Page 350 - Handbook of Battery Materials

320 11 Separators

These disadvantages have led to an extensive displacement of rubber separators
by polyethylene separators. Nevertheless, a few market segments exist, such as
golf cart batteries – which for statistical reasons due to their construction are
shown under SLI batteries – and traction batteries in severe heavy-duty service,
especially at elevated temperatures, where rubber separators continue to be used.
The reason is the top-of-charge behavior of traction batteries referred to above.
Rubber separators are able to delay the process of antimony poisoning significantly.
Its mechanism is based on uncrosslinked rubber components inhibiting hydrogen
evolution [69, 82] in a similar manner to that described for the water loss of starter
batteries. With a constant-voltage charging regime, this leads to a lower increase in
charging current and lower water consumption [79]. A comparative tabulation of
rubber separator properties can be found in Table 11.11 (see also Section 11.2.3.1.5
below).

11.2.3.1.3 Phenol–Formaldehyde–Resorcinol Separators (DARAK 5000) An
aqueous solution of phenol–formaldehyde resin and resorcinol (∼70 vol% water)
◦
is crosslinked by means of organic catalysts [18] at ∼95–98 C between two
continuous Teflon belts. With growing molecular weight, the water solubility of
the phenolic resin decreases and a phase separation occurs. A three-dimensional
phenol resin structure is generated, in the interconnected cavities of which
the water accumulates which is evaporated in a subsequent process step, thus
generating the porosity of some 70%. The Teflon belts mentioned contain grooves,
which determine the rib geometry of the separators. Phenolic resin is a duroplast
and thus brittle; a polyester fleece is incorporated into the separator backweb
during the production process, resulting in a sufficient reinforcing effect. In
contrast to the macroporous (phenolic-resin-impregnated) cellulosic separator,
the pore size of the present microporous phenolic resin–resorcinol system is
determined exclusively by process conditions and not by the reinforcing fleece.
A stiff, microporous separator is formed with a very narrow pore size distribution
with an average of 0.5 µm – about 90% of all pores being between 0.3 and 0.7 µm
in diameter!
The porosity, at 70%, is excellent, and it also achieves strikingly good values
for acid displacement and electrical resistance for an industrial battery separator
2
(0.12 cm ).
The stiffness of the duroplast is helpful in counteracting the tendency of
the negative active material to expand during cycling, even at larger rib spacing.
The prevailing profiles have vertical or diagonal ribs on the positive side, and on the
negative side a low rib is frequently added for better gas release from the negative
electrode.
Further details are given in the systems comparison under Section 11.2.3.1.5
below.
11.2.3.1.4 Microporous PVC Separators A mixture of powdered PVC, cyclohex-
anone as solvent, silica, and water is extruded and rolled in a calender into a profiled