Page 336 - Handbook of Battery Materials

306 11 Separators

Figure 11.16 Polyethylene separator: intermediate vertical ribs.

Figure 11.17 Polyethylene separator: cross-rib design.

processing problems in practice could not be justified by the minor cost advantages.
The trend toward thinner backwebs has already been mentioned several times; it
leads to a significant decrease in separator stiffness and thus possibly to processing
problems. This loss in stiffness concerns the cross-direction more seriously than
the longitudinal one, which is supported essentially by its high vertical ribs [41]. To
regain stiffness, additional small longitudinal ribs between the main ones have been
proposed (Figure 11.16) [47], and, for the far more seriously affected cross-stiffness,
additional flat ribs across the main rib direction can help (Figure 11.17) [59].
These enable the cross-stiffness of a backweb of almost twice the thickness to be
maintained without obstructing the escape of charging gases.
The selection of suitable profiles improves the efficiency of processing on
pocketing machines. Experience has shown that, even with an optimum machine
adjustment, the pocketing material, by its profile design and the strict adherence
to tolerances, contributes essentially to the quantity and quality of the pockets
produced.

11.2.2.1.5 Product Comparison Table 11.5 shows typical values for polyethy-
lene pocket materials; of course, for the various producers [60–64] (Grace Gmbh
(unpublished results)) they vary slightly owing to differences in formulation and
process. An exact comparison is also difficult, since not all producers stating
tolerances respectively clarify their statistical base.

11.2.2.2 Leaf Separators
The term ‘leaf separator’ characterizes the customary stiff version of a starter battery
separator that can be inserted individually between the electrodes on automatic
stackers, in contrast to pocket separators. This processing requires considerably