Paper Technology International 2020 - Journal - Page 63

PAPERTECHNOLOGYINTERNATIONAL
Figure 6: Zeta Potential Measurement.
Zeta potential measurement with SZP
To measure charge on a larger fiber, the technique is to use
a fine screen to fix the fiber and displace their counter ions in a liquid
flow induced by vacuum and thus generate the streaming potential,
which is used to calculate the Zeta Potential. So, by use of two lab
techniques, we can map both particulate and fibrous charge.
The importance of measurement
As we have seen, paper fibers are inherently anionic or
negatively charged, due to the carboxyl functional groups on the
fiber. Many functional chemicals will have the opposite cationic
(positive) charge and thus we have an electrostatic attraction
between fibers and chemical we can exploit to put the chemical
where it needs to be, i.e., on the fiber. Wet strength, some dry
strength, starch, coagulants and de-bonder chemicals all work
like this. Already we can see the complexity of several charged
functional polymers competing for space on the fiber!
However, the furnish stream may also have the so-called
‘anionic trash’ comprising pitch, stickies, fines or ash which will
quickly eat up the cationic additive before it can react with the fiber.
In the illustration below, we need an extra 50% of wet strength agent
just to overcome the anionic trash and reach the paper wet strength
target; but these wet-strength or trash combinations are also the
cause of foam and drainage issues on many paper machines.
The solution to this might of course be to neutralize the trash
with a non-functional cationic polymer, but how to do this in the right
place and right amount?
Using the measurements
The key to good chemistry management therefore is
objective measurement, and the techniques most often applied
measure the electrical charges on furnish and chemicals. For many
years, papermakers have known about particle charge detection
(PCD) measurement of the soluble or fine particle charge. PCD
can not only measure the residual chemical in solution but also the
troublesome anionic trash. System Zeta Potential (SZP) will directly
measure the charge on the fiber. The higher this negative charge
is, the better it will retain the cationic additive. Both measurement
techniques are now available in several easy-to-use commercial
options.
To exploit these techniques trained technicians would build
a map of SZP and PCD charge for all the stock and water streams,
noting chemical additions and modifying these to get the best
results. They ask the questions:
•
Which furnish stream has the highest chemical
retention potential?
•
How much anionic trash is present?
•
How much fixative to neutralize the trash?
•
Did my functional additive retain on the fiber?
And so a new chemical control strategy, based on
measurement, not guesswork, is built. By using these means we
can control chemical usage, optimize first pass retention and even
improve the crepe efficiency.
Figure 7: Impact of anionic trash.
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