Page 338 - Handbook of Battery Materials

308 11 Separators

Figure 11.19 ‘Japanese separators.’

and its brittleness leads to higher scrap rates. Tender treatment, preferentially by
manual labor, and an increased quality control effort may be justifiable at low labor
rates. Sintered PVC separators arc thus still widely used in China, India, Russia,
and some Asia-Pacific countries as well as around the Mediterranean Sea.
Sintered PVC is not at all one uniform product; large differences in properties
and quality are possible. Experience has shown that premium qualities require
significantly higher production costs.
The production process is comparatively simple, even though – of course – the
respective know-how is also decisive. The equipment for the production of sintered
PVC separators is suitable in size and production capacity to be operated on its
own by individual, medium-sized, starter battery plants, in contrast to the far larger
units required for the production of polyethylene pocket material.
Fine-grained PVC powder is spread onto a flat steel transport belt and, by means
of a doctor knife, brought into the desired profile, which is generally quite a thin
sheet of 0.3–0.6 mm thickness with vertical ribs. While passing through a sintering
oven the surface of the PVC grains is just barely molten, causing neighboring
particles to stick together (cf. Figure 11.2); the remaining void spaces within this
spherical packing are the resulting porosity. Finally the product is slit and chopped
into the dimensions required. In an alternative version of the process, the thin,
sintered sheet produced initially is embossed in a second step between heated
calender rolls to achieve the requisite total thickness.
Whereas a maximum number of contact points between PVC grains is desired
to achieve mechanical stability, this prevents higher porosities. Typical values for
porosity are 30–35%; therefore the electrical resistance is rather high, that is,
2
170 m cm , despite thin 0.3 mm backwebs for top qualities. As mentioned, the
range is very wide – even considerably higher electrical resistances are sometimes
acceptable, for example, in areas where cold crank performance is of no significant
importance.
Typical pore size distributions result in mean pore diameters of around 15 µm.
Even long and intensive efforts did not succeed in decreasing this value decisively