foam


In the process of industrial production, as long as it involves mixing, there will be foam problems, such as pulp and paper making, textile printing and dyeing, coating processing, sewage treatment, biological fermentation, oil extraction and refining, construction industry, adhesives and so on. Foam in these industrial processes may cause many problems, such as reduced production capacity, waste of raw materials, and prolonged reaction cycles; Product quality declines and so on.


It can be seen that the control and elimination of harmful bubbles has great technical and economic significance.


Foam is a dispersion system with a large number of bubbles dispersed in the liquid, the dispersed phase is a gas, and the continuous phase is a liquid. Because the relative density of the bubble is less than that of the liquid, the bubble in the liquid will rise to the liquid surface, forming a bubble aggregate separated by a liquid film composed of a small amount of liquid, that is, foam.


Foam is a thermodynamically unstable system. The factors affecting its stability include the surface tension of the liquid, the properties of the interfacial film, the repairing effect of the surface tension, the surface charge, the diffusion of the foam gas and the structure of the added surfactant.


defoamer


As the name suggests, it is an aid to remove foam. Foam elimination in the industrial process basically depends on defoamer.




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Defoaming mechanism


When a surfactant is added to the solution system, a layer of surfactant molecules in a directional arrangement is adsorbed on the surface of the bubble, and when it reaches a certain concentration, the bubble wall forms a solid film. The surfactant adsorbed on the gas-liquid interface causes the surface tension of the liquid surface to decrease, thus increasing the gas-liquid contact surface, so that bubbles are not easy to merge. As bubbles rise through the liquid surface, they adsorb a layer of surfactant molecules on the surface. Therefore, the bubble film adsorbed with surfactant exposed to air is different from the bubble film in solution, it is coated with two layers of surfactant molecules, forming a bimolecular film, and the adsorbed surfactant has a protective effect on the liquid film.


The function of defoamer is to destroy and inhibit the formation of the above liquid film. The surface tension of the defoamer is low, and it is easy to spread on the surface of the solution. After entering the bubble liquid film, it replaces the surface active molecules on the surface of the original liquid film, so that the surface tension at the contact point is lower than that at other parts of the liquid film. Due to the high surface tension of the liquid foam film, the contraction force will be generated, so that the liquid film with low surface tension will be pulled and extended around, forming a film with poor strength and gradually thinning. The effective elastic contraction force cannot be generated and the self-healing effect is lost. The viscosity of the liquid foam film is also greatly reduced, the liquid foam film discharge speed and gas diffusion speed are accelerated, and the life of the foam is shortened. Eventually the rupture was eliminated.


In short, the addition of defoamer can increase the surface tension of the foam liquid film in the solution system, lose the self-healing effect, and reduce the viscosity, and finally lead to the destruction of the mechanical balance of the liquid film and the foam rupture, so as to achieve the purpose of defoamer.


A good defoamer has the function of defoaming and defoaming at the same time, that is, it should destroy the foam quickly and prevent the formation of foam for a long time. It is often found that some defoamer loses its effectiveness after being added to the solution for a certain time, and some defoamer needs to be added to prevent foam formation. The reason for this may be related to whether the critical micelle concentration cmc of the foaming agent (surfactant) in the solution is exceeded. In the solution exceeding this concentration, the defoamer (generally organic liquid) may be solubilized, so that the effect of spreading on the surface is lost, and the defoamer effectiveness is greatly reduced. When the defoamer is added, its spreading speed on the surface is greater than the solubilization speed, showing a good defoamer effect. After a period of time, as the defoamer is gradually solubilized, the defoamer effect weakens accordingly.








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Type of defoamer


(1) The first generation defoamer


The first generation of defoamer is mineral oil, fatty acid and fatty acid ester, fatty amide, low alcohols and other organic matter.


Mineral oil such as fire oil, turpentine, liquid paraffin, etc. Can be used in printing paste, paper and other industries.


Fatty acids and fatty acid esters such as butter, lard, soybean oil, castor oil, glycol stearate, sorbitan monolaurate, etc. This kind of defoamer can be used in papermaking, pulp, dyeing, architectural coatings, fermentation and so on.


Alcohols such as coconut alcohol, hexanol, cyclohexanol. It can be used to make sugar, ferment petroleum refining and so on.


Amide such as distearyl ethylenediamine, dipalmitoyl ethylenediamine and so on.


