How to choose a degreasing agent and improve degreasing efficiency

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I have been working in the field of metal cleaning and rust prevention for 10 years now; it’s been 10 years since I graduated from school. From knowing nothing at the beginning to having some understanding now ； From having nothing when I first graduated to now having my own little family and daughter, looking back on the past ten years fills me with many emotions. After graduating, I continued to work in the laboratory, doing what I loved; there were moments of **, as well as periods of uncertainty, confusion, and monotony. It’s comforting that I’ve kept working hard and persevering for ten years, without giving up or settling for mediocrity. That is what brings me the greatest satisfaction. With nothing much to do, I’ve written this piece sharing my work experience in oil removal and degreasing; I welcome feedback and exchanges from those with more experience as well as my peers. How to choose a degreasing agent and improve the degreasing effect. Degreasing is also known as oil removal. In the pre-treatment of metal surfaces, degreasing is a very important step. Oil stains on metal surfaces arise from the material processing steps and various stages involved in the fabrication of parts. Anti-rust oils and similar products are used during storage and transportation to prevent rusting. Generally, metal materials and parts must have any oil stains on their surfaces removed before undergoing further surface treatments such as electroplating, coating, anodizing, or passivation; otherwise, it will affect the effectiveness of rust removal and scale elimination, and consequently impact the quality of the surface electroplating or coating layer. Due to the wide range of sources and types of oil pollution, as well as the significant variations in the degree of contamination, the problem of oil removal is very complex. There are also many items that require degreasing, including various metal materials, workpieces of different sizes and with varying degrees of complexity, as well as entire machines and equipment (such as machine cleaning and maintenance, and the cleaning and disinfection of food processing equipment). This illustrates how complex the task of degreasing is and how wide its impact spans. Therefore, many oil and fat removal processes require strict cleaning quality, even meeting ** or internationally established standards. 1. Factors affecting degreasing: The quality of degreasing depends mainly on four factors: degreasing temperature, degreasing time, mechanical action, and the degreasing agent. 1. Degreasing temperature: Generally speaking, the higher the temperature, the more thorough the degreasing. There are three reasons: (1) Temperature alters the physical properties of oils and greases; for example, rust inhibitors with high dropping points, vaseline, solid paraffin, etc., are difficult to clean even with high-concentration alkali solutions at lower temperatures. However, when the temperature of the oil spill is increased, their viscosity decreases, and they even form droplets, which facilitates their removal. (2) To accelerate chemical reactions, generally, for every 10-degree increase in temperature, the rate of chemical reactions doubles. (3) Accelerates the movement of surfactant molecules, thereby promoting processes such as wetting, emulsification, and dispersion. As the temperature rises, the solution’s ability to dissolve pollutants also increases. However, it is not true that the higher the temperature, the better in all situations; various degreasers have their appropriate temperature ranges. In degreasing solutions that use certain types of surfactants, excessively high temperatures can cause the surfactants to precipitate and aggregate, similar to oil droplets adhering to the surface, resulting in an uneven film formation. 2. Degreasing time: During the degreasing process, it is essential to ensure sufficient time for degreasing. The time required for high-pressure spray degreasing is generally 1.5–3 minutes, while immersion degreasing takes 3–5 minutes (depending on the type and amount of oil contamination). Increasing the degreasing time, that is, prolonging the contact time between the degreasing agent and the oil stains, improves the degreasing effect. The more oil contamination, the longer the degreasing time required. In automated production line operations, too long a time is often not allowed; therefore, degreasing is usually carried out first by spraying for 1 minute, followed by immersion degreasing for 3 minutes. 