Page 324 - Handbook of Battery Materials

294 11 Separators

[14] were used for separating the electrodes (the first separator in the narrow
meaning of the word), and for about 60 years the most successful material.

11.2.1.2 Starter Battery Separators
In the competition between the systems (electric motor versus combustion engine)
for vehicles, one essential disadvantage of the latter was the tedious and demanding
process of starting by muscular power. Only the development of the electric
starter by Kettering in 1911, and the battery accompanying it, changed this
situation suddenly. It is an irony of history that the starter battery contributed
essentially to the downfall of the early electric car. A rapid development of the car
industry, and of the battery industry in parallel with this, followed. Wooden veneer
became the standard separation of the lead–acid storage battery, be it in double
separation as wood veneer with rubber spacers or later as ribbed wood veneer alone,
when the electrodes became thinner; thus the required acid supply and also the
distance between electrodes decreased in order to increase the energy and power
density.
Wood veneers were produced preferentially from Port Orford cedar, primarily
domiciled in Oregon (USA). Trials with other types of wood, for example, poplar,
remained makeshift measures. The preparation of the wood veneer, that is, the
sawing and slicing of the trees, the dissolving of the lignin to achieve porosity,
and the almost complete leaching of resins which would otherwise accelerate the
corrosion, was quite difficult [15]. Wood veneer separators could be stored and
transported only when wet; dry-charged starter batteries could not be built using
them. Nevertheless, wood veneers remained the predominant separators until
about 1960!
In the meantime another development had decisively altered the outset sit-
uation; plastics had been discovered and synthesized, among them also some
acid-stable ones such as phenol–formaldehyde resin and poly(vinyl chloride) (PVC).
These opened up new possibilities: cellulose papers could be impregnated with
phenol–formaldehyde resin solution and thus rendered sufficiently acid-stable,
and sintered sheets from PVC powder were developed. Independent producers
of separators were founded, combining knowledge of the chemical industry with
experience of the battery industry and thus accelerating the development process.
During the first trials with synthetic separators around 1940 it had already
been observed that some of the desired battery characteristics were affected
detrimentally. The cold crank performance decreased and there was a tendency
toward increased sulfation and thus shorter battery life. In extended test series,
these effects could be traced back to the complete lack of wooden lignin, which
had leached from the wooden veneer and interacted with the crystallization process
at the negative electrode. By a dedicated addition of lignin sulfonates – so-called
organic expanders – to the negative mass, not only were these disadvantages
removed, but an improvement in performance was even achieved.
Larger vehicles with bigger engines required even higher cold crank performance.
In order to meet the resulting requirements for separators with lower electrical
resistance, around 1970 the polyethylene separator [16] and more or less at the same