Transient aggregation during dry-down of solvent-borne dispersions.
The present work investigates the role of changing solvency conditions on the stability of sterically stabilized colloids in evaporating solvent-borne dispersions and the recovery time for the redispersion of aggregates if destabilization occurs. Process conditions during the conversion of the coated fluid dispersion to a solid film must be carefully controlled to ensure that aggregation, leading to uneven pigment distribution, does not occur. Although a polymeric binder serves as a retention aid in a solvent-borne coating, it also act as a steric stabilizer during dry-down or curing of the coating. Because the dispersion medium in the coatings often is a mixture of solvents and nonsolvents, the binder's interactions with the dispersion medium govern whether aggregation is likely to occur. Tracking such an occurrence may be difficult due to rapidly changing solvent compositions. Light scattering is utilized herein to indicate if the conditions of a high speed coating and drying process (in this case, the production of magnetic data storage media) may lead to aggregation and, if aggregation is detected, the ability of aggregates to fully redisperse. Conformational changes of free polymeric binder chains in solution are followed to gauge the polymer-solvent interactions and, therefore, the binder's ability to prevent aggregation. A "danger zone" (conditions where aggregation is probable) is constructed from these measurements, and further dynamic light scattering measurements on binder-grafted-oxide particles detect aggregation in this zone. The aggregates are redispersed in a series of "good" solvents, and redispersion is found not to be instantaneous, suggesting that high-speed drying processes may not allow the flocculated colloids enough recovery time.
Huang
Alvin Y
AY
University of Washington, Department of Chemical Engineering, Box 351750, Seattle, Washington 98195-1750, USA.
Montrose
Donald K
DK
Berg
John C
JC