Martin A. Hubbe
Associate Professor, North
Carolina State University
This paper compares laboratory test procedures which
predict the performance of
chemicals used to enhance
retention or dewatering during the manufacture of
paper. Key points of difference among the various
laboratory methods include:
• the presence or absence
of fibre mat formation
during the test
• the optional application
of vacuum
• the presence or absence
of pressure or velocity
pulsations during dewatering
• the use of automation in
some test procedures.
A well-chosen laboratory
test can provide useful information without incurring the
high cost and risks associated with full-scale
evaluations of many different retention and drainage
programmes and dosage levels. However, it is important
to understand the compromises inherent in different
lab-scale tests to guard
against premature rejection
of specific chemical programme options.
Fillers & Pigments for
Papermakers,
Pira Conference, Barcelona,
2003
20
Selecting Laboratory Tests to
Predict Effectiveness of Retention
and Drainage Aid Programmes
“You have to run it on the machine to
really find out whether it works.” Those are
words you are likely to hear if you are responsible for the operation of a paper machine and
somebody is suggesting a change in the retention and drainage aid system. But you know,
deep down, that if you always followed the
train of logic suggested by the speaker, then
you would be overwhelmed by trials of new
chemical conditions on the paper machine.
The purpose of this article is to consider
how paper technologists can use lab tests to
increase their options. There has to be a middle ground between “no, we aren’t interested
in considering your chemical” and “sure, go
ahead, run your chemical on my paper
machine at different dosages and addition
points until we’re convinced it won’t work or
it’s absolutely not cost effective.”
Know Your Goals: Be realistic when
selecting lab tests to predict the on-machine
performance of retention aids and other
chemicals that affect fine-particle retention or
the rate of dewatering as paper is being
formed. There are some simplifying assumptions that you can make, and these
assumptions may allow you to use a relatively
simple and quick test. There are some more
sophisticated tests that you can run if you are
concerned about making increasingly accurate predictions about how different chemical
strategies will perform on you paper machine.
The “best” lab-based retention and drainage
assay may depend on which of the following
questions comes closest to your situation:
• Is it even worth considering this particular
chemical additive? Is there even a reasonable chance that you will see some
benefits – in retention or dewatering – if
this additive is used?
• What are the two or three top candidate
chemical programmes that would be most
likely to achieve our retention and dewatering targets without wrecking formation
uniformity?
• Will this suggested additive programme
stand up to the high levels of shear in
your paper machine system?
• Is this a test procedure that a new production team member or summer employee
without previous experience could run
over a couple of days?
• Is there any way that someone could automate the test procedures to get beyond the
“subjectivity factor,” especially when the
same test needs to be run by different
people on different shifts?
A brief history of lab-scale testing
In principle, a conventional handsheet apparatus(1) ought to be sufficient to study effects of
retention and dewatering additives. In practice,
however, the flows to which the fibres are subjected during the formation of a handsheet are
much gentler than those found on productionscale paper machines(2). Previous work has
shown that hydrodynamic forces during papermaking can be strong enough to tear mineral
particles from fibre surfaces, depending on the
types and levels of retention aid treatments(3-6).
As a consequence, handsheet tests can
grossly overestimate the first-pass retention
that can be achieved with a certain treatment.
Also, the stock used to form a standard handsheet is so highly diluted as to raise doubts
about the validity of using retention and
drainage results from those tests to make predictions about commercial paper machines.
Many of the test procedures to be described in
this review can be understood as different
attempts to overcome such limitations of handsheet testing.
A key milestone in the history of lab-scale
retention aid testing occurred in 1973, the year
that
Britt
unveiled
his
Dynamic
Drainage/Retention Jar(7). The “Britt jar”
achieved two important goals in retention aid
evaluation.
i) First, it broke ground in providing an
adjustable and reproducible agitation, something that might represent the net effect of unit
operations in the approach flow to a paper
machine. Pre-agitation tends to make test
results more credible, since hydrodynamic
forces can separate cellulosic fines or fillers
from fibre surfaces or from other fine particles
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
that have become attached by the action of a
retention aid treatment(8). Ideally, one would
like to use a range agitation level that would
effectively break chemical-induced attachments among the fibres in the slurry, leading to
a suitably low level of fibre flocculation during
the jar-type test. The minimum intensity of
hydrodynamic shear stress needed to detach
fibres from each other tends to be much less
than that needed to detach fine particles either
from each other or from fibre surfaces(5,8-10).
ii) The second area where the Britt jar broke
new ground was in evaluating colloidal retention, i.e. retention of fine particles onto fibres,
without allowing a fibre mat to form during the
test. History has been friendly to the Britt jar,
and it remains one of the most widely used
tests for screening of retention aids.
Several researchers have championed the
idea of testing retention and drainage aids
under conditions rather similar to the Britt jar
test, but with formation of a fibre mat(11-17). A
key benefit of forming a mat is the ability to
evaluate rates of dewatering, and differences in
these rates resulting from changes in the
dosages of chemical additives. Gess(15-17) and
others(14,18-28) advocated the use of vacuum during such tests. In some cases it was found that
one could draw conclusions about the effectiveness of different retention aid programmes
just by looking at how the measured vacuum
varied over time(14-17,26).
Unfortunately, procedures that merely form
a fibre mat on a screen, either by gravity or by
steady application of vacuum, can give misleading information. The magnitudes and
trends of retention and dewatering performance predicted by the lab tests often do not
match the results of trials carried out at industrial scale(21,24,29). For example, simple filtration
tests tend to exaggerate the importance of
chemicals on drainage rates and retention,
especially in cases where the furnish has a high
fines content(30).