The price of organic defoamer is low, and it is still widely used in the market. It is suitable for use under the condition that the liquid shear force is small and the foaming capacity of the surfactant is mild, but the elimination ability of dense foam is poor. Due to the diversity and particularity of the application field of organic defoamer, it is often specific, and its market share has been shrinking, so it has limitations in application. However, some special industries such as strong acid, strong alkali, etc., need organic defoamer that is resistant to acid and alkali like polytetrafluoroethylene. In recent years, due to the importance of environmental protection, the status of natural oil defoamer has been improved.






(2) Polyether defoamer


The second generation of defoamer is polyether defoamer.


Polyether is a block copolymer of polyoxyethylene polyoxyethylene propylene with low molecular weight. It is a water-soluble non-ionic surfactant with excellent performance. When in contact with water, the oxygen atom in the ether bond can combine with the hydrogen atom in water with weak chemical force to form hydrogen bond. The molecular chain joint becomes zigzag, the hydrophobic group is placed inside the molecule, and the chain around the chain becomes easy to combine with water. The combination with water is lost, and the temperature is heated from the low-temperature dissolution state to the turbidity temperature, which is the turbidity point of polyether, and only when the temperature of the foaming system exceeds the turbidity point temperature, the polyether defoamer plays a defoamer role.


The turbidity point can be changed by adjusting the ratio and molecular weight of oxyethylene and oxyepropylene in the preparation process, which is suitable for different occasions. Polyether defoamer has developed rapidly since 1954, when it was first put into production by Wyandott Company in the United States, especially in the 1960s, with the rapid development of polyether industry, polyether defoamer has developed more rapidly. Our country was successfully developed and put into production in 1967.


Polyether defoamer mainly includes straight chain polyether and polyether derivatives with alcohol and ammonia as starting agent or end group esterification.


Polyether defoamer has a wide range of applications, and it is an indispensable additive variety in the paper industry, and has important applications in many industries such as daily chemical, textile, petroleum, food, rubber and so on. However, due to the selective relationship, some areas still cannot be used. Therefore, it is necessary to improve the quality of polyether defoamer and develop polyether surfactants with different structures.


The biggest advantage of polyether defoamer is its strong anti-foam ability, which is the leading defoamer used in the fermentation industry. But it has a fatal disadvantage is the low bubble breaking rate, once a large number of bubbles are produced, it can not be effectively extinguished, but the need to add a certain amount of new defoamer to slowly solve the problem.


The first and second generation of defoamer do have a certain defoamer effect under certain conditions, but the structural characteristics of these substances determine that they have a certain foaming effect, when the conditions change, the first and second generation of defoamer is likely to become foaming agent or failure. Therefore, in order to adapt to the changing requirements of defoaming and operating conditions, silicone defoaming agent, the third generation defoaming agent, has been developed.






(3) silicone defoamer


Silicone defoamer is a kind of defoamer widely used in dyeing and finishing, food, fermentation, paper making, chemical production, adhesive, latex, lubricating oil and other industries.


In China, silicone defoamer has been developed and used since the 1970s, and has achieved good results in recent years. But on the whole, the grade is low, the variety is also relatively simple, and there is a big gap compared with foreign countries. At present, the high-efficiency silicone defoamer used in some imported devices mainly relies on imports.


Silicone defoamer due to its wide range of applications, its domestic and foreign commodity models are no less than dozens, generally polydimethylsiloxane, some occasions also need ethyl, hydroxyl, phenyl and other siloxanes. Dimethylsilicone oil belongs to the linear polymethylsiloxane of MDnM type, the molecule is easy to wind the "zigzag" chain, the molecular structure determines that it has the following characteristics:


Low surface tension: the surface tension of polydimethylsiloxane is lower than that of water, surfactant aqueous solution and general oil, so it is suitable for defoamer;


In water and general oil, the solubility is low and the activity is high: polysiloxane molecular structure is special, the main chain is silicon oxygen bond, is a non-polar molecule, and the polar solvent water is not compatible, and the affinity with general oil is also very small;


Low volatility and chemical inertia: dimethylsilicone oil is extremely low volatility, but also low surface tension, the combination of the two, so that the defoamer can play a defoaming role in a wide range of temperatures;


No physiological toxicity: dimethylsilicone oil generally used as a defoamer has a higher degree of polymerization. However, dimethylsilicone oil without oligomer is not physiological toxic.


