Surface degreasing
TO category:
Preparation for painting
Surface degreasing
During the degreasing process, various types of contaminants have to be removed from the metal surface. During stamping, drawing, and cutting, fats, soaps and oils are deposited on the surface. During subsequent mechanical processing, the fats burn out and, together with worn-out abrasive materials, fill the depressions and irregularities on the surface. During welding, marking, etc. operations, other types of contaminants appear that can firmly adhere to the metal surface.
The classification of contaminants depending on their chemical composition, the nature of the impact and methods of removal 17-19 is conditional, since most often contaminants are a mixture of substances of different composition and properties.
Of the variety of contaminants encountered that must be removed from the metal surface, the following can be distinguished.
Organic pollution. These include anti-corrosion lubricants and lubricating oils, which contain mineral oils, petroleum jelly, petroleum wax, paraffins, fatty acids, rosin, and tree resins. These contaminants are removed during the degreasing process.
Inorganic contaminants. These include carbon deposits and oxides deposited as a result of hardening or some
y other preprocessing operations. These opinions, as well as metal shavings and even large and small inorganic particles mixed with the lubricant remaining after machining, are removed with great difficulty during degreasing and etching.
Mixed pollution. These include lubricants used in metal forming, soluble oils* and emulsion compositions, which contain various pigments in the form of finely ground powders, which make cleaning very difficult, especially after heating.
Removing these contaminants, which have the greatest variety of properties, presents particular difficulties.
When removing contaminants from the surface of metals, it is especially important to choose the most effective method cleaning, which depends on the following factors: the nature of the contamination, the effect of the chemical components of the cleaning solution on metals, the required degree of cleaning, operational safety and cost.
The most widely used methods in the metalworking industry are physical and chemical methods for removing contaminants, which include: 1) cleaning with organic solvents; 2) water cleaning with alkaline, acidic and neutral compounds; 3) emulsion cleaning.
Cleaning with organic solvents
During degreasing, the solvent forms a homogeneous mixture or solution with the contaminant that it dissolves. Solvents used for cleaning must satisfy certain requirements, which include:
1. High dissolving ability in relation to the type of contamination being removed. Solvent power is the main technological property of a solvent. The more diverse the substances that a particular solvent can dissolve, or the greater the amount of a given substance dissolved in it, the higher the quality of the solvent.
Commonly used petroleum solvents (gasoline, white spirit) have weak dissolving ability; Aromatic hydrocarbons, ketones, alcohols and esters have a higher dissolving ability. The highest dissolving ability is characteristic of chlorinated hydrocarbons, and the most effective is methylene chloride, which is advisable to use for dissolving difficult-to-remove contaminants, such as old coatings with varnishes, enamels, resins, etc.
2. Stable in use. The solvents used must be resistant to various factors, affecting them. during the cleaning process. They must not decompose under the influence of the materials being cleaned, light, heat, water or react with steam. Otherwise, the solvent is either not used at all, or a stabilizer is added to it. Thus, trichlorethylene (TCE) under the influence of light and at elevated temperatures20 decomposes with the formation of hydrogen chloride, and upon contact with open fire, phosgene, therefore, to protect it from decomposition, stabilizers are used, in particular amines or cyclic and unsaturated hydrocarbons. There is evidence21 that TCE, when exposed to aluminum, magnesium and their alloys, can cause reactions accompanied by a large release of heat and even an explosion. In this regard, degreasing of aluminum and magnesium with TCE is not carried out. In contrast to trichlorethylene, perchlorethylene (tetrachlorethylene) can be used to remove contaminants from the surfaces of any parts, including those made of aluminum and magnesium.
3. Evaporation ability (volatility). To quickly dry the surface of the products being cleaned, especially recesses, joints, etc., solvents must have increased evaporation (volatility). The ability of a solvent to evaporate is determined by its saturated vapor pressure. However, in technology for evaluating volatile solvents it is convenient to use highly volatile ones; Solvents with a volatility of 8-13 are called medium-volatile, and those with a volatility of more than 15 are called low-volatile.
4 Low surface tension. It is desirable that the selected solvent has a low surface tension, quickly and well wets the surface to be cleaned and easily penetrates into the recesses, grooves and joints of parts.
5. Regenerability. This important requirement largely determines the efficiency of the entire process. In most cases, solvents are recovered by distillation.
High volatility, toxicity, tendency to electrify, flammability and the ability to form explosive mixtures with air - all these characteristics Most organic solvents require unconditional compliance with safety regulations and industrial sanitation.
Saturated hydrocarbons are, as a rule, not used as individual solvents. Most often these are mixtures of hydrocarbons obtained by distilling oil. These include gasoline various brands: gasoline for industrial and technical purposes, GOST 8505-57; gasoline solvent for the paint and varnish industry (white spirit), GOST 3134-52; gasoline solvent For the rubber industry (“Galosh”), GOST 56. These solvents dissolve well fresh and used mineral oils, greases, and preservative compounds. When cleaning products with gasoline, immerse them in a container with a solvent or wipe them with a rag moistened with a solvent.
Aromatic hydrocarbons, in their solvent properties, are significantly superior to saturated hydrocarbons, but are more toxic, therefore, for degreasing when preparing surfaces for paint and varnish coatings, they are used only in the form of mixtures with aliphatic hydrocarbons.
Chlorinated hydrocarbons play a particularly important role in cleaning metal surfaces. Abroad, in particular in the USA, there is experience working with these solvents. The most common solvents are:
Trichlorethylene - boils at 87.3 °C; has a very high dissolving power for the vast majority of contaminants encountered in the metalworking industry. Surface degreasing is carried out mainly in solvent vapors.
Perchlorethylene - boils at 120.8 °C; used for removing resins, paraffins and degreasing in printing. Perchlorethylene purification plants are usually slightly more expensive than trichlorethylene purification plants due to high costs heat to heat the solvent.