3. Mechanical action: In degreasing, mechanical actions such as pressure spraying or stirring are very effective. Because, during spraying, it ensures good contact between the fresh degreasing agent and the surface of the workpiece, and a uniform distribution of the degreasing agent throughout helps to improve the degreasing effect ； During spraying, mechanical force is used to drive the degreaser to penetrate and break down the oil film, thereby effectively forcing the oil stains to detach from the surface of the parts ； During spraying, the oil contaminants that have detached from the parts are emulsified and dispersed in the degreasing solution, preventing these contaminants from adhering again to the surfaces of the cleaned parts. In medium and low-temperature degreasing, mechanical action is particularly important. Generally speaking, pressure jetting is more than twice as fast as immersion degreasing. The jet pressure is usually (0.1–0.2) Mpa. Degreasing agents used for pressure jetting must be low-foam, as excessive foam can interfere with normal operation and lead to loss of the degreasing solution. Immersion degreasing cannot be considered a static process either; a circulation motor or ultrasonic equipment is necessary to keep the solution in motion, with the circulation rate being about 5 times the volume of the liquid in the tank per hour. 4. The impact of degreasing agents on degreasing efficiency: The composition and usage methods of degreasing agents have a significant influence on the efficiency of degreasing. For example, degreasing with an alkaline solution containing surfactants is more effective than degreasing with an alkaline solution alone. For solid or semi-solid fats with a high dripping point, cleaning with a solvent is more effective than other degreasing methods. To improve the emulsification and dispersion capabilities of oil stains, it is effective to increase the content of surfactants in the degreasing agent. Different types of surfactants as well as various alkaline substances can have an impact on the degreasing effect. Good degreasing agents are those that have been developed through numerous tests, with their component ratios being carefully selected over and over again. For alkali degreasing agents containing surfactants, the most effective cleaning action is achieved at the upper limit of the surfactant’s critical micelle concentration. During use, the degreaser is continuously consumed, causing its concentration to decrease; therefore, it is necessary to replenish the degreaser regularly in order to maintain the required concentration. The degreasing efficiency of a degreasing agent does not increase in a linear manner with its concentration; therefore, in cases where high standards are required for degreasing quality, it is not advisable to significantly increase the concentration. Instead, a two-step degreasing process should be employed, and the two degreasing solutions can be the same, without the need to raise the concentration further. In practical use, it is necessary to take various factors into consideration and keep them all at their optimal levels in order to achieve satisfactory degreasing results. Second, considerations for selecting a degreaser: Before carrying out the degreasing process, one must consider the choice of degreaser as well as the following factors: 1. The material of the parts to be treated. Different metals have different corrosion thresholds in alkaline solutions; therefore, it is necessary to select a degreaser with an appropriate pH level based on the material of the parts. See the table below: The critical pH values for corrosion of various metals are as follows: Metal – Zinc, Aluminum, Tin, Brass, Stainless Steel, Steel; pH values: 10, 11, 11, 5, 13, 14, 2. Regarding the type and quantity of oil contaminants, it is necessary to take into account the chemical properties of these contaminants on the parts (whether they can react with the surrounding substances during storage, thereby affecting the degreasing process, and whether they can undergo saponification reactions with the degreasing agents), the physical state of the contaminants (solid or liquid, as well as the amount of solid particles present in the oil), and the adsorption capacity of the contaminants toward the substrate (molecular polarity). Based on this information, an appropriate degreasing agent can be selected. For example, animal and vegetable oils are saponifiable, so degreasing with sodium hydroxide at higher temperatures is effective. Mineral oil does not undergo saponification, so degreasing with sodium hydroxide is not suitable; a degreasing agent with good emulsifying properties is required to be effective. It is difficult to remove semi-solid rust preventive greases using ordinary aqueous degreasers at low or moderate temperatures; in such cases, solvent degreasing is very effective. It is more difficult to clean rust preventive oil with strong adsorption properties than mechanical oil; therefore, a better degreasing agent should be used to clean rust preventive oil. 3. Correct selection of the degreasing process: Different degreasing methods require different degreasing agents. Pressure jet degreasing achieves good degreasing results due