisture content of not 
more than 0.3% shows a good leak-proof property. 
EXAMPLE 4 
Nylon 11 having a saturated moisture content of 2.5% was injection molded 
to make a gasket having an L-letter form section. The gasket was kept at 
30.degree. C. under 10 mmHg for 20 days, and then a 10% pitch solution in 
toluene was applied to the surface of the gasket to make a coating film of 
200 to 300 microns in thickness. The moisture content of the gasket 
immediately before assembly into a dry cell was 0.02%. By the use of this 
gasket, a button type alkaline dry cell was assembled according to a 
conventional manner. 
EXAMPLE 5 
In the same manner as in Example 4 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 3.5 hours and having a moisture 
content of 0.1%, a button type alkaline dry cell was assembled. 
EXAMPLE 6 
In the same manner as in Example 4 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 7 hours and having a moisture content 
of 0.3%, a button type alkaline dry cell was assembled. 
COMATIVE EXAMPLE 3 
In the same manner as in Example 4 but using a gasket kept at 20.degree. C. 
under a relative humidity of 65% for 22 hours and having a moisture 
content of 0.5%, a button type alkaline dry cell was assembled. 
COMATIVE EXAMPLE 4 
In the same manner as in Example 4 but using a gasket kept at 20.degree. C. 
under a relative humidity of 90% for 30 hours and having a moisture 
content of 1.0%, a button type alkaline dry cell was assembled. 
Each of the dry cells as prepared in Examples 4 to 6 and Comparative 
Examples 3 and 4 was cleaned at the surface and allowed to stand at 
45.degree. C. under a relative humidity of 90% for 50 days, 100 days or 
150 days. Then, an indicator prepared by dissolving cresol red (0.1 part) 
in a mixture of water (80 parts) and ethanol (20 parts) was dropped on the 
outer surface at the gasket part for detection of leakage. 
The results are shown in Table 2 wherein the numeral indicates the number 
of the dry cells having leakage in the tested 100 dry cells. 
Table 2 
______________________________________ 
Number of dry cells having leakage 
After 50 After 100 After 150 
Example days days days 
______________________________________ 
4 1 10 30 
5 1 12 35 
6 2 18 50 
Comparative 
3 3 30 80 
4 10 60 100 
______________________________________ 
From the above results, it is understood that a button type alkaline dry 
cell assembled by the use of a gasket made of Nylon 11 and having a 
moisture content of not more than 0.3% shows an excellent leak-proof 
property. 
EXAMPLE 7 
Nylon 11 was injection molded to make a gasket having an L-letter form 
section. The gasket was kept at 20.degree. C. under a relative humidity of 
65% for 25 hours, whereby the moisture content was raised from 0% 
(immediately after the molding) to 0.9%. Onto the surface of the gasket, a 
10% pitch solution in toluene was applied to make a coating film of 200 to 
300 microns in thickness. The resulting gasket was dried at 90.degree. C. 
under 10 mmHg for 4 hours to make a moisture content of 0%, and then it 
was assembled into a button type alkaline dry cell according to a 
conventional manner. 
EXAMPLE 8 
In the same manner as in Example 7 but using a gasket dried at 90.degree. 
C. under 10 mmHg for 15 minutes after application of the pitch solution to 
make a moisture content of 0.3%, a button type alkaline dry cell was 
assembled. 
Each of the dry cells as prepared in Examples 7 and 8 was cleaned at the 
surface and allowed to stand at 45.degree. C. under a relative humidity of 
90% for 50 days, 100 days or 150 days. Then, an indicator prepared by 
dissolving cresol red (0.1 part) in a mixture of water (80 parts) and 
ethanol (20 parts) was dropped on the outer surface at the gasket part for 
detection of leakage. 
The results are shown in Table 3 wherein the numeral indicates the number 
of the dry cells having leakage in the tested 100 dry cells. 
Table 3 
______________________________________ 
Number of dry cells having leakage 
After 50 After 100 After 150 
Example days days days 
______________________________________ 
7 1 9 38 
8 1 15 42 
______________________________________ 
From the comparison of the results in Table 3 with the results (Examples 4 
and 6) in Table 2, it is understood that the drying treatment of a gasket 
after application of the pitch solution and before assembly into the dry 
cell so as to decrease the moisture content is effective in enhancing the 
leak-proof property. 
EXAMPLE 9 
Nylon 11 was injection molded to make a gasket having an L-letter form 
section. The gasket was kept at 20.degree. C. under a relative humidity of 
65% for 25 hours, whereby the moisture content was raised from 0% 
(immediately after the molding) to 0.9%. Onto the surface of the gasket, a 
10% pitch solution in toluene was applied to make a coating film of 200 to 
300 microns in thickness. Separately, a negative can was applied with the 
pitch solution at the surface, which is to be brought into contact with 
the gasket, so as to make a coating film of 200 to 300 microns in 
thickness. Then, the gasket was fitted to the negative can and dried at 
90.degree. C. under 10 mmHg for 4 hours to decrease the moisture content 
of the gasket to 0%. The resultant integral body was assembled into a 
button type alkaline dry cell. 
EXAMPLE 10 
In the same manner as in Example 9 but using an integral body of the gasket 
and the negative can fitted therewith dried at 90.degree. C. under 10 mmHg 
for 15 minutes to make a moisture content of 0.3%, a button type alkaline 
dry cell was assembled. 
Each of the dry cells as prepared in Examples 9 and 10 was cleaned at the 
surface and allowed to stand at 45.degree. C. under a relative humidity of 
90% for 50 days, 100 days or 150 days. Then, an indicator prepared by 
dissolving cresol red (0.1 part) in a mixture of water (80 parts) and 
ethanol (20 parts) was dropped on the outer surface at the gasket part for 
detection of leakage. 
The results are shown in Table 4 wherein the numeral indicates the number 
of the dry cells having leakage in the tested 100 dry cells. 
Table 4 
______________________________________ 
Number of dry cells having leakage 
After 50 After 100 After 150 
Example days days days 
______________________________________ 
9 0 2 10 
10 1 12 35 
______________________________________ 
From the comparison of the results in Table 4 with the results (Examples 4 
and 6) in Table 2 and the results in Table 3, it is understood that the 
integration of the gasket and the negative can fitted therewith with 
intervention of the pitch solution, followed by drying to decrease the 
moisture content of the gasket is effective in enhancing the leak-proof 
property.