Deviations of this type are sometimes
explained by considering what happens during
industrial-scale forming of a paper sheet. Paper
machines use such devices as hydrofoils(2,31),
table rolls(2), forming blades(32), and vacuum
boxes(29). These devices create pulsations of
flow or pressure perpendicular to the wet mat
as paper is being formed. The “action”(31) created by such pulsations tends to increase both
the dewatering rate and mat uniformity, often
at the expense of lower first-pass retention(33-35).
Many of the unattached fines, instead of
blocking drainage channels in the wet web,
are merely washed out of the sheet as it is
being formed. In an effort to achieve more
realistic predictions, several innovative lab
test designs incorporate pulsations of reproducible frequency and amplitude(30,36-42).
Aside from issues of retention and formation, sometimes it is useful also to know how
much water is being tightly held inside the cell
walls of fibres. The easiest way to obtain such
information involves centrifugation of plugs of
damp fibres, the so-called Water Retention
Value (WRV) test(43-45). In the past it has been
assumed that results of WRV tests were unrelated to chemical flocculation effects occurring
between the fibres and at their surfaces; however, recent results showed a high correlation
between changes in WRV measurements and
freeness measurements, in response to treatment with dewatering aids(46).
Recently it has been shown that the effectiveness of different retention aid programmes
can be compared by measuring the size or
strength of fibre-to-fibre flocs in a treated
slurry of fibres(47-58). The measurement principles include light transmission(56-58) and
viscometry(47-51,53-56). In interpreting the results of
such tests it is important to bear in mind that
fibre flocculation, per se, usually is undesirable. Ideally one would prefer to achieve high
retention efficiency and increase dewatering
rates without harming the uniformity of sheet
formation. However, the fibre floc tests often
can be carried out more quickly than a typical
retention test.
Decisions in Selecting a Lab Test: Let’s
say that you are among the people responsible
for the performance of “Paper Machine 1” at a
certain mill, and it has been suggested that the
performance would be better with a different
programme of chemical additives for retention
and drainage promotion. Let’s assume that
there is a certain reluctance to go ahead with
full-scale trials of new chemistry – unless there
is a pretty good chance of substantial cost
and/or performance benefits. Your customers
are happy with the performance of your paper
product, and your management team is reluctant to make changes.
How, then, do you choose the most appropriate lab-scale test? As shown in the sections
that follow, the overall decision can be broken
down into a series of questions.
Question 1: Mat or no mat?
As illustrated in figure 1, one of the first considerations ought to be whether or not to allow
mat formation to occur during the lab procedure. The reasons can be subtle. If you are
concerned only about the effects of additives
on retention efficiency – not drainage rates or
any sheet properties – then there is potential to
save time and achieve good statistical results
with a method that avoids formation of a mat.
Tests with No Mat Formation: As shown
in figure 1, formation of a fibre mat, during a
retention aid evaluation, can be prevented by
continuous agitation. In practice no mat will
form as long as the stirring speed is above
some minimum value and the flow rate of filtrate through the screen is kept low. The
principle behind such a test is that the full-
21
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
length fibres in the slurry are too long to pass
through the openings in the screen.
By definition, the fine particles are small
enough to pass through those holes, but not all
of them do so. The fraction of the fines passing
into the filtrate depends on “colloidal retention”, i.e. the proportion of the fine particles
that remain attached to fibres. That fraction is
expected to depend on the previous chemical
treatments, in addition to any hydrodynamic
shear. By far the best-known test of this type is
the Dynamic Drainage/Retention Jar(7), or Britt
jar.
Essential features of the Britt jar are illustrated in figure 2. A sample of papermaking
furnish, usually having a consistency similar to
that of a headbox sample from a paper
Mat
Formation
No
Mat
Figure 1 Illustration of the first question, whether or not to form a fibre mat as part
of a test protocol to evaluate retention aid performance.
Impeller
Stand
“Jar” with
fibre slurry
Screen
Pinch
clamp
Beaker
Figure 2 Classic design of Britt’s “Dynamic Drainage/Retention Jar”
22
machine, is placed in a cylindrical plastic jar.
In the classic Britt jar test the screen consists of
a thin, machine stainless steel sheet, having
conical-section holes with a minimum diameter (at the top surface) of 76 µm. Subsequent
users sometimes have substituted a forming
fabric from a paper machine, or a woven stainless steel screen of a desired mesh size.
Agitation is supplied by an impeller, the
position of which needs to be kept constant
from test to test in order to achieve reproducible results.
The agitation speed is adjustable to whatever rpm level is selected by the user. The most
popular model of the Britt jar, in current use, is
fitted with small “baffles” at three locations on
the inner wall of the jar(59-60). These baffles
partly break up the vortex that forms within the
jar, and they also make it possible to use somewhat higher agitation speeds.
To carry out a test, the user closes a stopcock or pinch clamp below the jar, fills the jar
with slurry, starts the agitator, adds a selected
type and amount of chemical additive(s), waits
a chosen period of time, then briefly withdraws
a small sample of filtrate. An eyedropper tip,
below the stopcock or pinch clamp, restricts
the flow of filtrate, minimising the tendency of
mat formation. The mass of filterable solids in
the filtrate, per unit volume, is taken as an indication of the proportion of the fine particles in
the furnish that are not attached to the fibres
under the conditions of testing.
