The following is a slide show of the presentation I gave at the 1997 American Chemical Society Division of Colloid and Surface Science Symposium at the University of Delaware:
Click on title or image for full page scale slides.
We have a general interest in the behavior of
oil/aqueous interfaces, but today I will address the particular
subject of improving agglomeration processes for de-inking
toner-printed papers. Our most current work compares previous
agglomeration studies to a direct, single-particle analysis
accomplished with an atomic force microscope (AFM) set up for colloid
probing. This colloidal force microscopy is not a conventional use
of AFM and has only come into practice for fluid/fluid interfacial
studies in the last couple of years.
The critical separation step in toner de-inking
by oil-assisted agglomeration is the coalescence of dispersed toner and
oil into large aggregates which can be easily removed from the repulped
paper slurries leaving clean cellulose fibers. Furthermore, the key is to
create aggregates of adequate strength to remain stable against the high
shear forces experienced in the repulpers. Thesis work completed by Bret
Snyder showed that cationic polymers commonly present in most papers
inhibit oil-toner coalescence by electrosteric repulsion between starch
adlayers. Agglomerating agents of simple oils with surfactants are able
to overcome this steric stabilization to wetting the toner. It is the
addition of a surfactant into the oil phase which allows coalescence to
occur with starch-covered toners.
In recent tests, we used a commercial brand, modified
potato starch (STA-LOK 400--derivatized with quaternary ammonium groups)
as an adsorbing polymer similar to cationic starches present in many
papers known to inhibit toner aggregation (separation from paper slurries
by oil agglomeration processes). The positive charges in the cationic
starch chain (electrosterically) associate with the negative surface
charges on the toner particle and the oil-aqueous interface, forming
adlayers found to be 49 nm and 33 nm in thickness from dynamic light
scattering. Significant amounts of starch were also adsorbed by the
oil/surfactant blend, but with no change in droplet size. This, along
with zeta potential(from electrokinetic) measurements, reveals the
quite remarkable situation of hydrophilic starch engulfment into the
oil drop leaving no adlayer on the aqueous side. Our oil/surfactant
blend was a commercial product for toner-printed paper recycling from
Betz Paperchem, Inc., consisting of roughly 60% simple aliphatic oils
and 40% largely hydrophobic (HLB--low hydrophile-lipophile balance)
surfactants. The presence of the oil surfactants is enough to allow
starch penetration into the oil phase. I should note that the Betz
product requires elevated temperatures for maximum performance, but we
are only making room temperature comparisons at this stage in the
research.
Our main
objective was to use an atomic force microscope (AFM) to approach an oil
droplet under water with a toner particle at similar conditions found in
repulping agitation tanks and measure forces of interaction. The three
parameters we've chosen to focus on with AFM are the attractive force
between the toner and oil, the applied force required to induce toner
attachment or film rupture, and the induction time, which we estimate from
the time the toner encounters the interface until it enters the oil (first
repulsion to snap-in). These values are measured from the resulting
force-versus-distance curves acquired with the AFM and then compared with
advancing oil contact angles on toner flats under water. This
single-particle setup is also the first step toward a direct comparison to
bulk agglomeration studies which gauge the final degree of toner
dispersion (aggregation).
This schematic shows an unfused toner
particle roughly 10 microns in diameter epoxied at the end of a flexible,
(commercial) AFM cantilever which is lowered into the liquid cell for
probing. A Teflon tube (ID = 2.25 mm) glued to the bottom of the cell
anchors the n-hexadecane under water. The liquid cell sits on the AFM
scanner which uses piezoelectric transducers to move the cell,
specifically, the oil-water interface, toward and away from the toner.
When the toner is near the oil interface, it will experience attractive or
repulsive forces deflecting the cantilever down or up. The interface is
also free to deform as a result of long-range interactions and physical
contact with the toner. A laser reflecting off the back of the cantilever
is directed to a photodetector (PSPD) measuring the shifts in position.
The stiffness of the cantilever is known in terms of a spring constant, so
the measured deflection corresponds to a quantifiable force typically in
the nanoNewton range (from pico- to micro-Newtons).
We've observed three common interactions in our oil-aqueous-toner systems.
