Читать книгу Additives for Water-borne Coatings - Wernfried Heilen - Страница 53

Most foams are thermodynamically unstable and undergo structural changes over time. The effect of gravity on foams causes drainage of the liquid from the lamellas. This results in a reduction of the thickness of the lamella. Small gas bubbles disappear by diffusion, which causes a reduction in the number of lamellas and the nodes between them. As drainage continues, ball foam converts into polyhedral foam, causing a deformation of the lamella and an increase in their radius. In some foams drainage creates a sufficiently high tension in the lamella for them and the foam to collapse spontaneously.

Steric effects or electrostatic repulsion caused by surfactants can compensate the tension in the lamellas created by drainage. This counteracts further thinning of the lamella. Polyhedral foams stabilised in this manner are highly stable. To prevent the formation of stable polyhedral foams or to destroy them it is necessary to use defoamers [3] [6] [8].