Methylene chloride - boils at 39.95 ° C; used for special purposes when a solvent with a low boiling point or high solvent action is needed. Methylene chloride is used as a main component in paint remover formulations, including epoxy varnishes.
When degreasing metal surfaces with organic solvents, a distinction is made between: cold degreasing, which includes all methods of degreasing metal or metal products solvent or mixtures of solvents at a temperature close to room temperature; degreasing in solvent vapors.
When cold degreasing, metal products are usually immersed in a special bath filled with a solvent, or wiped with a rag soaked in a solvent; less often, they are processed in a jet. After cleaning using this method, a certain amount of contaminants remains on the surface in the form of a thin, uniform film of oil.
Degreasing in solvent vapors is modern method removal of lubricants, fats, oils, paraffins, resins, etc. from any surfaces of metals and non-porous materials. This process uses non-flammable chlorinated solvents that are heated to boiling point in a special bath. The cooling coil condenses the steam and creates a zone saturated vapors pure solvent. On the surface of parts located in this area, the solvent condenses and dissolves fatty contaminants. As the parts are washed off with the condensed solvent, they heat up to the vapor temperature; the parts are then raised above the vapor zone where they dry quickly.
Degreasing in solvent vapors can be carried out at any stage of processing of metal products, when it is necessary to quickly and completely remove oil contaminants.
At the Tallinn Electrotechnical Plant named after. M.I. Kalinin has been successfully operating for several years a degreasing installation designed and manufactured by this enterprise of the “heated liquid solvent - steam with periodic regeneration of the solvent” type.
Some experience in developing technological modes for degreasing in trichlorethylene vapor has been accumulated at NIITLP38.
A diagram of one of the industrial installations for degreasing metal surfaces with trichlorethylene, designed at NIITLP, is shown in Fig. 1. This installation uses a combination purification method, which can be divided into three hundred days. In the first of them, the parts are treated with trichlorethylene coming from a manifold with an excess pressure of 1.5-2 at. At the second stage, liquid-phase degreasing is carried out in a bath where trichlorethylene is heated to 40-45 °C. At the third stage, the parts are moved to the trichlorethylene vapor zone (bath 10) for final degreasing.
Rice. 1. Installation for degreasing a metal surface with trichlorethylene: 1- ventilation hood; 2 - conveyor; 3 - conveyor drive; 4 - viewing windows; 5 - door; 6 - refrigerators; 7 - settling tank; 8 - collection gutter (trichlorethylene condensate); 9 - coils; 10 - steam bath (degreasing in the vapor phase); 11 - liquid level regulator; 12 - overflow pipe; 13 - collector; 14 - bath for degreasing in the liquid phase; 15 - hatch; 16 - stretching device; 17 - basket.
The installation is completely sealed, bath 10 is equipped automatic regulators temperature, level of liquid trichlorethylene and its vapor.
All internal components of trichlorethylene vapor degreasing units exposed to solvent, water and air (for example, vapor condenser refrigerators) are made of galvanized steel, nickel, tin, copper or chromium-nickel of stainless steel. To pump both pure and contaminated solvents, it is advisable to use standard centrifugal pumps, for example TsNG-68. The sealing of files and sleeves along which the solvent moves, suckers, are made of solvent-resistant materials.
It is most rational to heat degreasing installations with steam. This makes it possible to more accurately control the heat supply, maintain low wall temperatures of the heat transfer surface, and make the installation relatively easy to maintain. Steam heating is carried out by multi-pass steam coils.
Gas-fired degreasing units use perforated pipes or burners located under the bath, or gas coils immersed in the solvent chamber.
Small degreasing installations can also be heated with electric strip heaters mounted under the side of the bath, and large installations can be heated with submersible oil heaters with a specific power of 1.55 to 2.3 W/dm2.
For safe work the installations are equipped with thermostats, which are located in the liquid phase and slightly above the level of solvent vapor. In the vapor zone, the thermostat is set to a temperature of 74 °C (for trichlorethylene) and 110 °C (for perchlorethylene). In the liquid phase zone, the thermostat is set to a temperature of 110 to 115 °C (not higher than 125 °C) for trichlorethylene and not higher than 145 °C for perchlorethylene. To reduce solvent losses, installations are equipped with a cooling system calculated based on the conditions of maximum heat input. Most often, coils are used for cooling, laid along the inner walls of the bath at the vapor level. On small installations Water jackets can also be used. Small jackets are required in all installations to cool the walls above the vapor level in order to maintain the temperature of these walls close to room temperature and prevent the vapor from rising above normal levels.
When choosing a solvent, degreasing method and designing appropriate equipment, it is recommended to use the following literature23’2428-42.
Cleaning in aqueous alkaline solutions
A significant part of cleaning operations during the preparation of metal surfaces for painting is carried out with aqueous solutions detergents 18. Water itself has a weak cleaning effect against oil stains. Small additions of surfactants (surfactants) can dramatically increase its cleaning ability. This is the result of the ability of a surfactant to be adsorbed at the interface and dramatically change the degree of interaction of contacting surfaces and the rate of exchange of substances between these surfaces (rate of evaporation, condensation, crystallization, etc.).
Surface treatment with detergents
The mechanism of washing action can be represented in the form of three main stages:
1) wetting the surface of the material with an aqueous surfactant solution;
2) removal of contamination from the surface by solubilization, emulsification, dispersion, suspension and wedging action of surfactants at the solid surface-contamination interface;
3) retaining contaminants in the volume of the washing solution and removing them from the washing bath in suspended, emulsified and solubilized states.
Therefore, in a rationally formulated detergent formulation for optimal manifestation technological properties a solution requires a combination of different surfactants with the most characteristic properties that are most pronounced for each (for example, one is a wetting agent and the other is an emulsifier).
Effective wetting agents are synthetic surfactants DB (polyoxyethylene ether of ditertiary butylphenol with a polyethylene glycol chain consisting of 6 units) and DS-PAC sodium, subject to activation with appropriate electrolytes.
