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Transient aggregation during dry-down of solvent-borne dispersions
Alvin Y Huang1, Donald K Montrose, John C Berg
1University of Washington, Department of Chemical Engineering, Box 351750, Seattle, Washington 98195-1750, USA.
Langmuir : the ACS Journal of Surfaces and Colloids
|October 19, 2005
Summary
Changing solvent conditions can cause aggregation in sterically stabilized colloids during coating drying. Aggregates formed may not fully redisperse, impacting product quality in processes like magnetic media production.
Area of Science:
- Colloid and surface science
- Polymer science
- Materials science
Background:
- Sterically stabilized colloids are crucial in solvent-borne coatings, where polymeric binders act as stabilizers during drying.
- The interaction between binders and solvent mixtures dictates colloid stability, but tracking changes during rapid drying is challenging.
- Aggregation during film formation can lead to uneven pigment distribution and affect final product properties.
Purpose of the Study:
- To investigate how changing solvency conditions affect the stability of sterically stabilized colloids in evaporating dispersions.
- To determine the recovery time for redispersion of aggregates if destabilization occurs.
- To identify process conditions that may lead to aggregation and assess the redispersion capability of formed aggregates.
Main Methods:
- Utilizing light scattering techniques to monitor colloid behavior during simulated high-speed coating and drying processes.
- Analyzing conformational changes of free polymeric binder chains to understand polymer-solvent interactions and stabilization potential.
- Constructing a "danger zone" indicating conditions where aggregation is probable.
- Employing dynamic light scattering on binder-grafted-oxide particles to detect aggregation within the identified danger zone.
Main Results:
- Measurements of polymer-solvent interactions revealed a "danger zone" where aggregation is likely.
- Dynamic light scattering confirmed aggregation of binder-grafted-oxide particles under these conditions.
- Subsequent attempts to redisperse the aggregates using "good" solvents were not instantaneous, indicating limited recovery time.
Conclusions:
- High-speed coating and drying processes may not provide sufficient recovery time for destabilized colloids to fully redisperse.
- Understanding and controlling solvency conditions is critical to prevent aggregation and ensure uniform film formation.
- The study provides insights into colloid stability during solvent evaporation, relevant for applications like magnetic data storage media production.