There have been many reported studies that
follow the essentials of the Britt jar procedure(21,24,59,61-64), though not all of them have used
the same equipment. For instance, the basic
Britt jar procedure even has been carried out
using a modified freeness test apparatus(14).
Some key assumptions inherent in the use
of the Britt jar test include (a) that it is possible
to represent the overall effect of hydrodynamic
shear in a real paper machine system by selecting a suitable agitator speed, (b) that
fine-to-fibre attachment is a dominant, or at
least very important contributing mechanism
responsible for the effect of the retention aid
under consideration, and (c) that retention
effects in the absence of mat formation will
follow the same trends as retention effects in
the presence of mat formation.
One recommended approach has been to
adjust the agitator speed such that the Britt jar
test results yield the same first-pass retention
as observed on the paper machine under consideration, when the same retention aid system
and dosage are applied in each case(65). Another
approach has been to adjust the agitator speed
to whatever level provides a strong differentiation among the different chemical conditions
being compared.
In addition to demonstrating different retention capabilities of treatment schemes based on
charge neutralisation, charge patches, and
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
polymer bridging(66-68), the Britt jar also has
been used to demonstrate the relative
reversibility of different treatments(56,61,69-70). In
other words, some flocculant systems seem to
“recover” after the speed of agitation is
increased, and then decreased. In contrast,
other retention aid treatments sometimes are
broken down irreversibly after exposure to sufficiently high rates of agitation. It has been
proposed that the chemical programmes leading to higher reversibility tend to be those that
also give better overall dewatering rates and
good formation uniformity(67).
Build a better mousetrap and the world will
attempt to improve upon it. Though most published work following the essential features of
Britt’s procedure(7,59,62-63,65) has been based on the
weighing of tared filter paper samples, to
determine the consistency of the filtrate (or
“white water”), it is possible to save time by
measuring the turbidity(19,56,71-73).
Despite the practical advantages of such
approaches, the results of optical tests have to
be regarded with caution. Though it is possible
to establish a calibration curve of turbidity (or
other optical measurement) versus consistency,
flocculants can be expected to agglomerate the
suspended particles, rendering the base-line
calibration curve inaccurate. Depending on the
purpose of the tests, sometimes such inaccuracies are ignored.
To provide a more convincing demonstration, it is possible to substitute a forming fabric
piece from the paper machine or interest in
place of the standard drilled screen(74). Another
trick has been to apply a small pressure below
the screen, before the start of an experiment, to
prevent any premature passage of liquid
through the screen(59).
Question 2: Mat formation with or without
vacuum?
Tests that involve the formation of a paper
sheet make sense in those cases where one
wants information about how chemicals affect
either dewatering rates or the resulting paper
properties. In principle, such tests also have the
potential to give realistic information about
retention, since a fibre mat has to play a role in
filtering some of the fine materials from the
process water during formation of a typical
paper sheet. We will return later to this issue,
since the relative importance of filtration by
the fibre mat will depend a lot on both the
paper machine situation, and also the selection
of a lab method.
Though there is potential to be more realistic, one has to be careful when making
assumptions about whether sheet-forming conditions prevailing during a particular
laboratory test adequately represent what is
happening on a larger scale.
To address Question No 2, above, let us
start out by considering laboratory test proce-
24
dures that use gravity alone to measure either
drainage rates or fines retention through a
forming mat of fibres.
Simple Filtration Without Vacuum: Simple is often better. This is especially true when
comparing test protocols intended for routine
use in the field. The Canadian Standard Freeness test(75-77) is among the most widely used
means of comparing dewatering tendencies of
different pulp samples. Though the dilute
slurry is agitated before the start of a freeness
test, the test itself is carried out in the absence
of stirring. The fibre mat that forms during a
freeness test tends to filter out cellulosic fines
and other fine particles very effectively, yielding filtrate that is much clearer than what
would be observed on a paper machine running
with the same furnish. Also, the mat tends to
have high resistance to flow, compared to what
might be assumed based on dewatering rates
observed on paper machines.
Nevertheless, in the hands of a careful
tester, freeness tests can provide quite good
reproducibility, with respect to dewatering
analysis. Reproducibility of test results is usually considered to be a priority, compared to
haw closely a test procedure mimics the conditions of hydrodynamic shear found on a paper
machine.
Certain modifications of freeness test procedures are especially worth noting by those
who may be interested in evaluating retention
and drainage chemical programmes. The basic
freeness test procedures involve passage of
some of the filtrate through a narrow orifice at
the base of the apparatus(75). The validity of the
results rests upon the assumption that the viscosity of the liquid phase does not change,
except for a small effect of temperature, which
can be corrected. However, it is well known
that addition of high-mass polyelectrolytes to
aqueous solution will tend to increase the
effect of temperature, which can be corrected.
However, it is well known that addition of
high-mass polyelectrolytes to aqueous solution
will tend to increase the effect of viscosity,
especially in cases where the fluid must converge and pass through a narrow hole(78-91).
Therefore, for evaluation of retention and
drainage chemical treatments it is recommended
to use a modified design of freeness apparatus
that has only one, relatively large outlet for the
flow of filtrate. Rather than measuring the final
volume of liquid collected from a side-port(75),
results are expressed as either the volume of filtrate in a pre-selected time or the time required
to collect a pre-selected volume. Some excellent
mechanistic studies have been carried out by
means of such modified freeness tests with automatic logging of filtrate mass versus time(80-81).
Figure 3, for instance, illustrates the system
used by Sampson, which incorporates a flow
spreader to separate air and water, giving a more
consistent flow to an electronic balance.
