Page 340 - Handbook of Battery Materials

310 11 Separators

is the coating of cellulose fibers with a very thin layer of phenol–formaldehyde
resin, largely protecting them from acid or oxidative attack. The pore structure
of the separator is predetermined by the paper; to increase the porosity further,
glass fibers may be mixed in during paper production. In a second step, inside a
curing oven the phenolic resin is crosslinked at elevated temperature, and finally
ribs of thermoplastic polymers are applied to achieve the desired total thickness.
Some versions have the backweb embossed with longitudinal corrugations with
plastic coated surfaces for better oxidation stability, since they are placed in the
battery directly against the positive electrode. Slitting and chopping processes set
the required dimensions for the product. Cellulosic separators show a high porosity
2
(70–75%) and thus also low electrical resistances (100–150 m cm according to
quality and producer). It is the preferred leaf separator in climatically more moder-
ate areas, where cold crank performance is of importance. The water-loss-reducing
properties of the cellulosic separator have made it possible to meet the criteria of
the standards for maintenance-free starter batteries, even when using easy-to-cast
antimony alloys (1.8–2.5%).
A thorough study (cf. Ref. [40]) of failure modes in practice has shown that
with this form of separation also the cause of failure has not been the separator;
the usual failure modes for leaf-type separators, as they have been described for
sintered PVC separators, apply here as well.
Table 11.7 shows typical values for different qualities of cellulosic separators
from various producers.

11.2.2.2.3 Glass Fiber Leaf Separators Glass fiber leaf separators in the
USA – especially at one large manufacturer – were for over a decade, between
1980 and 1995, an intermediate in the transition from conventional leaf separator
to microporous pocket. The web is produced from glass fibers of suitable quality
(C-glass) and of various diameters (mainly from 3 µm to around 10 µm) on a
special paper machine. Even though an impregnation for protecting the fibers is
not required, a small quantity of phenolic or acrylate resin is nevertheless applied
to achieve the desired bending strength. A thermoplastic rib is added in the usual
way. Glass fiber leaf separators are distinguished by very high porosity (80–85%)
Table 11.7 Cellulosic separators.

Brand name Backweb Electrical Porosity (%) Pore size Supplier
thickness resistance (average)
2
(mm) (mΩ cm ) (µm)
Arniorib-L 0.55 140 70 25 Daramic, Inc. [60]
Darak 101 a 0.50 110 75 22 Daramic, Inc. [60]
Axohm A 428 0.60 210 70 20 Iydall Axohm [72]
Axohm A 438 a 0.55 140 75 23 Iydall Axohm [72]

a
These types consist of a cellulosic/glass fiber blend paper.