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Factors affecting degreasing process

Views: 0     Author: Site Editor     Publish Time: 2024-05-27      Origin: Site

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Degreasing is also known as oil removal, the oil on the metal surface is brought from the processing process of the material to the completion of the processing of the parts, in the storage and transportation process in order to prevent rust. Metal materials and parts must be cleaned before further surface treatment (such as electroplating, coating, anodizing, passivation, etc.), otherwise it will affect the quality of rust removal and oxidation skin, and affect the quality of surface plating 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 of oil removal, there are various metal materials, parts made of various metals of different sizes and shapes with different degrees of complexity, and there are also overall machines and equipment (such as machine cleaning and maintenance, cleaning and disinfection of food equipment), which shows that the work of oil removal is complex and has wide influence, and in many cases, the cleaning quality is required to be strict. Even to meet national or industry standards. Therefore, enough attention must be paid to it.


1. Influencing factors of degreasing


The quality of degreasing mainly depends on four factors: degreasing temperature, degreasing time, mechanical action and degreasing agent.


1. Degreasing temperature


Generally speaking, the higher the temperature, the more complete the degreasing. This is because of three reasons, first, the temperature makes the physical properties of the oil change, such as high drop point of anti-rust grease, petroleum jelly, solid paraffin, etc., at a lower temperature even if the use of high concentration of lye is difficult to wash. However, when the temperature of the oil is raised, its viscosity decreases, and even droplets form to facilitate removal. Second, to promote the chemical reaction, generally speaking, every 10 degrees of temperature rise, chemical reaction speed doubled. Third, 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 phosphating film flower uneven.


2. Degreasing time


In the degreasing operation, it is necessary to ensure that there is enough degreasing time, the pressure injection degreasing time is generally 1.5 to 3 minutes, and the immersion degreasing is 3 to 5 minutes (depending on the type and amount of oil). Increase degreasing time, that is, extend the contact time between degreasing liquid and oil, so as to improve the degreasing effect. The more oil, the longer it takes to degrease. In line work, it is often not allowed to use too long, so it is generally used to spray degreasing for 1 minute, followed by dipping degreasing for 3 minutes.


In degreasing, the mechanical action such as pressure injection or stirring is very effective, because the fresh degreasing solution is forced to have good contact with the surface of the part when sprayed, and the whole degreasing liquid content 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. Generally speaking, pressure injection is more than 1 times faster than immersion degreasing. The injection pressure is usually (0.1~0.2) Mpa(the degreaser used for pressure injection must be low foam, so as not to affect the normal operation and the loss of degreasing fluid), impregnation degreasing can not be considered static immersion, must be equipped with a circulation pump, so that the solution does not stop stirring, the circulation amount per hour is about 5 times the liquid volume of the tank.


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 of lye containing surfactants is better than the degreasing 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 dispersion capacity of oil, it is effective to appropriately increase the content of surfactants in degreasing agents. Different varieties of surfactants and different alkaline substances make certain differences in degreasing effect. Good degreasing agents are determined by repeated screening of the proportion of their components in combination through a large number of tests. 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. The degreasing effect of degreasing agent is not directly related to the concentration, so for high degreasing quality requirements, the method of greatly increasing the concentration should not be taken, but the method of secondary degreasing should be used, and the two degreasing solutions can be the same, and there is no need to increase the concentration.


In actual use, we must consider all aspects of the factors, so that all factors are controlled in the best state, so that degreasing to achieve satisfactory results.


Second, the selection of degreaser should pay attention to matters


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. See the following table:


The critical PH value for corrosion of various metals is: 2. The type and quantity of oil pollution


It is necessary to consider the chemical properties of the oil on the parts (whether the storage can react with the surrounding medium and affect the degreasing, whether the saponification reaction can occur with the degreasing liquid), the physical state of the oil (solid or liquid, the solid dust particle content in the oil), the adsorbability of the oil to the substrate (molecular polarity), and then choose the appropriate degreasing agent. For example, animal and vegetable oils are saponifiable, and thus 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 choice of degreaser


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 phosphating, and whether there is a surface adjustment process before phosphating, 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-scrubbing, he is not only the requirements of degreasing, but also can significantly improve the quality of phosphating, pre-scrubbed phosphating film crystal fine, dense, and without pre-scrubbing crystallization is much larger.


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