Due to these advantages, there are more varieties of such defoamer and the amount of use is also large, and it has become the main variety of defoamer. At present, the silicone defoamer sold in the domestic market can be divided into four categories according to physical properties: oil, solution, emulsion and solid.






(4) polyether modified polysiloxane defoamer


The fourth generation defoamer is polyether modified polysiloxane defoamer, which is introduced into polyether chain segment through modification on the polysiloxane chain segment. In the siloxane copolymer molecule, the siloxane segment has a lipophilicity and the polyether segment has a hydrophilicity. In the polyether chain segment, the energy saving of the polyether chain provides hydrophilicity and foamability, while the energy saving of the polyether chain provides hydrophobicity and permeability.


Polyether modified polysiloxane defoamer not only has the characteristics of strong defoamer, low surface tension, low volatility, non-toxic, pollution-free, physiological inertia, but also has the characteristics of high temperature resistance and strong alkali resistance of polyether defoamer. Polyether modified silicone defoamer is a new kind of efficient defoamer which combines the advantages of both organically. Besides, it also has many excellent properties.


Strong defoaming effect: it has the characteristics of low surface tension and high activity inherent in polysiloxane, and has the advantage of polyether easily dispersed in water. Therefore, it is not only stronger than conventional silicone defoamer, but also can solve the problems that conventional defoamer can not solve in some occasions.


Reverse solubility: polyether modified polysiloxane at low temperature, polyether chain and water form hydrogen bonds, carrying polysiloxane chain evenly diffused into water, similar to the dissolved state; When the temperature rises, with the destruction of hydrogen bond, the hydrophilicity of polyether chain becomes worse, until the turbidity point, the hydrophilicity is lost, so that the whole polysiloxane polyether chain becomes insoluble state, and the defoaming effect is played. Polysiloxane polyether copolymer has low temperature solubility and high temperature insolubility, which effectively improves the application performance of polysiloxane.


According to the above characteristics, it is convenient to select the polysiloxane polyether defoamer with corresponding cloud point according to the required defoaming temperature of the system. In addition, the convenience of reverse solubility can also be used in a system that is not dissolved in water at low temperature, and the dissolved defoamer can be rinsed with cold water lower than its turbidity point.


Self-emulsion: When the polysiloxane polyether copolymer is added to water at an appropriate temperature, it will spontaneously form a dispersed state of polysiloxane chain segment curled inside and polyether segment stretched outside, forming micelles in water. This property is called "autolysis". The self-emulsion makes the polysiloxane polyether type defoamer disperse evenly in the foaming solution, which helps to give full play to the defoamer effect.


Stability: Polysiloxane polyether defoamer has good dispersion stability in water due to its good self-emulsion property. The copolymer of -Si-O-C - bond is easy to hydrolyze and has poor chemical stability, but it does not affect the defoaming effect. The copolymer connected by -SI-C - bond is more stable, and the thermal stability of all kinds of polysiloxane polyether defoamer is better. Within the validity period, it can be used for boiling water systems, and even for bubbling water systems at high temperatures of 130 ° C.


Non-toxic: polysiloxane polyether defoamer is allowed to be added to food packaging as an indirect food additive.


Donald et al in the early 1950s, the first synthesis of such copolymers, in recent years, this type of defoamer has developed rapidly, has been derived into a series of products, and polyether defoamer, polysiloxane defoamer, the formation of three major synthetic defoamer.








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Development trend of defoamer


With the rapid development of the national economy, a large number of auxiliaries are used in various industries, but this will produce a large number of harmful bubbles. In order to improve production efficiency, defoamer has become an indispensable chemical additive. Silicone defoamer, polyether defoamer and mineral oil defoamer are the main varieties of defoamer in today's era. How to further improve the performance of defoamer and reduce the amount of defoamer has become an important task for scientific researchers. The main development trend of these three types of defoamer should be as follows:


(1) Silicone defoamer has different emphasis in different industries, improving the quality of silicone defoamer is crucial, its quality mainly includes foam suppression performance, defoamer performance, stability, compatibility, anti-shear performance and so on;


(2) The foam inhibition performance of polyether defoamer was improved by studying the structure and composition of polyether;


(3) Improve the dispersibility, stability and anti-bubble performance of mineral oil products;


(4) Through the design and modification of silicone molecules, there are specific types of defoamer for specific industries.