In this paper, the development background, molecular structure and mechanism of polyether modified silicone oil defoamer are introduced, the structural characteristics of polyether modified silicone oil defoamer are briefly summarized, the main problems of defoamer are discussed, and the future research direction is put forward.






Foam exists in people's life and production all the time, and excessive foam will bring a lot of inconvenience to production and processing, and even affect the quality of products. At present, antifoam agent is mainly used in concrete admixture, admixture is an important component of concrete. The polycarboxylic acid mother liquor itself has a large gas content, which makes the concrete bubble phenomenon. If the combination of defoamer and polycarboxylic acid superplasticizer is used, it will eliminate many large bubbles in concrete, effectively alleviate the adverse effects of foam on concrete, and improve production efficiency, which are quite a concern.








1. Development background of defoamer






The development of defoamer is mainly divided into four stages.






The first type of defoamer is mainly composed of various organic compounds, such as fatty acids, mineral oils, fatty amides, low alcohols and so on. Most of these defoamants are natural substances, which have little pollution to the environment and low price. Due to the weak defoaming performance, it can not meet the production requirements for a large number of foams.






The second type is polyether defoamer CnH(2n+1)O(EO)a(PO)bH, polyoxyethylene segment (EO) is a hydrophilic group; The polyoxyepropenyl segment (PO) is an oleophilic group. This defoamer has good anti-foam performance, non-toxic, non-irritating, and has good dispersion. In actual production, the relative molecular weight of polyether and the proportion of EO and PO in polyether chain segments are adjusted to meet the needs of various production fields. At present, polyether defoamer in the market mainly includes linear polyether, ether amine, fatty alcohol polyether, fatty acid polyether and other polyether derivatives.






The third type is silicone defoamer, which has better defoamer performance than the first two. Usually, the silicone oil with a larger molecular weight is insoluble in the surfactant solution, and for small molecules, the silicone oil is easily soluble by the surfactant molecule, which changes the composition of the defoamer and weakens the defoamer performance. For the defoamer mixed with silicone oil, it is insoluble in water and mineral oil, so it has strong anti-foam ability and greatly enhances its defoaming effect. Because Si-O and Si-C bonds are relatively stable, they are allowed to be used in systems containing small amounts of acids, bases, and salts. Most of the silicone defoamer commonly used in industrial production is a class of silicone oil secondary processing products made by adding dispersion additives to silicone oil, which belong to liquid mixtures. In terms of appearance and fluidity, the mixture is mainly divided into five categories: silicone paste type, silicone oil type, emulsion type, solution type and solid type.






The fourth type of defoamer is polyether modified silicone oil defoamer. On the basis of the previous generations of defoamer, it strives to integrate the advantages of polyether and silicone, and introduces polyether chain segment into silicone oil chain segment, which has stronger defoamer and defoamer performance. Polyether modified silicone oil is a new type of special silicone surfactant, which is formed by addition reaction of polyether containing double bond and low hydrogen silicone oil. It is a kind of silicone non-ionic surfactant with unique properties, which not only retains the properties of silicone oil such as high and low temperature resistance, weather resistance, release property, hydrophobicity and physiological inertia. In addition, polyether has the characteristics of non-irritating, high surface activity and easy dispersion, polysiloxane chain segment is lipophilic, polyether chain segment is hydrophilic, which makes polyether modified silicone oil soluble in both water phase and oil phase, so it can be used as silicone surfactant; Polyether chain segments can also be used as emulsifiers to keep the properties of the emulsion stable and have good dispersion properties. Therefore, polyether modified silicone oil has a long-lasting defoaming effect in water system. Polyether modified silicone oil has good heat resistance, shear resistance, acid and alkaline resistance, non-toxic and tasteless, stable storage, so it is widely used, especially when silicone oil defoamer is difficult to solve.








2. Defoaming mechanism of defoaming agent






2.1 Causes of foam






Under the action of mechanical force, the insoluble gas enters the liquid, and is isolated by the liquid into a non-uniform system, thus forming a large number of gas-liquid interface results. Because the density of the gas phase is < the density of the liquid phase < the density of the solid phase, there will be a phenomenon of the gas phase moving to the liquid surface under the action of buoyancy, resulting in two-phase foam or three-phase foam. Foaming liquid is almost always a solution, pure liquid is generally not foaming. If the liquid is stirred, the liquid surface has to produce a relatively stable liquid film, then the gas can not escape outwards, the resulting bubble aggregation will become foam, this isolated bubble aggregation is commonly known as foam. The stability of the foam has a great influence on the viscosity, surface elasticity and surface rheology of the liquid phase, and it is generally difficult to obtain a stable and durable foam.






