Nonionic block copolymer antifoams.
Aqueous dispersions of alkoxylated alcohol block copolymer (BCP) drops are investigated as antifoams. A model aqueous nonionic surfactant solution of Polysorbate 20 and an industrial white water suspension are used as foaming systems. Visual evidence obtained using a two-bubble technique involving a CCD camera coupled with high magnification lenses clearly revealed the role of BCP droplets in the bubble coalescence process. The enhancement of bubble coalescence decreased as the temperature increased from 25 to 60 degrees C, which is due to the corresponding decrease in the rigidity associated with the weak interfacial structure and reduced viscosity of the BCP drops. The antifoaming efficiency measured in the macroscopic recirculation foam column increased with temperature from about 13 to 26 degrees C (attaining a maximum) and decreased as temperature increased further. Oscillatory thermo-rheometric measurements showed a sudden increase in the storage modulus (G') by several orders of magnitude, indicating gel formation initiated at about 13 degrees C and having a maximum at around 26 degrees C for an aqueous solution of the BCP above a critical concentration of around 20 wt %. Results obtained using small-angle X-ray scattering, micro-differential scanning calorimetry, and proton nuclear magnetic resonance confirmed the existence of ordered gel-like structures. Furthermore, macroscopic tests using a sparged air foam column showed a significant increase in antifoaming efficiency when highly hydrophobic particles are embedded in the BCP drops dispersed in water.