Based on the mechanism of the washing action, the composition of electrolytes is selected, which are not inert fillers, but active additives that significantly improve both the wetting ability and the overall washing effect of surfactant solutions.
The most widely used electrolytes in detergents intended for technical purposes are: caustic soda, soda ash, sodium silicates, phosphoric acid salts and sodium sulfate17’ 57>60. Their use is based on the beneficial effect of the alkaline environment on the cleaning process, as well as their high dispersing ability, which increases the stability of the suspensions formed during cleaning.
The introduction of silicates into the washing solution provides an inhibitory effect of the solution towards the metal being washed, especially aluminum.
Of the phosphate salts used as components of detergents: trisodium phosphate tetrasodium pyrophosphate Na4P207; sodium tripolyphosphate Na5P3Oio and sodium hexametaphosphate (NaP03)6. The effect of phosphates as fillers is explained by the ability to form complex compounds with alkaline earth metal ions and heavy metal ions, as a result of which water hardness is eliminated, polyvalent metal carbonates and calcium soaps are dissolved, and iron salts are neutralized by binding into complex compounds. These substances are also distinguished by their ability to suspend and peptize* solid particles; Thanks to this, contaminants are retained in finely dispersed form in the solution and do not settle on the surface being cleaned. The positive effect of phosphates is also manifested in the ability to regulate the pH of the environment. For example, when meta-phosphates are added, the pH of the solution decreases, and purification occurs in a neutral and slightly alkaline environment.
Except inorganic salts Synthetic detergents contain some organic products that improve their quality. These include primarily carboxymethylcellulose (CMC).
The main purpose of CMC is to prevent the reverse deposition of contaminants onto cleaned surfaces. In addition, CMC slightly increases the cleaning ability of the compositions. For example, alkylaryl sulfonates without the addition of CMC retain stains worse than soap, and with the addition of CMC they are better than soap44.
Analysis of numerous formulations 61-65 of detergents intended for metals showed that they are based on active inorganic additives, and the surfactant content in these compositions does not exceed 10%.
The pH value of the cleaning solution should be chosen so as to achieve the optimal effect of surfactants and fillers. For each electrolyte included in the cleaning solution, there is a characteristic pH region at which its properties are maximally manifested. For example, the optimal pH value for carbonates is 10.5-11, for sodium metasilicate - 11 - 11.5. The effect of phosphates does not depend on the pH of the environment.
For each surfactant, there are also optimal pH values of the washing bath, which favor a more complete use of the surfactant. It has been shown17 that alkyl sulfate at pH 7 and dodecylbenzene sulfonate at pH 10 have minimal ability to retain contaminants, and both synthetic surfactants are more effective in a very acidic environment than in an alkaline environment.
Soaps, on the contrary, have a maximum cleansing effect at a solution pH of 10.7. At a pH below 10.2 they hydrolyze, and at a pH of 8.5 they show practically no cleansing effect at all. Therefore, the pH of the washing bath is one of the parameters controlled during the degreasing process. The alkalinity of the solution is determined by titration with the bromocresol green indicator according to the standard method79.
Mechanical impact. The role of mechanical action during cleaning is reduced to ensuring rapid wetting of the surface to be washed, faster and more complete distribution of the cleaning solution, j separation of contamination particles under the influence of tangential forces arising from the relative movement of parts and liquid.
All this leads to the fact that cleaning can be carried out in liquids with reduced concentrations active component, with more low temperatures, and most importantly - in a shorter time.
Hardness of water. With high water hardness, degreasing deteriorates due to the formation of insoluble lime and magnesium soaps, deposited on degreased surfaces in the form of a film that is difficult to remove. The most effective water softeners are tripolyphosphate, tetrapyrophosphate and sodium hexametaphosphate.
These factors greatly influence the selection of the appropriate cleaning solution.
For example, aluminum, especially after polishing, is subject to corrosion in alkaline solutions. Zinc alloy castings also corrode in alkaline solutions and are therefore not recommended for processing in this environment. Bronze in an alkaline solution quickly tarnishes, and copper often turns black, magnesium corrodes in weakly alkaline solutions.
Washing the surface with water
The degreasing process ends by rinsing the products with water. If washing is poor, it is impossible to obtain a clean surface even with the best degreasers.
The transfer of residual contaminants and cleaning solution into the wash water depends on the shape of the parts, the duration of drainage of the solution and technological features the cleaning solution itself. It is obvious that parts that are complex in shape and have blind holes carry away more cleaning solution than flat products. Therefore, sometimes it is advisable to drill special holes to drain the cleaning solution. It takes time for the cleaning solution to flow back into the degreasing bath, during which rusting of parts or local drying of the cleaning solution and the formation of hardened, poorly soluble residues may occur. Accordingly, it is recommended to slowly remove parts from the bath after degreasing and quickly transport them for washing.
The amount of washing solution in the rinsing water should be minimal, which is achieved by a constant flow of fresh water into the bath. The maximum concentration of impurities in the wash water is established experimentally in each specific case. The contamination of the rinsing bath is measured by its electrical conductivity. Often several baths are used for washing. In this case, the concentration of contaminants and cleaning solution in the first washing bath should be 1/10 of the concentration of the cleaning solution; in the second bath this concentration will be even lower.
Great importance has a temperature rinsing water. If washing is carried out in hot water, then the solubility of the cleaning solution residues increases. Sometimes at high temperatures, simultaneously with dissolution, “fixation” of part of the residues of the washing solution may occur as a result of hydrolysis of alkaline salts contained in the washing solution, therefore rinsing in warm water at a temperature of 50-65 °C. These conditions ensure sufficient solubility of the cleaning solution residues without unnecessary heat loss.