2.2 Defoaming mechanism of defoaming agent






The addition of defoamer in concrete can prevent the formation of bubbles in concrete mix or reduce the original bubbles. The defoamer added in the test is a surfactant, which can reduce the surface tension of the liquid phase. When the wetting Angle is reduced to a certain extent, the bubbles escape or burst from the adsorption of the solid surface, so the addition of the defoamer can eliminate the bubbles in the concrete to a certain extent, effectively prevent or eliminate the honeycomb and pockmarked surface of the concrete. The surface of the concrete has a high flatness and gloss, and the surface performance is improved.






The meaning of defoaming mainly has two aspects, one is "bubble breaking", that is to eliminate the foam that has been produced; The second is "bubble suppression", that is, to prevent the regeneration of bubbles. The defoamer with excellent performance often has both excellent defoaming and defoaming ability.






Although defoamer has been used for a long time, different types of defoamer are needed for different defoamer systems, and the mechanism of action of defoamer is still uncertain due to different chemical structures and properties. There are three generally accepted defoamer mechanisms:






(1) Ross hypothesis When an antifoaming agent is added to the foaming system, the antifoaming agent molecules will be close to the foam.






First, the defoamer adheres to the foam liquid film and begins to invade, and gradually expands on the foam film, causing the local thickness of the foam film to decrease sharply, and finally causing the bubble to merge or burst.






(2) Defoamination mechanism of hydrophobic solid particles After adding hydrophobic solid particles to the foaming solution, the hydrophobic particles will immediately adsorb the hydrophilic end of the surfactant in the foaming solution, so that the outermost layer of the hydrophobic particles becomes the hydrophilic end of the surfactant, which has hydrophilicity and enters the aqueous phase, and the hydrophobic solid particles adsorb a large number of surfactants in the foaming system. The concentration of surfactant in the foaming solution plummets, causing the foam to burst. This defoaming mechanism cannot explain the action mechanism of other defoaming agents, which is too one-sided.






(3) Defoaming mechanism of polyether modified silicone oil To explain the defoaming process of polyether modified silicone oil as a defoaming agent, the most complete defoaming mechanism is: "bridging - stretching" mechanism and "bridging - dehumidification" mechanism.






"Bridging - stretching" mechanism: The surface tension of the defoamer is much lower than that of the liquid film. Droplets of the defoamer can continue to spread and deepen on the surface of the liquid film, and the local liquid film of the foam continues to thin and eventually form a bridge between oil and water. The surface tension of the oil phase and the water phase is very different. Causing the bubble to burst.






"Bridge - drainage" mechanism: After the solid hydrophobic particle defoamer is added to the foaming solution, the defoamer is immediately distributed in the foam system, and the hydrophobic particles are fixed on the surface of the foam liquid film. When there is enough hydrophobic Angle between the solid particles and the liquid film, the solid particles have opposite contact surfaces with the surrounding liquid film, and become a bridge between the surrounding liquid film, and finally can penetrate the foam liquid film and enter the foam.






Based on the unique low surface tension of silicone oil, the "bridging and stretching" mechanism points out that the droplet of defoamer can produce different degrees of deformation, but it is difficult to explain the difference between silicone paste and pure silicone oil. The mechanism of "bridging and expelling moisture" is based on the lipophilicity of silicone oil, which can explain the action principle of low viscosity polyether modified silicone oil.






Therefore, polyether modified silicone oil defoamer has three characteristics of defoamer: first, it is basically insoluble in the foaming solution (most of the dissolved can help the foaming effect); Secondly, the surface tension is lower than the foaming solution; Finally, it can be quickly dispersed in the foaming liquid. Only the substance with small solubility and large dispersion can become a defoamer with good defoaming and defoaming ability, so as to maximize its dispersion and achieve the purpose of both defoaming and defoaming.






Ross has clearly said: no one defoaming mechanism can cover all the defoaming phenomena, and various and complicated defoaming agents can correspond to a variety of defoaming mechanisms.








