Page 352 - Handbook of Battery Materials

322 11 Separators

against oxidation at continuous overcharge, because such batteries have a life
expectancy of 20–30 years.

11.2.3.2.1 Polyethylene Separators The production process for polyethylene
separators (Section 11.2.1.1) as well as the characteristic properties (see
Sections 11.2.2.1 and 11.2.3.1) have already been described in detail above.
Deviating therefrom, the desire for low acid displacement has to be added for
separators in open stationary batteries. This can be met either by decreasing the
backweb thickness or by increasing the porosity; the latter, however, is at the
expense of separator stability.
Stationary batteries, moreover, often have transparent containers, historically,
probably to allow observation of the electrolyte level or the extent of shedding.
Deposits of oily substances accumulating at the electrolyte surface due to their
stickiness could gather lead particles and produce an unpleasantly dirty rim, which
can be avoided by careful selection of suitable oils [53].

11.2.3.2.2 Phenol–Formaldehyde Resin–Resorcinol Separators (DARAK 2000/
5005) The production process and the principal properties of this system
have been described in detail in the section on traction battery separators (see
Section 11.2.3.1). The outstanding properties, such as excellent porosity (70%)
and resulting very low acid displacement and electrical resistance, come into full
effect when applied in open stationary batteries. Due to the good inherent stiffness
the backweb may even be reduced to 0.4 mm, reducing acid displacement and
electrical resistance to low levels that are not achievable by any other system.
Furthermore, the phenolic resin–resorcinol separator neither generates any
harmful substances nor is it attacked chemically or by oxidation. The sum of these
properties has made it the preferred separator for open stationary batteries.

11.2.3.2.3 Microporous PVC Separators Much of the above also holds true
for the application of microporous PVC separators (see Section 11.2.3.1) in open
stationary batteries. Very high porosity and thus low acid displacement and electrical
resistance are also offered by this system. The relevant properties are compiled in
Table 11.12.
Since the early days of using PVC separators in stationary batteries, there
has been a discussion about the generation of harmful substances: caused by
elevated temperatures or other catalytic influences, a release of chloride ions could
occur which, oxidized to perchlorate ions, form soluble lead salts resulting in
enhanced positive grid corrosion. Since this effect proceeds by self-acceleration,
the surrounding conditions such as temperature and the proneness of alloys
to corrosion as well as the quality of the PVC have to be taken carefully into
account.

11.2.3.2.4 Sintered PVC Separators Sintered PVC separators for open station-
ary batteries are produced in the same way as the corresponding starter battery
version (Section 11.2.2.2). Their brittleness and thus difficult processability are