Force profile "A" illustrates the main features for contacting oil and
toner in pure water, and curve "B" shows the decidedly different behavior
when cationic starch is added to the water. The horizontal line is the
undeflected cantilever signal, or zero interaction force, as the oil
approaches the toner. There is a (1) rapid increase in force at or near
contact with the oil/water interface. This initial repulsion due to
viscous draining increases to some maximum load when the intervening
aqueous film ruptures and the toner snaps into a new configuration
(equilibrium contact) partially engulfed by the oil. Further advancing
continues at a fairly constant compliance, indicating the cantilever is
deflecting linearly with displacement. Reversing the direction, (2) the
toner drags the interface along and must be pulled back more than 50
microns to detach it from the oil. This large pull-off force can only
correspond to a capillary force resulting from a three-phase contact line
with radius approaching the toner size. When cationic starch is present to
form steric adlayers, (3) the approach curve is monotonically repulsive.
No attractive snap-in is observed, and little or no pull-off force is
required to detach the toner. Thus, agglomeration will fail because the
oil and toner cannot coalesce in the presence of cationic starch.
All of
our experiments used a toner particle approaching and retracting from the
interface of interest at approximately 30 um/s to simulate the high-shear
environment experienced in repulpers. I've shown an example raw data
approach curve for the pure water case where the toner is engulfed by the
oil. The differences seen in the traces for pure water and one-tenth
weight percent (0.1 wt%) aqueous cationic starch are very remarkable but
we anticipated them from our bulk studies. For oil in pure water, the
induction time for the toner was 52 ms at an average applied force of 5.6
nN before the aggressive snap-in which was off scale and greater than 9
nN. A very rough estimate of the snap-in distance was calculated to be
>180 nm; this value mostly represents toner entry into the oil. The
addition of cationic starch at least delayed interfacial rupture beyond
191 ms and above 77 nN applied load. These pure oil results are in
agreement with the starch-induced stability against coalescence previously
observed. When the pure oil is replaced with the oil/surfactant blend,
noted as Betz, we once again record a measurable induction time, about 133
ms, at loads around 0.7 nN. The snap-in is much weaker than recorded with
pure oil (3.5 nN). This time the addition of cationic starch has a much
less dramatic effect on coalescence (151 ms, 1.4 nN, 2.5 nN) agreeing with
the past observations of the oil/surfactant's ability to imbibe the
cationic starch adsorbed to both toner and oil. The induction time and
load are slightly elevated with a smaller snap-in, suggesting that the
starch may still be working against complete wetting of the toner.
These
single-particle measurements in oil/aqueous/toner systems further
illustrate the roles of cationic starch and oil surfactants in the
coalescence of toner and agree with bulk agglomeration studies. Steric
stability against toner attachment to oil due to the adsorbed starch is
overcome by the addition of a largely hydrophobic surfactant to the oil
phase. A good qualitative comparison can be made between the snap-in
forces as measured by the AFM and the advancing contact angles of oil
against toner under water. Only the case of pure oil in the presence of
cationic starch showed interfacial stability (no interfacial rupture) and
a non-wetting contact angle (95°). Direct comparisons of oil and the Betz
product are not easily made because of the effect that differing
interfacial strengths have on the AFM measurements. Both pure water cases
show that toner is wet by the oil phase, but toner with cationic starch
more easily attaches to the oil/surfactant than the pure oil. Comparing
both Betz AFM experiments with contact angles suggests that the starch has
little effect on toner wettability with the oil/surfactant additive.
In order to
further substantiate our physical understanding of the force-distance
profiles, we want to obtain optical images of toner attachment to oil to
confirm wetting contact configurations. The next logical extension of this
work is to bridge the gap between single-particle coalescence studies of
toner and oil and the bulk agglomeration of toner. Not only do the
particles need to coalesce for successful separation from the repulped
slurries, but the liquid bridges between the toner particles must be of
sufficient strength to survive shear breakup. We'd like to look at the
interaction of an oil-covered particle with a toner flat in water and
cationic starch dispersions. Measuring and comparing pull-off forces of
attached particles will provide a quantitative and complementary study for
visual agglomeration observations. I mentioned at the beginning of the
talk that the commercial oil/surfactants require high temperatures for
maximum performance. Another research possibility would be to vary
oil/surfactant compositions to determine the best ratio for room
temperature operation.