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
Once one has modified the standard freeness device with a single, large outlet port, as
noted above, then, with a further modification
it is possible to evaluate effects of increasing
levels of hydrodynamic sheet(82-83). This is done
by adding a stopcock at the base of the apparatus and using a speed-controlled agitator, a
setup that resembles the Britt jar setup shown
in figure 2.
Most often, the point of running the experiment in a modified freeness apparatus, rather
than with a Britt jar, is to be able to compare
the results with routine freeness test results(75-77).
The same kind of experiment can also be
achieved by agitating the furnish samples in a
separate container, then quickly transferring
the material to a freeness test device immediately before drainage evaluation.
Speaking of simple, one of the most elegant
ways to evaluate dewatering effects of chemical additives is to use a “drainage tube”(84-86).
The tube consists of a clear PMMA cylinder,
one end of which is bonded to a piece of forming fabric or a standard screen. The user covers
one end and inverts the tube a specific number
of times – two or more – before allowing the
furnish to drain. Usually one measures the time
required for the level of stock in the tube to
reach a pre-selected mark.
The drainage tube can be an excellent
choice if either (a) you don’t have access to a
suitable freeness test device, or (b) you want to
carry something that is extremely portable and
durable. In one case it was reported that a
drainage tube was more sensitive to the addition of a drainage aid, compared to a freeness
test(85).
Laboratory handsheet procedures also have
been used to compare dewatering rates(84,87-88).
The advantage of this kind of approach is that
drainage times often can be recorded as a
Sealing cone
Slurry
sample
Screen
Spreader
cone
Balance
Figure 3 Modified Schopper-Riegler freeness tester with single, large outlet and
continuous recording of filtrate mass
byproduct of procedures that also result in
testable sheets of paper. For the tests to be useful it is important that the forming screen be in
excellent, clean condition. The gasket of the
handsheet device needs to be properly aligned,
avoiding leakage of air, which might vary from
test to test.
Also, especially when testing low basis
weight sheets, the results can be expected to be
dominated by the way that the initial fibre align
themselves on the screen(89), an effect that can
depend on just how the stock was agitated and
whether there is any residual swirling motion
of the stock when the drain valve is opened.
Some drainage tests have been automated
with level sensors(22,28,90-92), and similar methods
can be used with handsheet tests. Tanaka et
al.(93) carried out an elegant set of experiments,
using a handsheet procedure and sheet-splitting, to show the retention aids tend to make
paper more uniform in composition as a function of distance perpendicular to the plane of
the sheet.
Those with an eye for detail may have wondered why Britt called his now-famous device
the “Dynamic Drainage/Retention Jar.” Most
of the work done by others with that apparatus
has been in the presence of agitation. Usually
only a relatively small amount of filtrate is collected, and the filtrate flow is kept slow. All of
these factors tend to prevent the formation of a
fibre mat. One has to go back to Britt’s very
first report in the series(11) to appreciate that the
Britt jar originally was intended to be used in
two different ways:
i) Tests with the agitator turned on were
intended to emphasise retention effects.
ii) Tests with the agitator turned off before
dewatering, allowing all of the water to
drain under laminar conditions, were
intended to emphasise drainage effects.
One of the earliest experiments of this type
showed a strong effect of salt concentration on
retention of TiO2 particles(11). Subsequent
workers have used similar methods(13,59), often
substituting a coarser screen to make sure that
most of the flow resistance was due to the fibre
mat and not the screen.
Disputes about the meaning or validity of
the original Britt jar procedure(7) have
prompted further useful work. Davison(13)
found evidence that a retention aid can produce
agglomorates of filler particles that are too
large to fit through the holes in the screen of a
standard Britt jar. Li and Scott(94) showed that
the distribution of clay filler between the fibre
surfaces, agglomerates, and as dispersed particles during a Britt jar test was affected by both
the conditions of chemical treatment and by
hydrodynamic shear.
Finally it is worth mentioning that some
automatic pulp quality test devices provide
information that is closely analogous to
drainage rates, though the results may be
25
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
expressed as “permeability”(95). The advantage
of using such equipment is that the results can
be automatically interpreted in terms of an
equivalent hydrodynamic specific surface area,
based on a simplifying model of resistance to
flow through a uniform packed bed(96).
Simple Filtration with Vacuum Applied:
Anyone who has watched the change in
appearance of a paper web on a Fourdrinier
paper machine as it passes over vacuum boxes
will appreciate the importance of vacuum to
promote the release of water. There is a whole
list of test procedures that basically do the
same thing – dewater a sample of pre-agitated
papermaking furnish over a forming screen,
using vacuum to pull filtrate through the
screen(14-28,91).
Names of apparatus that have been coined
to describe such experiments and equipment
include the Water Release Analyser(1,23-24), the
Sample
Screen
Vacuum
pump
Vacuum
transducer
Valve and drain
Figure 4 Apparatus to evaluate dewatering rates and retention in the presence of
vacuum
Vacuum
Screen
is just
covered by
fibres
Dry line (air breaks through)
Final vacuum
value related
to porosity
Mat is
forming
A
B
0
0
Time
Figure 5 Interpretation of vacuum-time output from a DDA test(25-26)
26
C
Retention and Drainage Tester(20), the Dynamic
Drainage Analyser(14,25-26), the dynamic Filtration System(14), the Retention Process
Analyser(72,97), and the Gess/Weyerhauser
(G/W) system(15-17). Figure 4 shows the essential features of the G/W device.
