Foam is a colloidal system in which a gas is dispersed in a liquid. In a papermaking system, the gas is usually air and/or carbon dioxide and the liquid usually is water. Foam can exist either as bubbles of entrained air in bulk or as surface foam. Surface foam has a polyhedral shape (many-sided) which can be seen floating in chests, wire pits, trays, headboxes and on the machine wire. Surface foam is surrounded by a thin film of foam stabilizing particles of surfactant and water. Surface foam reduces drainage efficiency and causes rings, fisheyes and holes due to bursting of foam bubbles on the sheet. Surface foam also can foul papermaking equipment when surface foam collapses because it leaves behind finely divided solid particulates which are the foam stabilizing particles. Having a water shower on the surface may break the surface foam but does nothing for the entrained air.
Entrained air is the tiny spherical bubbles finely dispersed below the surface of the liquid and adsorbed on the surface of the fibers. Entrained air can greatly decrease the drainage rate of paper stock by blocking the voids in the fiber mat with tiny bubbles and hence machine speeds may have to be reduced to provide sufficient drainage time. Entrained air can also lower some of the sheet properties such as wet strength, tensile strength, smoothness, and also increase sheet porosity.
Most conventional antifoam or defoamer compounds consist of a hydrophobic material having a melting point greater than 40.degree. C. or an insoluble material such as silica, dispersed in an oil phase. The production of these compounds requires the distinct, consecutive steps of heating and cooling. Due to the use of high melting point hydrophobic materials, the resulting compound quickly becomes unstable and the propensity for undesireable deposition in the treated aqueous system is very high.