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.

ACS Slide 1: Colloid Force Measurements At Oil/Water Interfaces

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.

ACS Slide 2: Toner-Ink Oil-Assisted Agglomeration

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.

ACS Slide 3: Cationic Starch Adlayers

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.

ACS Slide 4: Objectives

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).

ACS Slide 5: AFM Cell For Liquid-Liquid Interfaces

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).

ACS Slide 6: AFM Force vs. Distance

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.

ACS Slide 7: Quantitative Results

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.

ACS Slide 8: Conclusions

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.

ACS Slide 9: Future Research

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.