Water hardness is also important for the rinsing process, especially when cleaning in caustic solutions or when using soaps, long-straight-chain alkyl sulfates or long-straight-chain alkylbenzenesulfonates as detergents. When washing surfaces from these solutions, insoluble calcium and magnesium salts are formed, which leads to a loss of solubility of detergents and the formation of a white coating on the surface of the parts, which is clearly visible after drying. Therefore, in areas with hard water, it is often necessary to use steam condensate for flushing or soften the water by introducing special additives, such as sodium phosphates.
Water consumption is essential when washing products. Calculations show that to reduce water consumption, the dimensions of the baths should be minimal, and the influx of fresh water should be as large as possible. Since washing requires smaller baths than cleaning, a large bath can be divided into several sections, thus increasing the productivity of the installation.
Multi-stage washing makes it possible to efficiently use water, passing it from the third bath to the second and from the second to the first. At the same time, the baths can be easily positioned so that the water in them is at different levels. With a level difference of 50 mm, an independent flow of water is ensured at a speed of 4 to 8 l/min. For warning reverse movement a water shut-off valve is installed. This system is known as cascade flushing. Water consumption during cascade washing is reduced by approximately 100 times compared to water consumption during single-stage washing.
The most effective and economical method is jet washing. At first glance, it may seem that disposing of waste water from jetting down the drain may result in excessive water consumption. But this is optional. For example, a quick rinse for 5 seconds with discharge into the sewer before a long jet rinse with recirculation will take away most of the contaminants and require significantly less water consumption in next bath. This way you can save significant amount water.
In jet washing installations, the nozzles must be located so as to ensure the treatment of products from all sides. When designing installations, it is necessary to pay attention Special attention to ensure that the jets are directed into the recesses of the products, as well as to select the type of nozzles and water pressure that provide the most effective washing.
Equipment used for cleaning in aqueous alkaline solutions
For alkaline degreasing, the following equipment can be used: stationary baths; blast cleaning units; steam jet installations; swinging drums or screw conveyors; circulation plants; electric cleaning baths; ultrasonic installations.
Due to the lack of standard equipment, in each individual case it is necessary to design and manufacture degreasing installations taking into account the technological features of a particular production: the required productivity available production area, material, dimensions and complexity of the shape of the parts being cleaned, the possibility of transporting the parts, etc.
Baths for cleaning parts using the immersion method are made from sheet metal carbon steel 4-6 mm thick with welded seams inside and outside; The capacity of the baths should not exceed 1900 liters. Large bathtubs must be reinforced on the sides with stiffening ribs.
To heat the washing solution, the baths are equipped with steam coils28, which ensure heating of the washing solution to operating temperature within 30-60 minutes. Steam coils are mounted on the working side of the bath and covered with a sheet metal screen. (In this case, the screen directs the flow of the heated solution up and away from the coil, ensuring constant circulation of the cleaning solution and carrying floating oil towards the far wall).
The coil is located at a distance of 7-8 cm from the bottom of the bath and at a distance of 7-8 cm from the level of the solution. The far wall of the bath is equipped with a drain partition to remove contaminants floating on the surface of the cleaning solution. To completely clean the bathtub when replacing the entire solution, a drain line is installed at the bottom.
Mixing of the washing solution is carried out either by moving the parts or by pumping the washing solution along the immersed products using a pump.
If the baths are installed on a conveyor and the conveyor speed is not sufficient to effectively move the cleaning solution relative to the surface being treated, the baths are equipped with special shakers or tippers, which the parts bump into as the conveyor moves.
A more advanced method of cleaning metals compared to the immersion method is jet degreasing. Due to significant capital investments, the use of this method is justified only in conditions of mass production.
Inkjet units come in a wide variety of designs80-81; they may represent simple devices for irrigation of parts in one bath or conveyor units in which parts are continuously moved in sequentially mounted sections of degreasing, washing, pickling, passivation, etc.
To move parts in inkjet installations, horizontal conveyor and overhead monorail lines are most often used. Common devices for supplying cleaning solution are: fixed nozzles or slotted nozzles, with the help of which the solution can flow onto the parts under different angles, rotating nozzles supplying solution to parts at a constantly changing angle; rotating wheels that create waves in the solution that wash over the parts.
The washing solution is supplied under an excess pressure of 0.5-2 at; at high pressures, special measures are required to prevent increased foam formation.
When using jet cleaning, cleaning solutions can be supplied at reduced pressure (0.1 - 0.7 at) and at increased flow rates; this often gives good results.
Blasting units must have two or more sections for cleaning and subsequent rinsing. At large quantities contamination, it is advisable to have two sections for cleaning and one for washing. In this case, the bulk of the contaminants are removed in the first section so that the uncontaminated solution in the second section can complete the cleaning at optimal temperature and with the greatest efficiency.
The American Metallurgical Society has compiled useful data on the management and application of blast cleaning, which is presented in a review paper by Spring18.
To clean large objects, the steam jet cleaning method is widely used in foreign practice. This method consists of supplying a hot washing solution together with a certain amount of superheated steam under an excess pressure of 3 to 10 at. The solution is supplied by a dosing device through a spray head, in which it is mixed with steam.
There are three types of steam jet cleaning units:
— installations with their own heating source (gas, fuel oil or butane) and with direct injection of cleaning solution;
- fireless installations operating on factory steam, with a washing solution supplied by a pump under high blood pressure;
— installations operating on factory steam or steam supplied from a separate source (steam boiler) with siphon suction of the cleaning solution.
Steam jet units can be portable or stationary.
Portable steam jet units are intended for processing large-sized products, metal structures on the installation site, as well as products that are painted after assembly in areas geographically remote from stationary surface preparation units.
The most convenient equipment for cleaning small parts complex shape are rotating drums and screw conveyors. The drum is completely or partially filled with an alkaline solution, which is mixed as a result of the rocking-rotating motion and friction of part against part. Steel balls and small crushed stone are used as auxiliary materials for cleaning.
