Defoaming mechanism of defoamer


1 causes of bubbles


Foam is produced by mechanical force, which causes insoluble gas to enter the liquid and is separated from each other by the liquid into an inhomogeneous system, resulting in a large number of gas-liquid interfaces. Because the density of the gas phase is less than the density of the solid phase, the gas phase moves along the liquid surface with the buoyancy force, producing two phase foam or three-phase foam. Foaming liquids are almost always solutions. Pure liquids generally do not foam. If the liquid is stirred, a more stable liquid film will be formed on the surface of the liquid. When the gas can not escape outwards, the bubbles will become foam. The isolated bubble aggregate is commonly known as foam. The stability of foam has great influence on the viscosity, surface elasticity and surface rheology of the liquid phase. It is generally difficult to obtain a stable and durable foam.


Defoaming mechanism of 2 defoamer


The addition of defoamer in concrete can prevent the formation of air bubbles in concrete mixtures or reduce the original air bubbles. The defoamer added in the experiment is a kind of surfactant, which can reduce the surface tension of liquid phase. When the wetting angle is reduced to a certain extent, the bubbles will escape or burst out from the adsorption of solid surface. Therefore, the addition of defoamer can eliminate the bubbles in concrete to a certain extent and effectively prevent or eliminate the surface of concrete. The honeycomb and linen surface of concrete have higher smoothness and glossiness, and the surface performance has been improved.


The meaning of defoaming is mainly in two aspects, one is "breaking bubbles", that is, eliminating the froth that has been produced; the two is "inhibiting bubbles", that is, preventing the regeneration of bubbles. The defoamers with excellent properties often have excellent foaming and anti-foaming abilities at the same time.


Although defoamers have been used for a long time, different types of defoamers are needed for different defoamers. Due to different chemical structures and properties, the mechanism of defoamers is still different. There are three widely accepted defoamer mechanisms:


(1) the Ross hypothesis adds defoamers to the foaming system, and the defoamer molecules will be close to the foam.


First, defoamer adheres to the foam liquid membrane and begins to invade and gradually expand on the bubble film, resulting in a sudden decrease in the local thickness of the bubble film, which eventually leads to bubble coalescence or disintegration.


(2) Defoaming mechanism of hydrophobic solid particles When hydrophobic solid particles are added to foaming fluid, hydrophobic particles will immediately adsorb the hydrophilic end of surfactant in foaming fluid, making the outermost layer of hydrophobic particles become hydrophilic end of surfactant, having hydrophilicity, entering water phase, and a large number of hydrophobic solid particles adsorb the surface of foaming system. Surfactant causes the concentration of surfactant in the foaming solution to drop suddenly, resulting in the burst of foam. This defoaming mechanism can not explain the mechanism of other defoamers, it is too one-sided.


(3) The defoaming mechanism of polyether modified silicone oil explains the defoaming process of polyether modified silicone oil as defoaming agent. The most complete defoaming mechanism includes bridging-stretching mechanism and bridging-dehumidifying mechanism.


The mechanism of "bridging stretching": the surface tension of defoamer is much lower than the surface tension of liquid film. The droplet of defoaming agent can be continuously spread and deep on the surface of liquid film, and the local liquid film of foam will continue to thin, eventually forming the bridge of oil in the middle of water. The surface tension of oil phase and water phase is very different, and the oil phase is constantly in the surrounding water. Under the traction of the ground, the elongated and thinned, the deformation is beyond a certain range, and the liquid film is destroyed, resulting in the burst of foam.


Bridging and draining mechanism: after adding solid hydrophobic particles to foaming liquid, defoamers are distributed immediately in the foam system, and hydrophobic particles are fixed on the surface of the foam liquid membrane. When there is enough hydrophobic angle between the solid particles and the liquid film, the solid particles have opposite contact with the surrounding liquid film and become the surrounding area. The bridging between the liquid membranes can eventually pierce the foam film and enter the foam.


Bridging-stretching mechanism is based on the unique low surface tension of silicone oil, which is easy to spread. It is pointed out that defoamer droplets can produce different degrees of deformation, but this mechanism is difficult to explain the difference between silicone paste and pure silicone oil. Bridging-dehumidification mechanism is based on the lipophilicity of silicone oil and can explain the effect of low viscosity polyether modified silicone oil. Principle.


Therefore, polyether modified silicone oil defoamer has three characteristics of defoaming: first, it is basically insoluble in the foaming fluid (most of the dissolved foaming aids); secondly, the surface tension is lower than that of the foaming fluid; finally, it can be quickly dispersed in the foaming fluid. Only the substances with small solubility and large dispersibility can be used as defoamers with better foam breaking and anti-foaming ability, so as to maximize their dispersibility and achieve the goal of strong foam suppression and anti-foaming.


Ross once said clearly that no defoaming mechanism can cover all defoaming phenomena. Various and complicated defoamers can correspond to various defoaming mechanisms.


Transfer from China Admixture Network