Page 220 - Lindens Handbook of Batteries

9.14 PRIMARY BATTERIES

Zinc chloride cells using plastic, resealable, venting seals utilize plugged, nonporous electrodes.
Their use restricts the venting of internal gas to only the designed seal path. This prevents the cell
from drying out and limits oxygen ingress into the cell during shelf storage. Hydrogen gas evolved
from wasteful corrosion of the zinc is safely vented as well.

9.5.8 Separator
The separator physically separates and electrically insulates the zinc (negative) from the bobbin
(positive), but permits electrolytic or ionic conduction to occur via the electrolyte. The two major
separator types in use are either the gelled paste or paper coated with cereal or other gelling agents
such as methycellulose.
In the paste type, the paste is dispensed into the zinc can. The preformed bobbin (with the carbon
rod) is inserted, pushing the paste up the can walls between the zinc and the bobbin by displace-
ment. After a short time, the paste sets or gels. Some paste formulations need to be stored at low
temperatures in two parts. The parts are then mixed; they must be used immediately as they can get
at room temperature. Other paste formulations need elevated temperatures (60 to 96°C) to gel. The
gelatinization time and temperature depend upon the concentration of the electrolyte constituents.
A typical paste electrolyte uses zinc chloride, ammonium chloride, water, and starch and/or flour as
the gelling agent.
The coated-paper type uses a special paper coated with cereal or other gelling agent on one or
both sides. The paper, cut to the proper length and width, is shaped into a cylinder and, with the
addition of a bottom paper, is inserted into the cell against the zinc wall. The cathode mix is then
metered into the can forming the bobbin, or, if the bobbin is preformed in a die, it is pushed into the
can. At this time, the carbon rod is inserted into the center of the bobbin and the bobbin is tamped or
compressed, pushing against the paper liner and carbon rod. The compression releases some electro-
lyte from the cathode mix, wetting the paper liner to complete the operation.
By virtue of the fact that a paste separator is relatively thick compared with the paper liner, about
10% or more manganese dioxide can be accommodated in a paper-lined cell, resulting in a dispro-
portional increase in capacity, because of the lower load per gram. 4,6,10

9.5.9 Seal

The seal used to enclose the active ingredients can be asphalt pitch, wax and resin, or plastic (poly-
ethylene or polypropylene). The seal is important to prevent the evaporation of moisture and the
phenomenon of “air line” corrosion from oxygen ingress. 4,5
Leclanché cells typically utilize thermoplastic materials for sealing. These methods are inexpen-
sive and easily implemented. A washer is usually inserted into the zinc can and placed above the
cathode bobbin. This provides an air space between the seal and the top of the bobbin to allow for
expansion. Melted asphalt pitch is then dispensed onto the washer and is heated until it flows and
bonds to the zinc can. One drawback to this method of sealing is that it occupies space that could
be used for active materials. A second fault is that this type of seal is easily ruptured by excessive
generation of evolved gases and is not suitable for elevated temperature applications.
Premium Leclanché and almost all zinc chloride cells use injection molded plastic seals. This
type of seal lends itself to the design of a positive venting seal and is more reliable. Molded
seals are mechanically placed onto a swaged zinc can. Many manufacturers have designed lock-
ing mechanisms into the seal, void spaces for various sealants, and resealable vents. Several
wrap the seal and can in shrink-wrap or tape to prevent leakage through zinc can perforations.
It is very important to prevent moisture loss in the zinc chloride system, and to vent the evolved
gases generated during discharge and storage. The formation of these gases disrupts the separator
surface layer significantly and affects cell performance after storage. Use of molded seals in the
zinc chloride cell construction has resulted in the good shelf storage characteristics evidenced by
this design.