The effectiveness of drum cleaning largely depends on the type of equipment used (rotating drum immersed in a bath; tipping drum; auger conveyor).
Cleaning in circulation units, in electrolytic baths, as well as in devices using ultrasound is very rarely used to prepare surfaces for painting, since in this case thorough removal of contaminants is not required, as is necessary, for example, when cleaning parts intended for electrolytic coating .
Emulsion cleaning
Degreasing with organic solvents, especially with the use of steam, occurs quickly, and the products leaving the installation are dry. However, if during the cleaning process it is necessary to remove not only oil, but also solid particles located on the surface (metal dust, residues after grinding, polishing pastes, etc.), additional cleaning is carried out in an aqueous washing solution.
For more effective surface cleaning, combined methods were required, involving cleaning in organic solvents and aqueous cleaning solutions.
Emulsion degreasing compositions have high dissolving, wetting and emulsifying properties, therefore, during the emulsion cleaning process, various oils, greases and inorganic contaminants are completely removed from the metal surface.
When cleaning using the emulsion method, the duration of cleaning products becomes shorter, and the service life of the washing bath is longer than when cleaning in alkaline solutions using the same surfactants. Moreover, emulsion degreasing can be carried out with room temperature without compromising the quality of surface cleaning.
When working with emulsion compositions, it is not necessary to control their composition; it is only necessary to maintain a constant level of detergent in the bath.
The advantage of the method is the safety of working with these compounds containing up to 90% water, since they are non-toxic and fire and explosion proof.
Cleaning emulsion compositions are emulsions of solvent in water, stabilized by surfactants. Hydrocarbons and their chlorinated derivatives are used as organic solvents. Of the emulsifiers, amine soaps, naphthenates, alkylaryl sulfonates, alkyl sulfonates and polyoxyethylene nonionic surfactants with low foaming ability are widely used. Petroleum sulfonates, used as emulsifiers, are highly soluble in organic solvents and have anti-corrosion properties. Special additives (alcohol, glycol ethers) are often added to the composition of emulsions for cleaning, which increase mutual solubility and thereby facilitate the combination of the emulsifier with the solvent, as well as corrosion inhibitors.
Emulsions for cleaning are usually highly dispersed and relatively stable. Most of them are oil-in-water emulsions, but special water-in-oil emulsions are also known57.
The stability of emulsions used in industry varies widely; in some emulsions, the solvent does not separate from the emulsions even after prolonged exposure at high temperatures; in others, the aqueous layer and the solvent layer are completely separated. For industrial purposes, it is desirable to have emulsions that are as stable as possible.
Cleaning in emulsion compositions differs significantly from cleaning in organic solvents and alkaline degreasing solutions. In emulsions, simultaneously with the dissolution of oil contaminants with organic solvents, these contaminants are emulsified. Reducing the viscosity of oil contaminants when they are dissolved with organic solvents facilitates the emulsification process, as a result of which emulsion drops do not contaminate an already washed surface.
The decisive role in emulsion cleaning belongs to surfactants; the presence of surfactants simultaneously in the form of an aqueous solution and a solution in an organic solvent promotes rapid and complete wetting of the surface with a detergent composition and the removal of contaminants from the surface in washing bath. As a result of simultaneous exposure to an organic solvent and an aqueous cleaning solution, cleaning efficiency is significantly increased.
Depending on the method of diluting the concentrate with water, two methods of emulsion degreasing are distinguished: one-stage and two-stage.
One-step method. The concentrate is diluted with water or a weakly alkaline solution in a ratio of 1: 10 to 1: 200. This forms either a stable emulsion of an organic solvent in water, or an unstable emulsion, which quickly separates into two layers, forming so-called two-phase cleaning solutions.
Regardless of the stability of the emulsion formed as a result of dilution, with the one-step method, the bulk of contaminants are removed by immersing the parts in the diluted preparation or during the process of spraying the emulsion.
When cleaning by immersion in two-phase solutions in the organic phase, partial dissolution of oil contaminants occurs with simultaneous adsorption of surfactants on the surface being cleaned; and in water - removal of inorganic contaminants and emulsification of oil residues. The organic solvent, depending on its density, can be located above or below the aqueous phase.
Of great interest are solutions in which an organic solvent (chlorinated hydrocarbon) is located under a layer of water. In this case, the quality of cleaning increases with alternate contact of contaminants with two phases of the solution; In addition, the upper aqueous solution helps to reduce the evaporation of the volatile solvent.
It is advisable to use two-phase effective cleaning solutions to clean the surface of difficult-to-remove contaminants, such as deposits, paint coatings, etc. However, compared to stable emulsions, two-phase solutions are less economical, since in most cases their use requires heating and frequent adjustment of the bath.
The concentrate intended for two-phase cleaning19,20 has the following composition (in g): kerosene - 89; sodium oleate - 7.2; triethanolamine - 3.8; tricresol - 1.
When the concentrate is diluted with water in a ratio of 1:10, cleaning lasts from 30 seconds to 3 minutes, depending on the type of contamination.
Two-stage method. Cleaning with this method is carried out by immersing the parts in a concentrated preparation for several minutes, followed by rinsing the products with water. When parts are immersed in the concentrate, the contaminants do not transfer into the washing bath, but the components of the detergent composition are adsorbed on the contaminants and on the metal surface. In the subsequent process of washing with water, the contaminants are quickly, in most cases spontaneously emulsified and removed from the metal surface.
Thus, with a two-stage cleaning method, there is no contamination of the detergent composition, and the concentrate is consumed due to adsorption and entrainment with the parts. Therefore, the service life of the concentrate without its complete replacement can be several years, subject to replenishment working bath fresh portions of the concentrate as it is carried away.
Emulsion preparations given in the specialized literature83’88-93 differ from each other in emulsifiers, solvents, methods of increasing the solubility of surfactants, as well as stability.
