What characteristics should a good defoamer possess?
Release Time:
2022-01-27
Source:
What characteristics should a good defoamer possess?
In mortar materials, the low viscosity of the paste allows air bubbles to rise due to buoyancy; as these bubbles migrate to the surface, the liquid film surrounding them experiences a pressure differential caused by gravity, leading to thinning of the film and eventual bubble rupture. However, this inherent defoaming mechanism in cement-based materials alone is insufficient to meet the comprehensive performance requirements of products such as self-consolidating concrete and self-leveling mortars, necessitating the addition of an appropriate defoaming agent for further defoaming treatment.
The functions of defoamers include foam inhibition and foam destruction. Foam inhibition refers to the phenomenon whereby, upon addition of a defoamer to a system, its molecules randomly distribute across the liquid surface, thereby suppressing the formation of elastic films and, consequently, the generation of bubbles. Foam destruction occurs when a large number of bubbles have already formed in the system; at this point, the addition of a defoamer causes its molecules to rapidly spread over the bubble surfaces, forming an extremely thin bilayer film that further spreads and penetrates, infiltrating layer by layer to replace the original thin wall of the bubble film. Due to their low surface tension, these molecules readily migrate toward regions of higher surface tension, causing the liquid film to thin rapidly. At the same time, the bubbles are subjected to strong tensile forces from the surrounding high-surface-tension film layers, leading to an imbalance in the stresses acting on the bubbles and ultimately resulting in foam destruction. Defoamer molecules that are insoluble in the system then migrate to the surface of another bubble film, and this process repeats continuously. During the water-mixing process for dry-mix mortars, the entrained air becomes encapsulated by the wet mortar to form bubbles; meanwhile, admixtures such as water-reducing agents, redispersible latex powders, and cellulose ethers contain hydrophilic groups that reduce the surface tension of the liquid, promoting bubble aggregation and enhancing bubble stability.
Defoamers exhibit a certain degree of specificity and application exclusivity. The foaming mechanisms and process conditions vary for each foaming system, necessitating the use of different defoamers; however, regardless of the type, a defoamer should possess the following characteristics:
(1) Strong defoaming power with low dosage;
(2) The addition does not affect the fundamental properties of the system;
(3) Low surface tension;
(4) Good surface balance;
(5) Good diffusivity and permeability;
(6) Excellent heat resistance and acid–alkali resistance;
(7) Chemically stable with strong oxidation resistance;
(8) Good gas solubility and permeability;
(9) Low solubility in foaming solutions;
(10) High physiological safety.
