Mini-Encyclopedia of Papermaking Wet-End
Part Two: Definitions and Concepts
The cationic demand of a sample of paper making furnish or process water equals the amount of highly charged cationic polymer required to neutralize its surface. The most common titratant for such tests is poly-DADMAC. The streaming current test is often used to determine cationic demand, based on the amount of cationic titrant required to reach a zero signal. Another way to determine the endpoint is by evaluating the zeta potential after each incremental addition of titrant. Data related to zeta potential can be evaluated by microelectrophoresis or streaming potential methods. Because polyelectrolytes can penetrate into the pore structure of fiber cell walls, the results of such experiments depend on the molecular weight of the titrant. Also it is necessary to use a reproducible, steady rate of titration in order to get reproducible results. Another strategy for determining cationic demand is to mix the sample with a known excess of cationic titrant, filter to remove the solids, and then back-titrate to a color endpoint (colloidal titration). It is a widely recommended strategy to maintain cationic demand of a papermaking furnish at a constant, but usually non-zero level.
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