Few investigators have considered the question about whether the presence or absence of
vacuum assistance affects the main conclusions and trends observed with simple
filtration tests(90). Ideally, the application of
vacuum ought merely to speed up the test. In
practice, however, one needs to be aware that
the stronger hydrostatic and hydrodynamic
forces in a vacuum-assisted test are likely to do
such things as compress the fibres, wash fine
particles out of the sheet in some locations, and
possibly cause premature “sealing” of the
forming fabric with an initial layer of fibres
draped over and into its openings(90-91). High
correlations have been reported between some
vacuum-assisted test results and results of freeness tests(15).
Most(14-18,20-22,24-26,28,91), but not all(27), vacuumassisted dewatering tests end up with air being
drawn through the sheet. As shown by Britt
and Unbehend(21), a flocculated sheet tends to
allow relatively easy passage for air to pass
through, resulting in less effective dewatering.
It is important to apply a sufficient amount of
agitation to break up such flocs, and ideally
one would want to break up the flocs to the
same degree as would happen on the full-scale
paper machine under consideration.
Most of the reported work with this kind of
method involves a variable or standardised agitation, either as part of the device
itself(21-23,25-26,28,98) or done separately in a beaker
just before the dewatering test(18,19).
Vacuum-Time Curves: As noted by Gess,
one can learn a lot about a given papermaking
furnish sample just by observing how the measured vacuum changes with time during a
vacuum-assisted dewatering test(15). Results of
such measurements depend on the pumping
capacities and recovery rates of the vacuum
systems employed(14). For instance, a constant
volumetric rate of pumping has been used in
the G/W procedure(17). Figure 5 illustrates the
kind of data that has been obtained with the
Dynamic Drainage Analyser(25-26).
Referring to figure 5, the sharp drop in vacuum, starting at time equal to zero, is
associated with the initial rapid flow of white
water through the forming fabric, i.e. free
drainage through the screen. It seems likely
that point “A” on the curve is related to the
point where the fabric effectively has become
covered with a layer of fibres. The rise in vacuum, going from point “A” to point “B”
appears to coincide with build-up of the fibre
mat. Point “B” appears to be associated with
the “dry line”, just before breakthrough of air.
The vacuum at the right-hand limit of the curve
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
(point “C”) can be used as a measure of the airpermeability of the damp sheet.
These principles have been used to predict
how various additives affect such things as the
position of the dry line on a paper machine(14,26).
At the other extreme, relative to the tests
just mentioned, is the concept of keeping the
applied vacuum nearly constant throughout the
dewatering experiment(90-91). Wildfong et al.
continuously monitored the level of stock, at
its upper surface, by means of a triangulation
method with a laser beam(91).
Stirring or Flow During Mat Formation:
During industrial-scale formation of paper it is
most common for there to be at least a slight
difference in speed between the jet of fibre
slurry and that of the moving fabric or fabrics
on which the paper is formed. The terms “rushing” and “dragging” have been used to
describe the situations where the jet is either
Stock inlet
Baffle
Stock
Forming
screen
Overflow
Drain
valve
Figure 6 Essential parts of an M/K sheet former, achieving some fibre orientation,
and using higher consistency than conventional handsheets.
Motor
Fibre
slurry
Truncated
10º cone
Forming
screen
Filtrate receptacle
and vacuum
Figure 7 Use of rotating cone to provide near-uniform average shear stress at
forming fabric surface during sheet formation(92)
faster or slower than the fabric speed at the
point of impact.
One of the effects of such speed differences
is to cause a preferred orientation of fibres in
the sheet, usually aligned preferentially with
the direction of manufacture. Though such
effects usually are associated with efforts to
improve the uniformity of paper, it is also
worth considering some lab-scale tests that
involve oriented flow.
Figure 6 illustrates a design concept of a
handsheet device that has been called the
“Minidrinier”, to emphasise the fact that the
sheets have characteristics related to those
from a Fourdrinier paper machine(85,100). The
version shown in figure 5 is the M/K Sheet
Former. From the standpoint of evaluation of
additives, the Minidrinier concept has the
advantage that the stock consistency tends to
be much closer to industrial practice, compared
to a conventional handsheet forming procedure. First-pass retention data obtained with
this device was lower, and closer to industrial
practice, compared to straightforward filtration
tests(85,100).
Despite these potential advantages, relatively little has been published with respect to
this method. Those who are interested in highquality paper samples with a controllable (by
trial and error) degree of fibre orientation
should also consider the use of a dynamic sheet
former that uses a centrifugal forming principle(101).
One of the most intriguing recent studies of
retention and dewatering involves the use of a
truncated cone to apply a nearly uniform average shear stress over the surface of a forming
fabric during formation of a sheet(92,102). The key
parts of this apparatus are shown schematically
in figure 7. Remarkably, a certain moderate
stirring action was found to increase, rather
than decrease the resistance to flow through the
mat of fibres that formed on the screen – even
in the presence of flow. Similar observations
were reported by Forsberg and Bengtsson(25).
A likely explanation for these effects is that
the flow tends to align the fibres so that they
can form a dense mat, not unlike well-combed,
wet hair. By contrast, non-aligned fibres are
expected to form a more bulky, porous mat.
One complication that resulted from continued
stirring during mat formation was the tendency
for non-uniform mat thickness as a function of
distance from the axis of the stirring
device(92,102).
Question 3: Whether to apply pulsations
During the industrial-scale production of paper
the continuous mat of fibres (or “wet web”)
passes over a series of dewatering devices,
which may include hydrofoils, blades, and
brief applications of vacuum. Such devices
give rise to pulsations of both vacuum and
pressure(2,29,31-32).