to its strong mechanical force; however, since foaming occurs easily in this process, it is necessary to use a degreasing agent that produces little foam. Immersion degreasing has a weaker mechanical effect, so it is necessary to use a degreasing agent with better degreasing capabilities, as well as to increase the amount of the degreasing agent used and extend the degreasing time. 4. Pay attention to coordination with the subsequent process. Whether the subsequent process is pickling or film formation, and whether there is a surface treatment step before film formation, all of these are related to the choice of degreasing agent. Because different pretreatments are used in combination with phosphating, the quality of the resulting phosphated coating varies. The phosphating crystals resulting from pickling and strong alkali degreasing are large and loose. For degreasing, if the next process is thin phosphating treatment, a degreaser with low alkalinity and surface conditioning properties should be used ； If there is a separate table tuning process, only the degreasing effect needs to be considered; the impact of alkalinity is minimal and does not require special attention ； If the next step is pickling for rust removal, there is no need to use a degreaser with surface conditioning; instead, surface conditioning should be carried out separately before phosphating ； If the time interval between processes is long, use a degreaser with a lower tendency to cause rusting after washing (such as a phosphate-type degreaser) or increase the wet spraying treatment. For surfaces with heavy oil contamination or those that are difficult to clean thoroughly, manual pre-cleaning is necessary. This not only helps effectively with degreasing but also significantly improves the quality of the phosphating process. The phosphating layer formed after pre-scraping is fine and dense in structure, whereas the crystals in layers formed without pre-scraping are much coarser. Third, the selection of surfactants for degreasing: There is no universal or fixed formula for degreasing agents. Metal workpieces require different types of working oils depending on the processing methods used, and different metal compositions or types of grease necessitate different degreasing agents in order to be cleaned effectively. The raw materials used in degreasing agents also vary greatly: (1) Light oils: Such as ordinary machine oils, rust preventive oils, and cutting oils – which are relatively thin greases – can use non-ionic fatty alcohol ethoxylates like AEO, phenol ethoxylates from the NP series, or fatty acid methyl ester ethoxylates like FMEE as their main components. In processes involving the use of light oils, it is possible to increase the proportion of FMEE, which has better dispersion properties. For anions, sodium dodecylbenzenesulfonate, which is inexpensive, can be used. (2) Heavy oil: Viscous greases used in industry such as butter; those with high emulsification properties are chosen as the main components, such as non-ionic phenolic alcohol ethers of the NP and OP series, as well as fatty alcohol ethers like AEO-3 or AEO-7 ； Regarding anions, replacing other anionic products in the formula with FMES, the sulfonate of FMEE which possesses the best emulsifying and dispersing capabilities among anionic surfactants, is also an effective way to improve the performance of the degreasing agent. (3) For some oils that have poor solidification or dripping properties, organic solvents can also be added appropriately; the choice can be made based on environmental and cost considerations. The organic solvents commonly used are mainly the following: propane, toluene, xylene, trichloroethylene, carbon tetrachloride, ethyl acetate, banana water, and anti-whitening water. Ranking of oil removal efficiency: xylene > toluene > carbon tetrachloride > trichloroethylene > propane > ethyl acetate > anti-whitening water. In general, degreasing agents available on the market today tend to focus only on their emulsifying properties, while neglecting their dispersing capabilities. This is an important point that many in the industry need to address; many people use high-quality raw materials yet still achieve unsatisfactory results. The key lies in the approach taken, and this is the conclusion drawn from my years of trial and error. Good dispersibility enables effective separation of oils from metal surfaces, thereby making degreasing and oil removal easier and more thorough. Especially in immersion processes, it also prevents various types of oil stains and dirt from reattaching to the surface of the objects being cleaned. For example, some manufacturers increase the amount of emulsifier used in order to speed up the degreasing process; this raises costs significantly without solving the problem. By improving the dispersion properties of the degreasing agent, the degreasing speed increases, and all related issues are resolved.