In industrial production and daily life, due to a variety of reasons will produce a large number of foam, foaming properties of good substances known as foaming agent.Some anionic surfactants such as sodium fatty acid, sodium alkyl benzene sulfonate and sodium alkyl sulfate have good foaming properties.It should be noted that foaming agent only under certain conditions (stirring, ventilation, etc.) to have good foaming performance, the formation of foam may not be durable.Generally speaking, any foaming agent that can reduce the surface tension of liquid and increase the film strength has higher foaming power, no matter whether the generated foam is stable or not.In this paper, foam and its stability principle, foam elimination method are summarized in detail, and the mechanism of action and performance evaluation method of defoaming agent are introduced.
1 Foam stability principleThe study of foam can be traced back to Plato’s time, but the definition of foam has not been unified for hundreds of years.American colloidal chemists l.i.ossipow and r.f.smith of dow corning defined foam in terms of its density;In Japan, koichi ITO defined foam from the perspective of foam structure, but ignored the interrelation between bubbles.The surface of the famous physicist professor zhao of foam is defined as: bubble is dispersed in the liquid gas dispersion system, gas is dispersed phase (discontinuous phase), liquid is dispersive medium (continuous phase), bubbles rise to the level of the liquid, separated with liquid membrane formation consisting of a small amount of liquid gas bubbles aggregates.At present, scholars at home and abroad agree that foam itself is a thermodynamic unstable system. When the gas enters the solution containing surfactant, the foam system will be stable for a long time.
1.1 The attenuation mechanism of
Under the combined action of gravity and pressure difference, the foam liquid film will flow unevenly to drain the liquid, and the gas in the bubble will continuously diffuse and permeate due to the pressure difference between the two sides of the bubble film. The instability of foam is mainly reflected from the perspective of dynamics.The attenuation mechanism mainly includes two aspects: gas diffusion through liquid film and liquid film drainage. These two properties are inherent properties of the foam itself and have nothing to do with the presence or absence of surfactants.Gas diffusion through liquid film and liquid film drainage only play an obvious role in the initial stage of formation of the foam system. With the attenuation of the foam system, these two functions gradually weaken, making the foam decay rate gradually slow down.
1.2 Stable factors
Bubble is the direct cause of due to surfactant to reduce surface tension of solution, the stability of the different foam system show different, the main influence factors include: foaming solution viscosity, surface tension, foam surface solution viscosity, surface tension of self-repair function (i.e., Gibbs surface elasticity and Ma – rangoni effect), liquid membrane surface electric double layer repulsion repulsion and entropy, surfactant hydrophobic structure, space steric effect and so on.The main factors influencing the stability of foam are different in different foam systems.In addition to the above factors, foam size, temperature, pH, solvent evaporation rate, foam impact degree and surfactant adsorption rate will also affect foam stability.
2 Foam elimination method
2.1 Physical methods
Physical methods mainly include placed baffle or mesh, mechanical stirring, electrostatic, refrigeration, heating, steam generating, radiation, high speed centrifugal, pressure relief, the high frequency vibration, instantaneous discharge and ultrasound (acoustic liquid control) and so on, these methods to different extent promoted the liquid film on both ends of the through rate and of the gas bubble film drainage, the foam stability factor is less than the attenuation factor, so that the number gradually reduced.The disadvantages of these methods are that they are strongly restricted by environmental factors and the defoaming rate is not high.
2.2 Chemical methods
Chemical method mainly includes chemical reaction method and adding defoaming agent method.Chemical reaction method means that by adding reagents to make it and foaming agent chemical reaction, the formation of water insoluble substances, thus reducing the concentration of surfactant in the liquid film, promote foam burst.Because the composition of foaming agent is uncertain, chemical reaction method will produce insoluble substances, there are disadvantages such as harm to equipment.At present, the widely used defoaming method is to add defoaming agent, the advantage of this method lies in the high efficiency of defoaming, easy to use, but the key is to find the appropriate, efficient defoaming agent.
