390
17.
BLEACHING AND PULP PROPERTIES CALCULATIONS
17.8 PROPERTIES OF DILUTE PULP
SLURRIES, FLOCCULATION
The behavior of dilute slurries of pulp (0.1 to
3 %
or higher) is critical to paper formation on the
paper machine. (It is also important in pulp
pumping, mixing, and screening, but receives less
attention in these applications.) This area is
complex and there are few resources that give a
good overview to it. The results of some studies
are given in this section, but this is only a starting
point of a complex area.
One important aspect of
dilute
pulp slurries is
the tendency for pulp fibers to flocculate, which
leads to poor paper formation on the paper ma-
chine. Flocculation is a nonuniform fiber distri-
bution, or clustering, in the slurry. Kerekes
(1983) gives a review with 32 references on fiber
flocculation with particular attention to the pres-
ence of decaying turbulence, which
is
applicable to
many areas of pulp processing such as flow be-
yond the holey roll in the headbox, at impeller tips
of mixers and pumps, and perforated screens.
Mason (1954) showed that the critical con-
centration
(CQ)
for flocculation is inversely depen-
dent upon the square of the axis ratio (the length
divided by the width) of the fiber. For example,
the calculated c^ on a volume basis for a L/w of
100 is 0.0015% and for a L/w of 60 is 0.0042%.
Jokinen and Ebeling (1985) list some ways of
decreasing flocculation. These include reducing
the pulp consistency (which is why headbox
consistencies are about 0.5%), lowering tfie
temperature of the pulp suspension, reducing fiber
length (such as with increased refining or using
hardwood instead of softwood), increasing the pH,
using anionic polymers, and using high stock
velocities and turbulent flow (such as holey rolls
in headboxes). The authors experimentally evalu-
ated the relative importance of these factors and
concluded that fibers flocculate for mechanical
reasons. Flocculation is most effectively de-
creased by decreasing the consistency, using
shorter fibers (but this decreases paper strength),
and adding anionic polymers of high molecular
weight such as poly aery lamide.
Kerekes and Schell (1992) indicate that
uniformity of fiber distribution and mobility of
fibers in suspension both contribute to good
formation. Superposition, the piling of fibers on
the wire during sheet formation, also affects
formation. The authors develop a mathematical
model called the crowding factor (the number of
fibers within the spherical volume formed by the
diameter of a fiber) to describe fiber flocculation.
They tested their model with a variety of fiber
types and include some stunning pictures of fibers
in slurries.
Gorres et al. (1989) used a simulation model
to predict paper formation based on fibrous floe
characteristics. Smith (1986) compared hand-
sheets as a way to rank formation potentials.
17.9 STRENGTH OF WET FIBER MATS
Introduction
In their classic study, Lyne and Gallay (1954)
measured the wet strength of pulp webs and
various types of glass fibers to determine the
relative effect of surface tension and hydrogen
bonding on the wet web strength. Their results
are shown in Fig. 17-3 for groundwood and sulfite
pulps and Fig. 17-4 for glass fiber webs.
Based on their results, the authors conclude
that up to 20-25% solids, the fibers are held
together by surface tension forces. Surface ten-
sion forces decrease with increasing solids beyond
this point (as observed by the unmodified glass
fibers), but hydrogen bonding begins and the
strength increases as the water is removed.
The authors also demonstrate that papers
formed from liquids of lower surface tension than
water do not achieve nearly the strengths of paper
formed from water. It would be ideal if paper
could be dry-formed like most fiberboards, but
this is not possible when hydrogen bonding pro-
vides the interfiber bonding in the final product.
However, pulps are sometimes dried
ft^om
acetone
in the laboratory so that they remain bulky for
subsequent work. Even just lowering the surface
tension of water to 33 dyn/cm (from 73 dyn/cm at
18°C) by the addition of 0.1
%
surfactant decreas-
es the ultimate breaking length of a groundwood
paper from 300 meters to 95 meters. Making the
surface of
pulps
hydrophobic with mineral oil also
drastically decreased the strength of paper made
from them. (Freeze-drying of pulp slurries also
gives a very weak fiber mat.)