Introduction to Alkaline Degreasing

The saponification reaction is a process where saponifiable oils react chemically with alkaline substances in the degreasing solution to form soap and glycerol. When oil-contaminated products are immersed in an alkaline degreasing solution, the above saponification reaction removes the oil stains.

Mineral oils or other non-saponifiable oils cannot be saponified by alkalis, but they can be emulsified into an emulsion by surfactants and removed. Emulsification is the process of dispersing one of two immiscible liquids into extremely fine droplets within the other to form an emulsion. Surfactants with emulsifying properties are called emulsifiers. In chemical degreasing, anionic or non-ionic surfactants such as sodium silicate, sodium stearate, and OP emulsifier can be used.

During degreasing, emulsifiers first adsorb at the interface between oil and the solution. Their lipophilic groups interact with the oil on the part surface, while the hydrophilic groups bind to the aqueous degreasing solution. Under the action of emulsifiers, the adhesion of oil to the part surface gradually weakens. Driven by hydrodynamic forces, the oil detaches from the metal surface, disperses into fine droplets in the degreasing solution, and forms an emulsion, thus removing the oil. Heating and stirring the degreasing solution accelerate oil dissolution, enhancing degreasing speed and efficiency. Therefore, chemical degreasing typically uses elevated temperatures, stirring, or ultrasonic waves to expedite the process.

Surfactants are the core components of degreasing agents. Early degreasing agents relied primarily on the emulsification of emulsifiers, such as fatty alcohol polyoxyethylene ether (AEO) series and alkylphenol polyoxyethylene ether (TX, NP) series. Excessive emulsifiers can solubilize detached oils into the working solution, gradually reducing its degreasing capacity and requiring frequent replacement.

With rising surfactant costs, there is an increasing need to reduce surfactant usage while improving degreasing efficiency. This requires degreasing agents to have excellent dispersion and anti-secondary deposition properties, stripping detached oils from metal surfaces without emulsification or saponification. The oils should float on the solution surface, keeping the bath clear and maintaining continuous degreasing ability.

On the other hand, non-ionic surfactants are generally preferred for degreasing due to their effectiveness, though they are costly. To reduce degreasing agent costs, anionic surfactants—especially those with non-ionic properties like fatty acid methyl ester ethoxylate sulfonate (FMES)—are incorporated. FMES exhibits superior "dispersion and rolling separation" characteristics, aiding in non-emulsifying oil stripping.

Chemical degreasing solutions mainly contain alkalis or alkaline salts such as sodium hydroxide, sodium carbonate, sodium phosphate, sodium pyrophosphate, sodium silicate, and sodium borate. They often include metal chelating agents like EDTA and sodium citrate, as well as organic additives such as emulsifiers. Alkaline degreasing solutions typically contain two or more alkalis or alkaline salts:

Degreasing solution formulations are selected based on the substrate material properties and degreasing requirements.