Freeze-thaw (FT) cycles can aggregate particles in aqueous paint suspensions. To understand the mechanism of particle aggregation, cryogenic scanning electron microscopy (cryoSEM) was used to visualize the microstructure after the freezing and thawing steps of the FT cycle. After the freezing step, cryoSEM images show that the microstructure contains ice crystals and particle-rich regions. Adding propylene glycol, a FT stabilizing additive, leads to formation of larger ice crystals. After thawing, the dispersion structure revealed by cryoSEM shows that the particles redisperse only in the paint with the highest amount of propylene glycol. The other paints contain clusters that are different from the particle-rich regions found after the freezing step. Increasing the thawing rate leads to a more dispersed microstructure even in the absence of propylene glycol. Analysis of the cryoSEM results shows that particle aggregation into these clusters occurs during the thawing stage, and slow thawing conditions lead to more aggregation. The cryoSEM results reported here are used to propose a mechanism of aggregation of particles in the paint.
Bibliographical noteFunding Information:
The authors thank The Dow Chemical Company for material donation and supporting the research at UMN. M.M. thanks Chris Frethem for help with cryoSEM. C.L.J. thanks George Charles for help with DSC measurements. Parts of this work were carried out in the University of Minnesota Characterization Facility, which receives partial support from NSF through the NNIN program. Appendix A
- Cryogenic SEM
- Freeze-thaw stability