The Development Trend of Surface Active Agents for Metal Cleaning

The development trend of surface active agents for metal cleaning
Metal cleaning is an indispensable process in metal processing, such as cutting, electroplating, and coating processes, which must be cleaned before processing to remove rust, oil, scale, and dust. Metal cleaning agents can be broadly divided into solvent-based and water-based categories. Solvent-based cleaning agents include petroleum solvents and halogenated hydrocarbon solvents, but due to problems such as flammability, environmental pollution, and energy waste, their use is greatly restricted, and they are gradually being replaced by water-based cleaning agents in many cases. Water-based cleaning agents have become the main development direction of the metal cleaning industry.

Water-based cleaning agents are mixtures of substances, where the main active component is a surfactant, and other auxiliary agents are added. The surfactant has a significant impact on the cleaning process, such as:

① The surfactant can break the intermolecular bonds between the metal surface and the dirt;

② It can wet the metal surface, allowing the cleaning solution to surround the dirt particles;

③ It can emulsify animal and plant oils into small droplets, dispersing them in the solution;

④ It can dissolve solid dirt particles;

⑤ It can disperse various types of dirt, preventing re-deposition. Changing the chemical composition of the surfactant will change its ability to function in each step of the cleaning process. As the surfactant in cleaning agents can break the interaction between dirt particles and the metal surface, but the wetting, penetration, and emulsification ability of these surfactants may not be good enough, therefore, water-based cleaning agents formulations often include several different types of surfactants. The selection of surfactants is also related to the cleaning process. For example, low-foam cleaning agents are needed for spray cleaning processes; cleaning agents with good resistance to re-deposition are needed for ultrasonic cleaning; and high emulsification and wetting properties are needed for pipe cleaning.

Metal surfaces cleaned with water-based cleaners often adhere to a layer of surfactant molecules, which may affect the next process and pose a risk of fire for oxygen system components. Therefore, the metal parts after cleaning must be rinsed thoroughly with deionized water to remove the adhered surfactant. It is best to clean oxygen system components with surfactants that are easily removable.

The widespread use of water-based cleaning agents has also become a serious problem in wastewater treatment. In the past, our focus was on the cleaning performance and lifespan of cleaning agents, but now the formulations of spray cleaning or ultrasonic cleaning agents are completely different from the past. It is not the best to completely emulsify organic contaminants into the cleaning liquid components. Instead, it is hoped that organic contaminants can be released from the emulsified liquid after stirring stops or cooling, so that when fats or contaminants are separated, the cleaning agent can be regenerated. Membrane filtration technology can separate cleaning agents and contaminants, and this filter can separate them based on the size, shape, and charge of the molecules.

In recent years, the properties of microemulsions have also attracted attention. Microemulsions exist in the phase region of a multi-component system consisting of water, oil, surfactants, auxiliary surfactants, and electrolytes, with ultra-low interfacial tension and high solubilizing power for both hydrophilic and lipophilic substances. Therefore, microemulsions can be used as a medium in fabric cleaning to combine the organic solvent's solubilizing effect with the traditional wet cleaning characteristics. Raney et al. have used a typical multi-component system consisting of water, surfactants, auxiliary surfactants, and electrolytes for standard cleaning tests. They believe that the cleaning power of the system reaches its maximum value when the cleaning medium forms a microemulsion with the oil stain during the cleaning process and the cleaning temperature is close to the phase transfer temperature of the microemulsion.