Substances that can reduce the surface tension of water, solution and suspension, prevent foam formation, or reduce or destroy the original foam.  

The defoamer should have the following properties: ① strong defoaming force and less dosage; ② adding to the foaming system does not affect the basic properties of the system, that is, it does not react with the defoaming system; ③ the surface tension is small; ④ It has good balance with the surface; 5) good heat resistance; 6) good diffusion and permeability; 7) high positive spreading coefficient; 8) stable chemical property and strong oxidation resistance; 9) good gas solubility and permeability; 10) low solubility in foaming solution; 11) no physiological activity and high safety.  

Defoamers are widely used in food industry, paper industry, water treatment industry, oil extraction industry, printing and dyeing industry, coating industry, detergent industry, rubber latex industry, aerosol industry, daily chemical industry, pharmaceutical industry, dairy industry, etc.

Brief introduction

Defoamer is an additive to eliminate foam. In the fields of coating, textile, medicine, fermentation, papermaking, water treatment and petrochemical industry, a large amount of foam will be produced, which will affect the quality and production process of products. Based on the inhibition and elimination of foam, special quantitative defoamer is usually added during production.

Foam generation and stability factors

The study of bubbles can be traced back to the Platon era. But for hundreds of years, the definition of bubbles has not been unified. The American colloid chemist L I Osipow and R F Smith Smith of Dow Corning company have defined the bubble from the density of foam. Itou Koichi of Japan has defined the bubble from the perspective of foam structure, but has ignored the correlation between bubbles. The famous Chinese surface physicist professor Zhao Guoxi has defined foam as the dispersion of gas dispersed in the liquid. In the system, the gas is the dispersed phase (discontinuous phase), and the liquid is the dispersive medium (continuous phase). The bubbles in the liquid rise to the liquid surface and form a bubble aggregate composed of a small amount of liquid separated by a liquid film. Domestic and foreign scholars agree that foam is a thermodynamic instability system. When gas enters the solution containing surfactant, it will form a long time stable foam system.

Attenuation mechanism of foam

Under the condition of gravity and pressure difference, the liquid film of the foam will not flow evenly, and the gas in the bubble will continue to diffuse and permeate because of the different pressure difference between the two sides of the bubble film. Therefore, the instability of the foam itself is mainly reflected in the dynamics.  

The mechanism of attenuation is mainly the two aspects of the diffusion of gas through the liquid membrane and the liquid discharge of the liquid membrane. These two properties are the intrinsic properties of the foam, and it doesn't matter whether the surfactant exists or not. However, the two attenuation mechanisms are more obvious in the initial stage of the foam system formation, and the two functions gradually weaken with the decay of the foam system. The rate of foam decay gradually slowed down.

Stabilizing factors of foam

The direct cause of foam production is the presence of surfactants, which reduces the surface tension of the solution. Due to the combined effect of foam attenuation mechanism and different reasons, different foam systems exhibit different stability properties which are mainly related to the following factors: the surface tension of the foaming solution, the surface viscosity of the foam, the viscosity of the solution, and the self repairing effect of the surface tension (i.e. Gibbs These factors do not exist independently. The change of one factor will change other factors. The most important factor affecting the stability of foam is the elasticity of liquid film and the rate of liquid drainage. From this point of view, we can see that the main factors affecting the stability of foam in different foam systems are different, and sometimes there are several influencing factors at the same time.  

In addition to these factors, some factors also affect the stability of foam, such as the size of the foam, the compatibility of solute and solvent, temperature, pH value, the evaporation rate of the solvent, the impact degree of the foam, and the adsorption rate of the surfactant.

Elimination of foam

(1) Physical methods

From the point of view of physics, the methods to eliminate foam mainly include placing baffles or filters, mechanical agitation, static electricity, freezing, heating, steam, radiation, high speed centrifugation, pressure reduction, high frequency vibration, instantaneous discharge and ultrasonic (acoustic liquid control). These methods promote the permeation rate of gas at both ends of the liquid film and the drainage of bubble film to varying degrees. The stability factor of foam is less than the attenuation factor, and the number of bubbles gradually decreases. However, the common disadvantages of these methods are that they are strongly restricted by environmental factors, and the defoaming rate is not high. The advantages of these methods are environmental protection and high reuse rate.  

(2) Chemical methods

Chemical methods to eliminate foam include chemical reaction and defoamer.

Chemical reaction means that chemical reaction is made with some reagents to form insoluble substances, thus reducing the concentration of surfactants in the liquid membrane and promoting the rupture of foam. However, this method has the disadvantages of indeterminate components of foaming agent and insoluble substances which cause harm to the system equipment. At present, the most widely used defoaming method in all walks of life is the method of adding defoamer. The biggest advantage of this method is that it has the advantages of high efficiency and easy to use, but the key is to find suitable and efficient defoamer.

Defoaming mechanism of defoamer

There is no unified understanding of the action mechanism of defoamers. According to the mechanism of defoamers proposed by predecessors, there are roughly the following types:

Defoaming mechanism with generality

The typical defoaming mechanisms are Robinson defoaming mechanism and Ross hypothesis. The Robinson mechanism is the basis of the Ross hypothesis. It mainly emphasizes the defoamer to destroy the foam drainage and Marangoni effect to defoaming. The hypothesis of Ross is that the defoamer particles are carried out on the basis of non soluble droplets. In fact, the defoaming effect of some defoamer is carried out under the condition of dissolution, so the defoaming mechanism of the Ross hypothesis is not comprehensive.