The parts are washed in this preparation at normal temperature. The duration of cleaning depends on the type and degree of contamination and ranges from 30 seconds to 10 minutes. The disadvantage of the drug is its volatility; Therefore, good ventilation is necessary. The solution is harmful to the skin of your hands, so you should work with protective gloves. Since the solution contains water, there is little risk of ignition, but it is not recommended to work with open fire in the room where the bath is located.
Mixtures of surfactants of ionic and nonionic types were used as emulsifiers for preparing the emulsion. The introduction of a surfactant mixture into the emulsion helped to increase the cleaning power of this composition. The cleaning process with this composition is carried out both when heated to 60 C and at room temperature.
Determination of surface cleanliness
Methods for determining the amount of contaminants on the surface of metals are summarized in a number of works. However, most of them provide only qualitative information about the degree of grease-freeness of the surface; only in rare cases is it possible to obtain quantitative data about the cleaning process.
When assessing surface cleanliness, the following methods are used: gravimetric, ferrocyanide, rubbing, wetting the surface with water, spraying water with pigment or fuchsin, fluorescent, contact metal deposition, as well as the method of residual radioactive impurities.
Weight method- a simple and convenient way to determine the degree of cleaning of small parts. In this case, the oil is washed off from the metal surface with an effective solvent (such as chlorinated hydrocarbons), then after the solvent evaporates, the dry residue is weighed. You can also weigh the part being tested before and after solvent rinsing.
Ferrocyanide method - used to determine the degree of surface cleaning of steel and copper. Paper saturated with a solution containing NaCl - 50 g/l, KgFe(CN)e - 10 g/l and HC1 - 1 g/l is left for several minutes in a wet state in contact with the test sample. Clean surface corresponds to the painted part of the paper, the greasy surface corresponds to white spots on the paper.
The wiping method is used to determine the cleanliness of the entire surface of parts and its individual sections. For wiping, you can use filter paper, paper napkins and a white cloth. Determining cleanliness by rubbing is especially useful for detecting contaminants consisting of tiny particles that are difficult to detect by other methods.
The method of wetting the surface with water is the fastest method, most often used in practice, especially in industry. When wetting, a continuous film of water remains in areas free of grease, and its rupture is observed in areas that are not completely degreased.
Method of spraying water with pigment or magenta - a fuchsin solution prepared by dissolving 2 g of fuchsin, 10 mg of phenol and 100 mg of glycerin in 200 g of distilled water while heating is sprayed onto the surface under study. 5 minutes after spraying the solution, the surface to be tested is thoroughly washed with water. Traces of fats and oils leave bright red stains.
Fluorescence method - based on the fluorescence in the dark of mineral oils when exposed to ultraviolet light; in this case, the fluorescence intensity is directly proportional to the oil content on the metal surface. The degree of purity is determined by this method using an ultrachromic microscope UI-N. or an apparatus for fluorescent analysis (model 833).
Method of contact deposition of metal - an indicator of degreasing is the mutual substitution of metals in the voltage series, i.e. deposition of a more electropositive metal on the degreased surface of a less noble metal.
To determine the degree of cleaning of the metal surface, samples are immersed for 1-2 seconds in the following compositions: 3% solution of ZnS04 (steel, aluminum, aluminum alloys), 1% solution of CnS04 (steel, aluminum, aluminum alloys and zinc), 3 % solution of Hg(N03)o (copper and brass).
The method of residual radioactive impurities is rarely used; is based on the determination, after purification, of the radiation intensity using a Geiger-Muller counter of traces of a previously deposited substance, which includes labeled atoms.
None of the methods listed are perfect.
The choice of method for assessing the quality of degreasing depends on the requirements for the surface being treated, i.e., on the amount of contaminants acceptable for subsequent technological processing.
For example, if degreasing in an alkaline solution is followed by electrolytic degreasing, an external inspection of the surface is sufficient to determine the presence of a large accumulation of contaminants on the surface. After electrolytic cleaning, the effectiveness of degreasing is checked more carefully by the method of wetting with water.
During laboratory testing and research work The method of fluorescence, contact deposition of metal and the method of residual radioactive impurities have proven themselves well.
Degreasing is nothing more than the removal of any types of contaminants, such as synthetic oils and dust. It is necessary for better adhesion of paint, primer and other substances to any surface, be it a car body or a wooden table.
To degrease the surface, three types of chemicals are usually used: organic solvents, emulsion compounds and alkaline or acidic solvents.
Organic solvents
The most simple means surfaces are degreased using organic solvents. White spirit or acetone are most often used. Their operating principle is as follows. By applying the product to the surface with a brush or rag, the organic impurities on it dissolve. After this, all that remains is to remove them using a clean, dry cloth or compressed air.
A significant disadvantage of this degreasing method is the high risk of fire. In addition, organic solvents may not completely remove contaminants.
Emulsion compositions
This type of product consists of surfactants added to aqueous emulsions of solvents. It has much higher wetting and dissolving abilities, which allow you to completely remove any organic and inorganic contaminants from the surface.
The advantage of such compositions is non-flammability, lower toxicity than individual solvents, and higher efficiency of the degreasing process due to the simultaneous dissolution and emulsification of contaminants.
However, after completing the procedure, it is necessary to rinse the surface more thoroughly than in other cases. In addition, this type of degreasing cannot be carried out without special equipment. First of all, we are talking about the availability of disposal facilities Wastewater. Otherwise you may cause serious damage environment.
Alkaline solutions
The most common types of this degreasing agent are weak or medium alkaline solutions, which consist of various salts of mineral acids and additions in the form of active detergents. The most commonly used are solutions of the CM type.
As in the case of emulsion compositions, this product requires special conditions and equipment, since
Preparing metal products for the application of paints and varnishes ensures better adhesion of the substrate to the paint film and corrosion resistance of the metal, which increases the service life of the painted product. According to many, rust removal is a more important issue in the preparation process than the need to degrease the metal before painting, which is often either forgotten or simply neglected.