3 Type and application of defoaming agent
3.1 species
Defoaming agents can be divided into solid particle type, emulsion type, dispersion type, oil type and paste type 5 categories;According to the application in different industrial production can be divided into textile industry defoaming agent, paper industry defoaming agent, coating industry defoaming agent, food industry defoaming agent and oil industry defoaming agent;According to chemical structure and composition, it can be divided into mineral oil, alcohol, fatty acid and fatty acid ester, amide, phosphate ester, organic silicon, polyether, polyether modified polysiloxane defoaming agent.
3.2 application
Mineral oils, amides, lower alcohols, fatty acids and fatty acid esters, phosphate esters and other organic defoaming agents have been applied earlier, which belong to the first generation of defoaming agents, which have the advantages of easy access to raw materials, high environmental performance, low production cost and so on.Polyether defoaming agents are the second generation of defoaming agents, mainly including straight chain polyether, polyether starting with alcohol or ammonia, polyether derivatives with end-group esterification.Polyether defoaming agents are widely used in cleaning, paper making, fermentation and other industries. The advantages are strong anti-foaming ability, and some polyether defoaming agents are resistant to high temperature, strong acid and strong alkali, etc. The disadvantages are that the application conditions are limited by temperature, narrow application field, poor defoaming ability and low defoaming rate.Silicone defoaming agent (the third generation of defoaming agent) is made of silicone grease, emulsifier, thickener, etc. with an appropriate amount of water through mechanical emulsification, the characteristics of which are: small surface tension, high surface activity, strong defoaming force, low dosage, low cost, most bubble media can defoaming;It has good thermal stability and can be used in a wide temperature range.Good chemical stability, can be used in acid, alkali, salt solution.Organosilicon defoaming agent has both anti-foaming and defoaming functions on all bubble systems, and belongs to the broad-spectrum defoaming agent.It is widely used in the petroleum industry for desulfurization of natural gas, acceleration of oil and gas separation, drying of ethylene glycol, extraction of aromatic hydrocarbons, processing of asphalt, dewaxing of lubricating oil and other devices to control or inhibit bubbles.Used in textile industry for defoaming in dyeing, refining, sizing and other processes;In the chemical industry for synthetic resin, latex, paint, ink and other defoaming;In the food industry for a variety of concentration, fermentation, distillation process of defoaming.Different fields of application, silicone emulsion defoaming agent or focus on defoaming or focus on foam suppression or focus on compatibility.Non-silicon defoaming agents are mainly composed of fatty amides, metal soaps, fatty alcohols, fatty acid esters and other organic compounds, which have the advantages of large diffusion coefficient and strong ability to break bubbles, but the disadvantages are poor ability to inhibit bubbles.For the use of silicone defoaming agent is likely to lead to silicon spots, shrinkage, the impact on product quality of the industry will often use non-silicon defoaming agent.Polyether modified polysiloxane defoaming agent has the advantages of polyether defoaming agent and organosilicone defoaming agent, and has the unique advantages of strong inverse solubility, good self-emulsification, high chemical stability and high thermal stability, which is the development direction of defoaming agent.In general, in polyether chain segment, the increase of polyoxyethylene chain segment will improve the hydrophobicity of the copolymer, while the increase of polyoxyethylene chain segment will improve its water solubility.If the proportion of polysiloxane chain segment and polyether chain segment is fixed, and the proportion of polyoxy propylene in polyether chain is increased, the solubility of the copolymer in water will be reduced, the turbidity point will be reduced, and the defoaming performance will be improved.Polyether modified polysiloxane defoaming agent is widely used in polyester fabric high temperature dyeing process and fermentation process defoaming, in addition, it can also be used in diethanolamine desulfurization system defoaming and various oil agents, cutting fluid, non-freezing liquid, water-based ink system defoaming and printing industry photosensitive resin plate after washing the defoaming of uncured resin.At present, there are few kinds of this kind of defoaming agent, which is still in the research and development stage, and the production cost is relatively high.
Post time: Dec-17-2019