27
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
Pulsation (thickening)
Mode
Filtration
Mode
Velocity
vs. time
Figure 8 The absence or presence of velocity or pressure pulsations during forming is expected to affect the way the fibre mat is formed.
Motor
Control
Unit
Motor
Main Control Unit
House air
House vac.
Drain
Figure 9 The “Turbulent Pulse Sheet Former”(30)
Impeller
Mixing
chamber
Stock
valve
Test
chamber
Screen
Rotating
hydrofoil
Vacuum
transducer
Vacuum
Drain
Figure 10 Pulsed Drainage Device
28
(38-40)
Motor
The pulsations also result in rippling
motions at the surface of a Fourdrinier sheet, a
phenomenon that has become known as
“action on the table”(31,34). Though it is usually
much easier to form a sheet of paper in the laboratory without attempting to reproduce such
effects, it might be argued that generation of
“action” has the potential to make a lab test
more realistic.
In principle, pressure or velocity pulsations
perpendicular to the plane of the wet web of
paper are expected to decrease retention by
“washing” fine particles out of the sheet. Pulsations also are expected to make the paper
slightly more uniform in the plane of the sheet
by loosening up some of the fibre flocs. Figure
8 illustrates the principle of using pulsating
flow during a suitable laboratory test.
Figure 9 shows one of the best-known
portable systems for evaluation of retention
and dewatering in the presence of pressure pulsations(30). This “Turbulent Pulse Former”
employs vacuum regulators, valves, and timing
adjustments to superimpose a series of pulses
during drainage of the sheet. The design also
allows for the jar to be quickly dissembled,
making it possible to weigh the damp sheet.
The moisture content after vacuum application
sometimes is used to predict trends of moisture
after the couch on a paper machine.
The type of device shown in figure 9 can
produce a well-defined frequency of pulsation.
Because the turbulent pulse sheet former
design can yield a high quality sheet of paper
the method has been used for the evaluation of
starch products for dry-strength, as well as for
retention effects(103), and also for evaluation of
sizing agent systems(104).
One might argue that, if you want to mimic
the effects of a hydrofoil, then you ought to go
ahead and use one. The apparatus illustrated in
figure 10 does just that(38-40). The “Pulsed
Drainage Device”, a patented design, has been
used within Betz-Dearborn and now Hercules
for evaluation of their products in the field.
A suspension of stock is treated under continuous agitation in an upper chamber. At a
signal from a computer, a pneumatically activated stock valve – an inverted stopper with a
conical top – is opened, allowing the sample to
fill a lower chamber. Though rotation of the
foil device under the forming fabric at the base
of the lower chamber is continuous, the vacuum can “kick in” after an adjustable delay,
allowing the user to draw conclusions about
the gravity drainage rate vs. the vacuum
response.
Various adjustments in design have made it
possible to increase the accuracy with which
the test results match what is seen later when
the same chemical programmes and dosages
are tried out at a commercial scale(40). These
improvements included an increase in the consistency (and reduced volume) of the slurry
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
samples, use of a coarser forming fabric, and
application of a moderate vacuum as soon as
the sample dropped down into the lower compartment.
The “peak to equilibrium vacuum level”
ratio was found to be a useful parameter when
comparing the relative effectiveness of different treatments. Useful results were obtained at
basis weight values of about 45 g/m2 or higher,
and much of the work with the device has been
in the development of paperboard grades.
The device illustrated in figure 11, the
“Moving Belt Drainage Tester”, was designed
especially to mimic some of the effects of vacuum boxes on a paper machine(41-42). The
moving belt, running continuously under a stationary forming fabric, is covered with holes
that momentarily allow vacuum to reach different areas under the forming fabric. The
duration of pulses can be approximately 2 mS
or longer.
By means of this test the inventors showed
that there can be an optimum condition of both
frequency and amplitude of vacuum pulses to
achieve the most effective dewatering, depend-
Fibre slurry
Forming
screen
Belt with
holes
Vacuum
Figure 11 Moving Best Drainage Tester(41-42) schematic diagram
Fibre slurry
Movement
of piston
Forming
screen
Water
Hydraulic piston
Figure 12 Pulsating flow drainage test device with hydraulic piston
ing on the basis weight and other properties of
the wet paper mat(41).
The results also were consistent with a
model in which the fibres nearest to the forming fabric can act as a kind of “pump”, helping
to dewater the rest of the paper as it goes
through cycles of compression and release(29,41).
This “pumping” action also helps to explain
the absence of filler particles in the part of the
sheet adjacent to the fabric, as shown by sheetsplitting experiments.
The “High Speed Retention Tester”,
unveiled in 2000(105-106), combines some of the
essential features of the moving belt tester, but
the belt is replaced by a series of parallel
hydrofoils.
A key deficiency of the “pulsation” devices
described up to this point in this review is that
one does not know the average velocity of flow
through the forming screen as a function of
time. In the case of valve-actuated puffs of vacuum it takes an undefined length of time for
pressure beneath the forming fabric to come to
equilibrium. Nobody has attempted to make a
detailed prediction of pressure or velocity fluctuations resulting from the hydrofoils or
moving belt motions.
The most elegant effort to overcome the
challenge of precisely defining the average
velocities associated with pulsating flow in a
lab-scale retention test was reported by Persson and Osterberg(36), whose publication
actually predates other developments mentioned in this review, with the exception of
handsheet and freeness tests. Essential features
of their apparatus are shown in figure 12. A
pneumatic control system was used to drive a
hydraulic piston, controlling the flow through a
screen. The resulting pressure pulses were
evaluated with a transducer. Pulse frequencies
could be adjusted up to 100 Hz. It is worth noting that this is one of only a very few
publications in which retention or drainage
were tested with a continuous water column
(no air) below the forming screen(36,107).