What is defatting
The degreasing process involves removing fatty substances from the surface of the substrate, which may be contained in polishing compounds, preservative lubricants, mineral oils, cooling emulsions. In addition, fingerprints and sweat, residues from washing and etching also need to be degreased. The presence of such contaminants significantly worsens the conditions for wetting the surface with paints and varnishes, and also adversely affects film formation and other properties of the coating.
According to the degree of surface contamination, depending on the amount of fatty impurities on square meter, distinguish:
- Weak – up to 1g;
- Medium – from 1 to 5g;
- Increased – more than 5g.
The chemical effect on fats consists of the following processes:
- Solvents;
- Emulsifying;
- Saponifying (destructive);
According to the ability of fats to be destroyed using degreasing agents:
- Saponifiable (fingerprints and skin marks, polishing materials, lubricant residues);
- Non-destructible (preservative lubricants, emulsions, etc.).
Chemical methods for degreasing metal surfaces
Organic solvents
Using organic solvents, you can clean oily and greasy areas of metal parts in a short period of time. They are more often used in individual production, less often in serial production (due to explosion and fire hazards). When exposed to organic solvents, oil and fat deposits dissolve. The quality of work directly depends on the degree of contamination of the solvent, since the ability to dissolve sharply deteriorates with a fat content of 5 g per 1 liter. The most common are aliphatic (gasoline, white spirit) and chlorinated (non-flammable, but more toxic) solvents. Among the shortcomings, one can note the lack of cleaning of surfaces from abrasive materials and other mineral contaminants.
Aqueous solutions of alkaline and acidic nature
It's no secret that by the water low performance cleaning properties, the reason for which is high surface tension and incompatibility with fats, as a result of which it poorly wets the greasy surface and does not form stable emulsions. An increase in pH, a temperature range of 50-65C, and the addition of surfactants significantly increases the washing ability of an aqueous solution.
These compositions are distinguished by high cleaning properties, fire safety, environmental friendliness, and a variety of application methods. The effect of aqueous solutions on saponifiable fats and oils is destructive, and unsaponifiable contaminants are emulsified. The emulsification process can be described as the peeling of fat layers from the surface into a working fluid and subsequent removal along with the solution. Among the disadvantages, we can note the need for mandatory anti-corrosion treatment of the cleaned surface.
One of the most effective components of alkaline cleaning solutions are surfactants. Surfactants form foam on the surface being cleaned, reduce surface and interfacial tension, increase wettability and have a dispersing effect on solid contaminants and an emulsifying effect on liquid contaminants. The surfactant content in grease cleaners is no more than 10%.
If, along with degreasing, it is necessary to remove thin oxide or hydroxide films, use acidic solutions containing 1-3% phosphoric acid.
After the cleaning agents have been used, the surface must be thoroughly rinsed with water. Salt residues negatively affect the properties of the paint film, increasing moisture permeability and promoting under-film corrosion.
Emulsion products
To remove stubborn contaminants, grease, oil deposits, and old paint and varnish coatings, emulsion degreasing is used. This is a mixed version that takes advantage of organic solvents and aqueous alkaline solutions. Such compositions contain emulsions of solvents in water with surfactant additives. Solvents can be aliphatic or chlorinated hydrocarbons.
Ultrasonic and electrochemical degreasing method
To improve the cleaning properties of detergent compositions, special baths with an ultrasonic field are used. This method is used for small-volume products that have a surface of increased complexity and require ideal surface cleaning quality (parts of watches, devices, tools). For large parts, this method is unjustified from an economic point of view due to increased input power requirements.
Electrochemical degreasing also occurs in specially equipped baths, where gas bubbles formed on the electrodes play an active role. This reduces the consumption of chemical components and improves the quality of cleaning.
How to degrease metal before painting
IN Everyday life the choice of degreasing agent most often falls on the old “old-fashioned means” - gasoline, kerosene, acetone, alcohol. Nefras (White Spirit), Solvent 646 are considered more modern and technologically advanced. They are distinguished by their low price, better environmental friendliness and form films that are neutral to paint coating, which prevents corrosion. In order for the substrate to match the selected paint after degreasing, it is advisable to buy the solvent that is suitable for the coating (the manufacturer writes the numbers of suitable solvents on the label). At the same time, according to consumer reviews, No. 646 remains the most universal. Special solvents Antisilicon is used by craftsmen to degrease the car body before subsequent painting. Of course, we should not forget about safety - work with organic solvents should be carried out in a room with good ventilation, away from sources of ignition, using personal protective equipment.
Video: Degreasing a car body
Special concentrated solutions – Chistomet, Docker Dekamet and others – boast the best efficiency. These are alkaline products containing inhibitors, surfactants, additives and other components. The concentrate dissolves in water in different proportions, depending on the degree of contamination of the surface to be painted. Among the operational properties are environmental friendliness and fire safety. In addition to degreasing, these products provide anti-corrosion protection for the metal. Suitable for use in industrial production.
Performance preliminary preparation surfaces - one of necessary conditions obtaining the best gluing results The strength of the bond depends largely on the adhesion between the adhesive and the surfaces being joined. The more thorough and complete the preliminary cleaning of the working surface is, the greater the strength of adhesive joints (Fig. 19).
Adhesion is enhanced:
- when removing unwanted surface films after mechanical cleaning or defatting;
- when creating new active surfaces as a result of applying primers;
- when surface activity changes after plasma treatment of metal, corona treatment, etching of metal surfaces and similar operations.
If the bonded surfaces are dirty, adhesion decreases.
Degreasing surfaces before gluing
To get the best results, you should completely remove oil, grease, dust and other dirt residues from the surfaces to be bonded. Solvents that evaporate without leaving residue can be used for this purpose. The table lists the main solvents and their cleaning properties.
