Degreasing, in the metal surface pretreatment, degreasing is a very important step. The oil on the metal surface is brought from the processing process of the material and the completion of the processing of the parts, and anti-rust oil and other products will be used in the storage and transportation process in order to prevent rust. But how to choose degreasing agent and improve degreasing effect?
Influencing factors of degreasing
The degreasing effect mainly depends on four factors: degreasing temperature, degreasing time, mechanical action and degreasing agent.
1.Degreasing temperature
In general, the higher the temperature, the more complete the degreasing. There are three reasons:
The temperature changes the physical properties of the oil, such as anti-rust oil with high drop point, Vaseline, solid paraffin, etc., which is difficult to wash at lower temperatures even if a high concentration of lye is used. However, when the temperature of the oil is raised, its viscosity decreases, and even droplets form to facilitate removal.
Promote the chemical reaction, generally speaking, every 10 degrees of temperature rise, chemical reaction speed doubled.
Accelerate the movement of surfactant molecules, thereby promoting the effects of infiltration, emulsification, dispersion, etc. With the increase of temperature, the solubility of the solution to the dirt also increased. However, not all occasions are the higher the temperature the better, a variety of degreasing agents have their suitable temperature range, in the use of some kinds of surfactant degreasing solution, too high temperature will make the surfactant precipitation aggregation as oil beads attached to the surface, resulting in uneven formation of the skin film.
2. Degreasing time
In the degreasing operation, it is necessary to ensure that there is enough degreasing time, the high pressure spray degreasing time is generally 1.5-3 minutes, and the immersion degreasing is 3-5 minutes (depending on the type and amount of oil). Increase degreasing time, that is, extend the contact time between degreasing agent and oil, so as to improve degreasing effect. The more oil, the longer it takes to degrease. In automatic production line operations, often not allowed to use too long, so generally first with spray (spray) degreasing for 1 minute, then with dipping degreasing for 3 minutes.
3. Mechanical action
In degreasing, mechanical action such as pressure injection or agitation is very effective. Here's why.
When spraying, the fresh degreasing agent is forced to have good contact with the workpiece surface, and the content of the whole degreasing agent is uniform, which is conducive to improving the degreasing effect.
When spraying, mechanical force is used to promote the degreaser to penetrate and destroy the oil film, so as to effectively force the oil from the surface of the part;
When spraying, the oil from the parts is emulsified and dispersed in the degreasing solution to prevent the oil from adsorbing to the surface of the cleaned parts. In the middle and low temperature degreasing, the mechanical action is particularly important.
4. The effect of degreasing agent on degreasing effect
The composition and application of degreasing agent have great influence on degreasing effect.
For example,
The degreasing effect of lye containing surfactants is better than that of alkaline solution alone.
For solid or semi-solid grease with high dripping point, solvent cleaning is more effective than other degreasing.
In order to improve the emulsification and dispersing ability of oil, it is effective to increase the content of surfactant in degreaser appropriately.
For lye degreasers containing surfactants, the most effective cleansing is performed at the upper limit of the critical micelle concentration of the surfactant. In the process of use, the degreaser will be continuously consumed, so that the concentration is reduced, therefore, it must be regularly supplemented with degreaser to maintain the necessary concentration.
Matters needing attention when choosing degreaser
Before degreasing, the choice of degreasing agent and the following aspects should be considered
1. Material of processed parts
Different metals have different corrosion limits in lye, so it is necessary to choose the right pH degreaser according to the material of the part.
2. Type and quantity of oil pollution
It is necessary to consider the chemical properties of the oil on the parts, the physical state of the oil, the adsorption of the oil to the substrate, etc., and then choose the appropriate degreaser.
Such as:
Animal and vegetable oils are saponifiable, so degreasing with sodium hydroxide is effective at higher temperatures.
Mineral oil does not saponification reaction, degreasing with sodium hydroxide is not suitable, the use of good emulsifying properties of degreasing agent can be effective.
