Evaluating the effectiveness of chemicals for stickies control.

Application: This study develops a simple procedure for screening the effectiveness of agents used for stickies control.

Mixing an aqueous suspension of stickles under turbulent conditions induces agglomeration, since the particles collide strongly enough to overcome their mutual repulsion. The turbidity of the water progressively decreases at a measurable rate, which increases with increasing turbulence and decreasing pH. Stickies control additives can accelerate agglomeration, and the clarification rate can be used as a measure of the effectiveness of the additive. This study evaluates talc and a chemical additive. We found that while talc did not alter the agglomeration rate, it led to a detackified agglomerate. In contrast, the chemical additive greatly enhanced agglomeration but did not reduce tack.

Experimental

Agglomeration studies were run in a 5-L temperature-controlled, glass mixing chamber (Bioflow 3000). A stirrer with a radial flow impeller was controlled at 50-1000 rpm. Four removable metal baffles reduced solid-body rotation and minimized vortex formation. Agglomeration kinetics were measured under acidic conditions, since agglomeration either did not occur or occurred very slowly at pH>4.

Measurements were usually made at 50[degrees]C and at 1000 rpm by periodically withdrawing samples and measuring their optical density at 650 nm, the wavelength typically used to measure turbidity. Zeta potential and particle size were measured with a Malvern Zetasizer 3000. Samples containing talc (whose particle size of >1.0 [micro]m exceeded that of the acrylate stickles used) were allowed to settle for a day prior to the zeta measurement; the absorbances of the other samples were taken immediately upon sampling. Rate constants were calculated from 10-20 equally spaced absorbance measurements taken for each run and were usually of the first order ([r.sup.2]>0.99); the uncertainty was about 20% for the uncured stickies but was much higher (at about 100%) for the cured material.

A stable suspension of cured acrylate was prepared as follows. The latex (10 mL) was spread evenly on a glass plate, dried with a stream of hot air, and cured overnight. The film was then scraped off and added to 600 mL of hot deionized water with 1 mL 1.0N potassium hydroxide and 50 [micro]L BRD surfactant, a blend of fatty acids and a nonionic surfactant obtained from Buckman Laboratories. The suspension was then chopped into smaller particles with a high-speed homogenizer. Most of the particles were quite large and settled to the bottom of the beaker, and the yield of dispersed fine material was quite small. After cooling to room temperature, the solids were further homogenized to produce a milky suspension, from which some further settling occurred. The decanted liquid was then used for the kinetic work. A comparison of the particle size distribution of cured and uncured material is provided in Fig, 1.

[FIGURE 1 OMITTED]

Conclusions

In summary, we used turbulent mixing to accelerate the tendency of stickles to agglomerate. This allows conditions that promote agglomeration, e.g., pH, to be better defined. It also enables measurement of the efficiency of chemicals and minerals used to control stickies through their effect on the agglomeration rate or on the quality of the agglomerate. These chemicals are meant to deactivate stickles through various mechanisms. For example, some cause detackification, others agglomeration. Our technique allows some of these mechanisms to be delineated and provides an objective means of selecting the dose.

Banarjee and Merchant work at the Institute of Paper Science and Technology, 500 10th St. NW, Atlanta, GA 30318, USA. Address correspondence to Banerjee by email at [e-mail].