Choose considerations
The physicochemical properties of an ideal defoamer must meet the requirements of the system used. In general, the following requirements must be considered when selecting a defoamer:
(1) Strong defoaming ability, can effectively eliminate foam when using a very small amount;
(2) It has a lower surface tension than the defoaming system;
(3) After the defoamer is added, it will not affect the basic performance of the defoamed system;
(4) It is insoluble in the defoaming system and is not easy to be soluble by the surfactant in the system;
(5) The surface tension balance should be good;
(6) It does not react with the defoamed medium, nor will it be decomposed and degraded by it, and has good chemical stability;
(7) It has good diffusion and permeability, has a positive diffusion coefficient in the foam medium, and has a fast spreading ability on the foam surface;
(8) Good heat resistance, will not lose effectiveness at high temperature;
(9) Good gas solubility and permeability;
(10) It has high physiological activity and safety in the defoaming system, and the defoamer itself is non-toxic or low-toxicity substance;
(11) Low chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total oxygen demand (TOD);
(12) Good storage stability;
(13) It has a long-term defoaming effect, and some defoamers can quickly defoam, but they will fail after a long time;
(14) Low cost;
(15) Do not increase the surface viscosity of the aqueous surfactant solution.
It is difficult for the defoamer to meet the above requirements at the same time, and can only meet the requirements of the defoamed system in terms of main properties. Each defoamer can only be effective for one or several systems, and the same foam system can also use a variety of different defoamers, because of this, users should conduct tests to choose the most effective and economical defoamer.
Turbidity problems
The main components of the defoamer are generally hydrophobic particles, silicone oil and emulsifier, and the hydrophobic particles adsorb the silicone oil to achieve the maximum effect with as little as possible. Silicone oil as the main defoaming medium, the surface tension is very small, neither lipophilic nor hydrophilic, suspended in the system, when the defoamer exists in the middle of the foam wall, the oil-water phase is discharged to produce a defoaming effect, and at the same time, the silicone oil has a small amount of consumption, when the silicone oil outside the hydrophobic particles is completely consumed, resulting in the turbidity of the foam system. Therefore, the defoamer is selected with different amounts of hydrophobic particles, silicone oil, emulsifier dosage and color, resulting in a wide variety of defoamer properties. When the defoamer has a good defoaming effect and a long inhibition time, turbidity will generally not occur in the system.
Oil bleaching problem
Since the defoamer is not dissolved in the system, but dispersed in the system, the uniformity of the dispersion of the defoamer in the system is critical. When the defoamer is evenly dispersed in the system, it has little effect on the transparency of the system, and the time for agglomeration into larger particles is relatively long, and it can be maintained in the system for a long time. When the defoamer is not evenly dispersed in the system, but agglomerates together with many small particles, on the one hand, it will affect the transparency of the system, and on the other hand, the time for the defoamer to agglomerate into large particles will be shortened, which will lead to turbidity after the defoamer is added to the system, and the oil will be bleached the next day. In order to avoid oil bleaching, the following methods can be adopted: move the order of adding defoamer forward; Dilution is performed before being added to the system, either as water or as a surfactant in the system.
Bubble inhibition time issues
The properties of the silicone oil in the defoamer determine the inhibition time of the defoamer, the content of the silicone oil determines the consumption cycle of the defoamer in use, the amount of silicone oil added too little will make the defoaming performance of the defoamer fail to reach the required value, and the addition of too much amount will affect the performance of the defoamer, and the defoaming property of the defoamer will be reduced at the same time; The particle size of the defoamer determines the filtration resistance of the defoamer, and too large particle size may cause the defoamer to be easily filtered, resulting in oil bleaching and affecting foam inhibition. Stirring time is also an important indicator of the defoamer's foam inhibition ability, and insufficient stirring may produce turbidity, oil bleaching, weakened defoaming ability, and shorter foam inhibition time.
Failure issues
Acid-base stability. Silicone oil can destroy the surface tension of the liquid, to eliminate the effect of foam, if the defoamer has poor acid and alkali resistance, it will lead to the decomposition of silicone oil, which will lead to the reduction of defoaming ability, or even failure, adding silicate to the system will generally inhibit its decomposition; Defoamer solubility. Certain chemical components dissolve the silicone oil into the system, so that the defoamer no longer has a defoaming effect, but exists in the system as a surfactant, and the foam in the system is higher than when no defoamer is added.
