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Introduction of alkaline degreasing
Time:2019-3-6 15:51:45  Read:486
I. Principles of Chemical Degreasing

_1) Saponification

Saponification reaction is a chemical reaction between saponified oil and alkaline substances in deoiling solution to produce soap and glycerol. When the product with oil contamination is put into the alkaline deoiling solution, the oil contamination is removed due to the saponification reaction mentioned above.


_2) Emulsification

Mineral oils or other unsaponifiable oils cannot be saponified by alkali, but they can be removed by emulsifying to form emulsions under the action of surfactants. Emulsification is the process of dispersing very small droplets in two dissimilar solutions to form emulsions in another liquid. The surfactant with emulsification is called emulsifier. Anionic or nonionic surfactants, such as sodium silicate, sodium stearate and OP emulsifier, can be used in chemical deoiling.


In the process of oil removal, the emulsifier is adsorbed on the interface between oil and solution, in which the hydrophilic group is affinity to the oil on the surface of the parts, while the hydrophilic group is affinity to the oil removal solution. Under the action of emulsifier, the adhesion of oil stain to the surface of parts gradually weakens. Under the combined action of hydrodynamic factors, oil stain gradually separates from the surface of metal parts, and the tiny droplets are dispersed in the degreasing liquid and become emulsion to remove oil stain on the surface of parts. Heating and stirring the oil removal solution will accelerate the oil pollution into the solution, so it can increase the speed of oil removal and improve the effect of oil removal. Therefore, in chemical oil removal, higher temperature and stirring measures are generally adopted, and ultrasonic wave can also be used to accelerate the oil removal process.


_3) Dispersion


Surfactant is the most important component of degreaser. The early degreasers mainly used emulsifier emulsification, such as aliphatic alcohol polyoxyethylene ether (AEO) series, alkyl phenol polyoxyethylene ether (TX, NP) series and so on. Excessive use of emulsifiers will emulsify and solubilize the fallen oil in the working fluid, resulting in a gradual decline in the oil removal capacity of the working fluid, which requires frequent replacement of the working fluid.


However, with the rising price of surfactants, it is increasingly required to reduce the use of surfactants and increase the degreasing rate. This requires that degreasers have good dispersion and anti-secondary deposition properties. The degreasers will be peeled off the metal surface, not emulsified or saponified in the solution, but floating on the surface of the solution to maintain the clarity and sustained degreasing capacity of the tank liquid.

On the other hand, surfactants suitable for deoiling are generally non-ionic products. Non-ionic products are generally of high price. In order to reduce the cost of deoiling agents, anionic products will also appear in deoiling agent formulations, especially anionic surfactants with non-ionic properties, fatty acid methyl ester ethoxylate sulfonate (FMES), which have excellent dispersion. The "roll-off" feature is conducive to the non-emulsifying peeling removal of grease.


II. Technological Conditions for Chemical Deoiling


_The main components of chemical deoiling liquid include alkali or alkaline salts such as sodium hydroxide, sodium carbonate, sodium phosphate, sodium pyrophosphate, sodium silicate, sodium borate, etc. In addition, metal chelating agents such as EDTA, sodium citrate and various organic additives such as emulsifiers are usually included. Generally, alkaline deoiling liquids contain two or more alkali or alkaline salts. Sodium hydroxide and sodium carbonate are two inexpensive alkaline substances. Sodium phosphate and sodium pyrophosphate are not only the source of alkali, but also play the role of chelating calcium and magnesium ions to soften hard water and buffer the pH value of solution. The colloidal film formed by silicate can protect amphoteric metals such as aluminum and zinc from corrosion. Organic additives play an auxiliary role in emulsifying and dispersing oil pollution.


_Chemical deoiling liquid formula should be selected according to the different properties of matrix materials and deoiling requirements.