Foam Control of Trisiloxane Alkoxylate Superspreaders with Modified Trisiloxane Alkoxylates: A Mechanistic Study

摘要

Foam control of trisiloxane alkoxylate (TSA) superspreader solutions with traditional antifoams (typically silica-filled polydimethylsiloxane) is often poor or not possible. To solve this problem, a new foam control method was developed for TSAs in which the superspreader is blended with a low-foam component that is also a TSA but with hydrophobic end groups. These new low-foam TSAs that act like foam control agents (TSA-AFs) are soluble in the TSA superspreaders, and the two components are preblended (5-20 % TSA-AF) before dilution. The behavior of this low-foaming blend is also unusual. There is a delay in the rupture of the superspreader foam during which seemingly nothing happens, but then the foam collapses so fast that only a high-speed video can show how the collapse progressed (avalanche effect). Nevertheless, TSA-AFs break only foams from trisiloxanes. To understand this new phenomenon and its possible mechanism, we studied foams of TSA solutions with TSA-AFs created by bubbling. The collapse times were random, and after a time lag a metastable state was reached, after which the slightest mechanical shock could trigger the catastrophic collapse of the foam. We found that the foam collapse was caused by the insoluble drops that contained both TSA and TSA-AF, and the effect was strongest when the drops contained a mixture of both. Very small drops (homogenized) did not break the foam as expected. In the proposed mechanism the drops collect inside the Plateau borders of the foam with the highest capillary pressure at the top layer of foam, and then the collapse starts there, creating a mechanical shock, which then rapidly propagates downward.