With the general water-based degreaser at low or medium temperature is difficult to wash the semi-solid anti-rust grease, then degreasing with solvent is very effective.
Washing the anti-rust oil with strong adsorption is more difficult than washing the mechanical oil, so a better degreaser should be used to clean the anti-rust oil.
3. Correct selection of degreasing process
Different degreasing methods have different requirements for degreasing agents. Pressure injection degreasing due to the mechanical force is large, degreasing effect is good, but because it is in a state of easy foaming, so the selection of low-foam degreasing agent is necessary, the mechanical effect of immersion degreasing is weak, so the choice of degreasing performance is better degreasing agent, appropriate increase degreasing agent content and extend degreasing time.
4. Pay attention to the cooperation with the next process
Whether the next process is pickling or skin film formation, and whether there is a surface adjustment process before skin film formation are related to the selection of degreaser. Because of different pretreatment and phosphating supporting, the quality of the resulting phosphating film is different, and the quality is different. The phosphating of pickling and strong alkali degreasing is large and loose.
For degreasing, if the next process is thin phosphating, a degreasing agent with low alkalinity and surface conditioning should be selected;
If there is a separate table adjustment process, only the degreasing effect is considered, and the alkalinity has little impact, so there is no need to worry about it.
If the next process is pickling and rust removal, it is not necessary to choose degreaser with surface adjustment, and the surface adjustment treatment is carried out separately before phosphating;
If the process interval is long, choose a degreaser with a small tendency to rust after washing (such as phosphate type degreaser) or increase the spray wet treatment.
For the surface of heavy oil or not easy to clean in place, to use manual pre-cleaning, which can not only effectively help degreasing, but also significantly improve the quality of phosphating, pre-scrubbed phosphating film crystal fine, dense, and not pre-scoured crystallization is much larger.
Selection of surfactants for degreasing
There is no universal and invariable degreasing cleaning agent formula, the metal workpiece will choose different working oils due to different processing processes, different metal components or oils need different degreasing agents to please wash clean, the raw materials of degreasing
agents are also very different:
1. Light oil
Light oil: general oil, rust prevention oil, cutting oil and other thin grease, can choose non-ionic fatty alcohol polyoxyethylene ether AEO, phenolic polyoxyethylene ether NP series or fatty acid methyl ester polyoxyethylene ether FMEE as the main component, in the removal process of light oil, can appropriately improve the proportion of high dispersion FMEE. Anions can be used inexpensive sodium dodecyl benzene sulfonate.
2. Heavy oil
Heavy oil: butter and other industrial viscous oils, choose high emulsifiability as the main component, such as non-ionic phenolic alcohol ethers NP, OP series, fatty alcohol ethers AENO-3 or AENO-7 and other products; In terms of anions, choosing sulfonated salt FMES of FMEE, which has the best emulsification and dispersion ability among anionic surfactants, to replace other anionic products in the formula is also an effective shortcut to improve the performance of degreasers.
3. Other oils
For some oil with poor coagulation or dripping, organic solvents can also be added appropriately, which can be selected according to the requirements of environmental protection and cost. Usually, organic solvents are as follows: acetone, toluene, xylene, trichloroethylene, carbon tetrachloride, ethyl acetate, banana water and white water.
In SUMMARY:
In general, metal materials and parts must be cleaned before further surface treatment (such as electroplating, coating, anodizing, passivation, etc.), otherwise it will affect the effectiveness of rust removal and oxide removal, and then affect the quality of surface electroplating and coating layer. Due to the wide variety of sources of oil pollution, the degree of pollution is very different, so the problem of oil removal is very complicated. In addition, there are many objects that need oil removal, various metal materials, workpieces with different sizes and shapes of complexity, and overall machinery and equipment (such as machine cleaning and maintenance, cleaning and disinfection of food equipment), which is enough to show that the work of degreasing oil removal is complex and has a wide impact. Therefore, many degreasing processes must require strict cleaning quality, even to meet national or international standards.