Question 4: What can one learn from water
retention?
There is a long history behind the use of centrifugal dewatering tests to determine the
relative amounts of water held within swollen
pulp fibres(43-45). As shown in figure 13, the test
is carried out by placing a sample of wet pulp
in a centrifuge tube insert that has a fitted glass
filter at its base. The tubes are accelerated at
900 g’s(43-44) or 3000 g’s(108) for 30 minutes. The
wet mass of the fibre plug is determined after
centrifugation and again after oven-drying.
The Water Retention Value (WRV) is usually
expressed as the mass of water, divided by the
mass of solids.
Usually it has been assumed that WRV tests
mainly show effects due to water held in tiny
slit-like pores within the cell walls. In particu-
29
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
lar, it has been assumed that “external water” is
mostly removed by centrifugation(45). Recent
results reported by Ström and Kunnas(46) make
it necessary to question that assumption, especially if the WRV test is used to assess affects of
drainage-aid chemicals(27). Contrary to some
theories, these researchers found that the ability of a highly charged cationic polymer to
decrease the WRV increased with increasing
molecular mass.
To explain these results it was proposed that
the drainage aid mainly acts by precipitating
microfibrils flat onto the fibre surface, and that
the WRV contribution due to water within fine
pores probably is less affected by the polymer
treatment. The possibility of water being held
Centrifuge
tube
Filter insert
Damp plug
of fibres
Sintered glass
Absorbent felt
Figure 13 Centrifuge tube insert, as used in Water Retention Value (WRV) tests(43)
Motor &
load cell
Jar
Rotor,
top view
Slurry
sample
Figure 14 Floccky Tester to evaluate effects of additives on strength and persistence of fibre flocs(54).
30
in the regions between fibres during WRV tests
was proposed earlier by Abson and Gilbert and
by Maloney et al.(109-110).
In summary, one can view the WRV test as a
way to estimate the maximum amount of water
that can be removed from a certain furnish
before the wet web leaves the press section of
a paper machine.
There has been a debate, over the years, on
whether it is better to use the centrifugal test
just described, or rather to carry out “fibre saturation point” measurements based on the
inability of very high mass sugar molecules to
enter small pores within the cell walls of
fibres(111). Points raised in the previous paragraph help add ammunition to the argument
that WRV is not always a true or accurate measure of cell wall water.
However, other questions have been raised
regarding the meaning of solute exclusions
tests(112), since the main work in this area did
not take into account osmotic pressure effects
and excluded volumes of solution very close to
surfaces, regions having a low probability of
being occupied by polymer segments in the
absence of molecular attractions(113). Further
work is needed in this area.
Question 5: Can fibre-flocculation tests be
used to evaluate retention aid programmes?
In addition to promoting retention of fine particles(68), and speeding up dewatering(114-116),
addition of chemical flocculants to papermaking furnish usually makes the resulting paper
less uniform due to increased fibre flocculation(33,117). Though fibre flocculation, per se, is
not usually considered to be a desirable effect,
it sometimes can be used as an indicator of the
relative effectiveness of retention aid treatments.
Figure 14 illustrates an innovative test procedure developed in Japan and recently
introduced into the US for lab-scale testing(53-54).
The “Flocky Tester” senses the resistance to
rotation of a specially shaped rotor in the presence of a fibre slurry. The signal becomes
higher after the stock is treated with a flocculating agent. The effect appears to be due to
friction caused by fibre flocs being pressed
between the rotor and the walls of the cylindrical beaker. The maximum torque can be
compared to either the control – untreated fibre
slurry – or to the final torque after a specified
duration of shearing. The rate of decay of the
resistance can be interpreted as an indication of
the durability of the extra degree of flocculation induced by the chemical treatment.
Because the tests are quick, this test has been
proposed for field evaluations of flocculating
chemicals(54).
One of the potential drawbacks of the previously described test is that it gradually
destroys the fibre flocs as it senses their presence. Recently Hubbe(56) demonstrated a
related test procedure that was designed to
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
minimise disruption of the flocs. As shown in
figure 15, pencil-like probes were slowly
rotated through the fibre slurry samples. Resistance to the motion of the probes was detected
with a digital viscometer. Treatment of the
slurry with flocculating agents appeared to
increase the size of fibre floc able to remain on
the front surface of a probe element without
coming apart.
The viscometric test made it possible to distinguish the separate contributions of time and
hydrodynamic shear in breaking down fibre
flocs(56,118). Such results find use in helping to
justify decisions about whether to add retention aids before or after pressure screens(118).
Rotation
Spindle No. 1
Probe
Baffle
Sample
Figure 15 Viscometric test of effects of chemicals on fibre flocculation in a pulp
slurry(56).
Light
Photodetector
Flow
direction
Pump
Stirrer
Figure 16 Fibre floc evaluation by means of a low-gain Photometric Dispersion
Analyser(56).
Another approach to quantifying fibre flocculation
involves
monitoring
light
transmission or reflection as dilute stock flows
through a transparent tube or past a window in
a channel. One of the easiest ways to achieve
such measurements is to use a flow scheme as
illustrated in figure 16, which is based on the
use of a Photometric Dispersion Analyser
(PDA)(56). To study papermaking fibres it is necessary to use a “low-gain” version of the PDA,
with a fitting to accommodate a larger-thanusual tubing size, having an internal diameter
of about 6 mm(56-58).
