2001 - Volume 2 - Journal of Engineered Fibers and Fabrics

index failed to predict the rate of drainage of fiberglass slurries.
It is believed that the fundamental differences between
the WFGM and the papermaking processes are among the
probable causes, assuming that drainage index correlates well
in a papermaking process. A paper furnish typically uses short
cellulose fibers with high content particulate fillers at relatively
high consistencies, while a wet process slurry usually consists
of very long glass fibers with nearly zero percent particulate
substances at very low consistencies. A high concentration
of long molecular chain polyacrylamide in the white
water also makes the wet process different from the papermaking
processes. Johnson conducted [16] simulated experiments
with fiber lengths of 1 to 4 mm and reported that [17]
drainage was proportional to the fabric drainage index in a
papermaking process. In this study, the input materials were
~100% 1.25 inch (~32 mm) glass fibers with no particulate
additives at all. The PAM concentration was also higher than
in a papermaking process.
Conclusion
Wet process drainage is a complex filtration process
depending on both the characteristics of a fiberglass slurry and
the structures of a forming fabric. By employing a recently
developed wet process mimic device, a lab tester, the present
investigation has successfully conducted drainage experiments
of fiberglass slurries under simulated dynamic conditions
with a real (commercial) forming fabric in position. The
effects of wire parameters and white water characteristics
were examined.
The drainage experiments have shown that in a typical polyacrylamide
(PAM) white water, a higher PAM concentration
significantly reduced the rate of drainage, presumably due to
a higher viscosity. The PAM-based white water was also very
sensitive to shearing (mixing) effect. So, an increase in input
mixing energy, either by a higher mixing speed (RPM) or by
a prolonged mixing time, has reduced the white water viscosity,
and resulted in a substantial increase in wet process
drainage.
Mat basis weight also had a strong impact on wet process
drainage. Although an increase in mat basis weight has always
reduced the rate of drainage, its influence was stronger on the
wires with a higher air permeability and a lower drainage
index than on the wires with a lower air permeability and a
higher drainage index.
Another important conclusion of the study was that
drainage index did not predict wet process drainage, and the
main causes are believed to lie in the fundamental differences
between the WFGM and papermaking processes.
This investigation has also showed that the correlation
between air permeability and wet process drainage was complex.
While the drainage rate of pure water and of the wet
process white water (without fibers) correlated well to the initial
flow resistance (the air permeability) of a forming fabric,
for fiberglass slurries, however, the correlation failed under
some circumstances. It is generally true that while air permeability
may be used as an empirical parameter for light weight
mats at lower PAM concentrations, the higher the web basis
20 INJ Summer 2001
weight and the higher the PAM concentration, the more likely
it would fail to predict wet process drainage
Acknowledgements
The author would like to acknowledge Howard Ruble for
his assistance on the drainage experiments. He also wishes to
thank David Mirth, Thomas Miller, Robert Houston, David
Gaul and Warren Wolf for their support to publish this work.
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