3. Structural characteristics of polyether modified silicone oil defoamer






At present, polyether modified silicone oil PESO is currently produced and used the most a class of silicone oil, which through the linear dimethyl silicone oil main chain or side chain and polyether chain connected polymerization, is currently the market production and the most widely used modified silicone oil. The oxygen atoms in the polyether chain segment in the molecular structure of the silicone oil can form hydrogen bonds with water, so that it can be dissolved in aqueous solution. The polysiloxane segment is neither hydrophilic nor oil-philic, and the hydrocarbon group on the main chain is also not hydrophilic. Therefore, polyether chain segments and polysiloxane chain segments are linked together by chemical bonds generated by hydrosiloaddition reaction to form a non-ionic surfactant with unique properties. Due to the wide variety of polyethers, a series of different polyether modified silicone oils with different properties and functions can meet the needs of diversified markets.






The main polyether modified silicone oil structure of PESO defoamer has five types: AB type, ABA or BAB type, (AB) type, side chain type and branched chain type, and the A and B division n indicates the polysiloxane chain segment and polyether chain segment. Among them, Si-O-C is the condensation of silicone oil and polyether, which belongs to the hydrolytic type. Literature studies [19] show that even if it is hydrolyzed, it is still significant in defoaming performance. Its typical molecular structure is shown as follows (R=H, alkyl, acyl oxide, etc.) :








PESO defoamer combines the water solubility of polyether with the low surface tension of silicone. It has a hydrophilic group EO in the polyether chain segment, hydrophobic group PO, appropriate adjustment of the proportion of the two, there will be a strong low surface tension, the formation of a strong defoaming ability, will increase the concrete surface water absorption, surface carbonization depth, thereby improving the surface performance of concrete as a whole.








4. Problems with PESO defoamer






After the 1970s, China began to engage in the development and application of polyether modified silicone oil, but the defoamer industry started late, and the replacement of domestic defoamer still cannot keep up with the pace of industrial development. Domestic supply of Si-C polyether modified silane is mainly dependent on the import of this channel, because compared with foreign Si-C polyether modified silicone oil research, China's research on it is lacking a lot, basically no new products and new processes appear, on the one hand, because, can produce special polyether and special hydrogen-containing silicone oil manufacturers are less. Limited selection of raw materials; On the other hand, it is because the synthesis of polyether modified silicone oil process or the traditional synthesis process, so the conversion rate of the product is low, there are more crosslinked products in the product, the product quality is poor, and its economic benefits are difficult to be guaranteed.






According to the original synthesis process to prepare polyether modified silicone oil, the cost of catalyst chloroplatinic acid in addition reaction will be very high, and the reaction conditions are very harsh. If the amount of catalyst in the reaction is small, the side reaction is difficult to control, and the appearance color of the product will be deepened and the reaction time needs to be extended. The solvents in the reaction (toluene and isopropyl alcohol) are harmful to the environment and human body. If the solvent remains, the surface tension of the product will increase at the same time. In addition, in order to remove the remaining solvent from the reaction, additional processing processes are required, and the equipment is complex.






On the market at present is a wide variety of defoaming agent varieties, defoaming agent defoaming effect can be mostly, inhibit effect is not strong, make customers spent money, they want to reach the defoaming effect, that is not safe to use, or simply give up the use of defoaming agent. Usually, many admixtures in concrete have a certain air entraining effect, and the bubbles introduced have certain side effects on the strength of concrete. Good defoamer is incorporated into concrete to inhibit or eliminate excessive or harmful bubbles in concrete, which can improve the density of concrete and thus improve the strength of concrete. Therefore, it is undoubtedly very important and urgent to develop a defoamer with excellent performance.








5, the development trend of polyether modified silicone oil defoamer






With the continuous development of PESO defoamer, many new active defoamer components have emerged, which makes the study on the reaction mechanism of the compound defoamer and the synergistic effect of each component more in-depth. Therefore, the defoamer with single composition and poor economic benefit will be gradually replaced by multi-functional and high-efficiency compound defoamer, which will be the future trend of the defoamer market.






The introduction of new groups such as -NH2 (amino group), Cn(H2O)m (sugar group), C6H5COC6H5 (benzophenone) and -COOH (carboxyl group) on the basis of the original PESO can give PESO more reactivity and versatility. With the increasing demand for cleaner production in the world, the development of new varieties of efficient, green and multi-functional surfactants has become the leading direction of the development of PESO defoamer.