The addition of a chemical flocculant to a
dilute fibre slurry – paper machine headbox
solids level or lower – has been found to
greatly increase the root-mean-squared variation in intensity of the transmitted light through
the clear tubing. Though such results clearly
indicate changes in the degree of fibre flocculation, more work is needed to understand how
the signals depend on either the size or the density of fibre flocs(57-58).
More sophisticated studies, using the scattering of laser light, have achieved more
success in separately determining the effects of
various treatments on floc size and on different
degrees of flocculation, in cases where the floc
sizes were similar(119).
Some of the most sophisticated evaluations
of retention aids recently have been carried out
by means of a “Focused Beam Laser Reflectant Measurement” (FBLRM or SLM)
method(47-52). This method works by estimating
the “cord lengths” of objects that pass in front
of a sapphire window.
The device rotates a focused beam of laser
light in a circular pattern and detects the
lengths of time over which some of the
reflected light either does or does not return to
the window after being reflected from objects
in the path of the light. Because the light follows a circular path at a known rate of speed, it
is straightforward to convert time periods of
continuous reflected signal to equivalent linear
lengths, i.e. cords of the circular path. FBLRM
measurements have been used, for instance, to
compare the effectiveness of different
microparticle retention aid programmes(48).
Question 6: Can automation help eliminate
operator bias and achieve more reliable
retention and drainage test results?
Though automation certainly will affect the
amount of effort required to run a lab evaluation of retention and drainage chemicals, the
important goal should be reproducibility, i.e.
the precision of the results. This is especially
important if one needs to rely on results
obtained by different staff members. Experience has shown that results of jar-type
retention and drainage tests can be sensitive to
subtle differences in mixing practices, the time
intervals between chemical injections, and
31
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
anything that one does at the moment when
water starts to flow through the forming fabric
or screen(89).
Issues related to portability and cost are
expected to become less important with the
passage of time, especially in systems that can
be run with a laptop computer.
Though automation could, in principle, be
applied to just about any of the tests described
in this review, and some have been(90), let us
focus on one example. As shown in figure 17
the Dynamic Drainage Analyser(14,25-26), which
was discussed earlier, incorporates automatic
control of such things as chemical additions,
stock agitation, opening of the value to start
drainage, and acquisition of dewatering rate
data. As had been pointed out(25), it is often difficult to add chemicals quickly enough in
manual tests to fairly mimic what happens on a
real paper machine. Highly precise repetition
of experimental conditions is also essential
when one wants to distinguish subtle differences between paper sheets that result from
different levels of chemical treatment.
Conclusions and recommendations:
1) Though only a few of the test devices
described in this review are currently available for sale(7,25,120-121), many of them are
simple enough to be reproduced in a
machine shop or the essential features can
be assembled from conventional laboratory
equipment, such as a Büchner funnel.
Since different kinds of tests have different
strengths and weaknesses, it is best to start
out by deciding which type of test is most
suitable for obtaining the information that
you need.
2) For quick, relatively reproducible comparisons of retention effects, not considering
3)
4)
5)
Additives
Impeller
Slurry sample
Screen
Valve
Vacuum
Transducer
Figure 17 Automatic features of the Dynamic Drainage Analyser (DDA)(14,25-26).
32
6)
dewatering effects, it is recommended to
first consider the classic Britt jar test(7). The
equipment can be ordered from Paper
Research Materials(60). Alternatively, similar tests can be carried out with fairly
straightforward modifications of freeness
tests or with a homemade drainage jar.
In cases where one is most interested in
obtaining highly reproducible dewatering
rate evaluations, following treatment with
different chemical recipes, it is recommended to consider the Dynamic Drainage
Analyser (DDA), which also is currently
available(122). The DDA offers advantages of
automation. However, for short-term, lowbudget needs, one should be aware that
similar information could be achieved with
various related devices, some of them
essentially homemade, as described earlier.
If you facility has either freeness testing
equipment or handsheet forming equipment available for your use, such devices
can be used possibly with some modifications, for evaluation of dewatering rates
under simple filtration conditions.
None of the pulsation type of test apparatus described in this review is currently
available for sale as a portable test. It is
recommended that only serious
researchers, willing to devote considerable
development time, attempt to go down this
road. Although pulsations can, in principle,
make tests more representative of what
happens during industrial-scale formation
of paper, there is also a danger of increasing the noise-to-signal ratio.
Water retention value (WRV) tests offer a
way to obtain supplemental information,
requiring very little sample and fairly conventional centrifugation equipment.
Assuming, as noted earlier, that the WRV
results predict the maximum solids that
can be achieved by mechanical means,
there may be situations where such information could help in deciding between
which of two alternative chemical programmes to evaluate first.
Depending on what kind of information
you are looking for, it is recommended to
view fibre-flocculation-sensing tests with
caution. It is important to bear in mind that
a chemical programme that produces a
high degree of fibre flocculation is not necessarily the most desirable programme for
increasing retention efficiency or dewatering rates in a given paper machine
situation. But as long as those cautions are
well understood, then either the Floccky
Tester(54) or the FBRLM(47-52) test equipment
can be obtained. Also, the non-destructive
floc test described earlier can be carried
out with a conventional Brookfield viscometer and a simple modification of one
of the standard probes(56).
PAPER TECHNOLOGY OCTOBER 2003 LABORATORY TESTS
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by Akribi Kemikonsulter AB, Sweden.