Alkaline and acidic water systems almost always contain corrosion inhibitors. Their residues on the cleaned bonding surface can reduce adhesion or slow down the polymerization process of the adhesive. Before using such cleaners, preliminary tests should be carried out. In each case, the surfaces must be completely rinsed and dried.
Cleaning solution: Dependence of final strength of adhesives on concentration
Cleaning solution: Dependence of the final strength of adhesives on the drying position (horizontal or vertical).
When using special degreasing baths on large production lines, heavily soiled surfaces should be pre-cleaned to avoid contamination of the baths. Vaporizing cleaners are often used. When using them, the solution is heated to boiling temperature and evaporates. When contacting cold surfaces, steam from the cleaning composition condenses on them. The resulting liquid removes grease and dirt particles. The degreasing process is carried out using degreasing compounds inside hermetically sealed mechanisms.
As a rule, pre-treating surfaces with fast-acting cleaners is sufficient. With their help, oil, grease, and dirt residues are removed, thereby preparing the surface for gluing. If solvent cleaning is carried out in addition to the chemical degreasing process, good result Removal of dirt from the surface is achieved by using mechanical treatment with brushes and wiping cloths.
Mechanical treatment of surfaces before gluing
Contaminated metal surfaces often contain oxides that cannot be removed by degreasing. Here it is necessary to carry out preliminary mechanical treatment: wire brushing, grinding or sandblasting.
Sandblasting is good for cleaning large surfaces. The surface roughness achieved in this way has a positive effect on the quality of gluing if the sand used is not very coarse. Sanding also creates a good surface roughness. With this method, it is very important to use a suitable roughness grain (for aluminum, for example, 300-600, for steel - 100). At the end of sandblasting, as well as after brushing or grinding, the parts must be degreased in order to completely remove processing residues. Before machining It is also necessary to degrease heavily soiled parts. This is done so that the abrasive or sand used does not leave solid particles on the surface. As practice shows,
The mechanical pre-treatments used to create adequate bond strength are readily available and very simple.
When gluing plastic or rubber parts, all contaminants in the form of surface films, mold deposits and vulcanization films must also be removed first. Abrasives made from aluminum oxide or cast iron balls are effective for processing plastic parts. Rubber surfaces can be cleaned by sanding or solvents.
Etching surfaces before gluing
When etching surfaces, chemicals with relative aggressiveness are used. Depending on the type of surface being treated, strong alkalis or acids are used. When etching, a change in the surface structure occurs as reactive groups are added and cavities are formed, providing space for mechanical fixation adhesive composition. The results of such processing will directly depend on the type of surfaces. The use of the etching method at the industrial level is limited by its high cost.
Ionization preparation of bonded surfaces
During ionization pretreatment of the surface, its potential energy and polarity change, as with wet chemical preparation. The choice of processes used will depend on the material of the product, its configuration, the number of parts and the sequence of their assembly.
Using primers to prepare bonded surfaces
Primers usually contain reactive soluble chemical compounds. They are applied with a brush or sprayed onto the surface of the material. Solvent particles remaining after treatment can be neutralized. When certain types of primers are used, the surface can be ready for bonding immediately, as is the case with polyolefin primers in cyanoacrylates. Thanks to surface primers, surface adhesion improves. They act like a chemical bridge between the adhesive and the surface. In a primer, the reactive moiety is usually multifunctional with respect to a specific set of reactive groups that react with the working surface as well as with additional adhesive-like groups.
x = additional or alternative method
xxx = most suitable method
Surface testing by wettability test
The quality of the preparedness of the surface for gluing can be determined using a fairly simple method of repelling liquid. Drops are applied to the cleaned surface clean water. If the surface is not sufficiently cleaned, the spherical shape of the drop remains, therefore, this surface must be cleaned again. On a sufficiently clean bonding surface, water spreads.
The liquid repelling method cannot be used on anodized magnesium and aluminum surfaces.
The main advantage of the water repellency test method is its ease of availability.
In addition, the use of this method is not always acceptable due to different hardness water, creating different surface tensions. Even the use of distilled water during water repellency tests sometimes does not provide reliable results. Therefore, for special occasions It is necessary to use surface-stressed liquids with precisely defined surface tension. It should also be taken into account that carrying out water repellency tests only allows you to check the preparedness of the surface, and not the possibility of gluing as such.
Surface preparation can be checked using the "water repelling" method.
The problem of increasing the corrosion resistance of painted surfaces has not lost its relevance to this day. The first and most necessary operation preparatory process surfaces before painting are degreased.
If you do not degrease the surface, you will not be able to achieve good adhesion, and the paint will begin to peel off very quickly in those places where there were stains and dirt. In most cases, surface degreasing is performed using organic solvents. But what solvent should I choose for this and how is this work done correctly? Let's take a closer look at this question: does solvent 646 degrease and is it always possible to use it?
Degreasing metal surfaces.
Solvent 646 is a multicomponent composition, which includes several simple substances. It is considered one of the most popular solvents in modern market. It is very often used in car repair shops, since the physical and chemical properties of this composition are excellent for diluting car paints and degreasing car bodies. However, you should remember that this substance can very easily dissolve varnish and paint, so you need to use it very carefully and only before applying putty.
Before degreasing with a solvent, all visible contaminants must be removed from the surface. We should not forget that when working with solvents, you must use personal protective equipment, such as a respirator, goggles and gloves. Otherwise, organic solvent poisoning is quite possible.
Stages of work:
- Cleaning the surface grinder or regular sandpaper. At this stage, you need to clean the surface of dry large debris, rust and dust.
- We wet a cloth with solvent and thoroughly wipe the entire surface to be treated. It is best to use solvent 646 as it provides maximum cleaning and excellent adhesion.
- After cleaning with solvent, leave the surface for a few minutes to allow it to dry completely. Now you can start priming and painting.
If someone has not yet understood whether it is possible to degrease with solvent 646, then the answer is obvious - it is possible, but very carefully. Don't forget that it can easily dissolve putty, varnish or paint.