New building materials and technologies have radically changed the attitude to finishing work, including the application of paint and varnish coatings. A new level of approach is provided, first of all, by a significant improvement in operational and decorative properties, as well as by expanding the types of surfaces suitable for applying paint and varnish coatings.
As statistics show, the main part of the defects appears due to improper preparation of the mowing base. It is important to choose the right paint system and follow the technology. The most unlikely defect is poor quality paint.
To understand the nuances of the above, we will understand the features of each type of possible defect, we will note the actions that need to be taken to avoid these consequences.
Substrate preparation
Before performing work, it is necessary to assess the quality of the base visually, as well as pay attention to damage. The base can be made of inorganic and organic materials, and also have a dense or porous structure. Concrete bases must be free of formwork lubricant. The base surface must be clean and dry. If the application of the paintwork is planned on the old base, then its reliability can be checked in the following way. A masking tape is glued onto the old coating, after which it abruptly comes off. If, as a result, the paintwork material does not come off, then its strength will be sufficient.
The absorbency of the surface is also evaluated. If water is quickly absorbed, then the applied compositions (diluted with water) will not gain sufficient strength. In this case, the surface is pre-treated with special primers. If shedding or uneven absorption is detected, special primers are also used.
Choice of paint system
The service life of the coating will depend on the choice of painting system. In addition, the right choice will avoid additional costs.
When choosing one or another system, pay attention to the operational requirements for the surface, the physical properties of the system and possible color solutions.
The most popular systems are:
- Acrylic (suitable for almost any surface; have rich color options);
- Silicate (the best permeability of water vapor and carbon dioxide; the color gamut is very limited);
- Silicone (high permeability of water vapor and carbon dioxide; high water repellency; suitable for almost any mineral surface; best performance and decorative properties; the only negative is high cost).
Application
In order for the coating to provide these properties, it is necessary that the thickness of the dried film be equal to 100-120 microns (for an area of 1 m 2, about 200 ml of paint). In the case of using liquid paint on a vertical surface, it will be necessary to apply about 4-5 layers. The result can also be obtained in one pass using high quality thixotropic paints. The paint liquefies under mechanical action and thickens at rest, moreover, when using such a paint, it becomes possible to use Airless spray technology, which allows you to create an ideal surface.
Summing up, it should be said that the application of paint and varnish coatings is a rather simple process, but this process has many nuances, which were mentioned in the article. And the more correctly all the steps are completed, the longer the service life will be. It will also help to avoid high costs later, when carrying out repairs in the future.
The main purpose of paint and varnish coatings is surface protection and its decorative finish. A coating system is a combination of layers of sequentially applied paintwork materials for various purposes (covering, priming, intermediate layers). The properties of complex coatings depend both on the quality of coatings and on their compatibility.
By appropriate surface preparation, the choice of primers, putties and top coats, the performance and durability of coatings can be varied. First, a coating material suitable for the given service conditions is selected, and then a primer is selected that has good adhesion to the surface to be painted and is compatible with the coating material for the given service conditions.
Scheme of a protective coating based on paints and varnishes.
1. Protected surface (metal, wood, concrete, etc.)
2. Priming layer;
3. Putty layer. When painting porous materials (wood, concrete, etc.) it can be applied first without a primer;
4. Protective and decorative layer of paint, enamel or varnish.
What are the requirements for paintwork?
Basic requirements for protective coatings- high adhesion to the substrate, gas impermeability and water impermeability, mechanical strength, wear resistance and resistance to operating conditions (weather resistance, chemical resistance, etc.).
Coatings can be transparent and opaque (opaque); transparent are obtained when applying varnishes, opaque - when applying primers, putties, paints and enamels.
The total thickness of the coating in the case of using traditional paints and varnishes is usually 60-100 microns, sometimes up to 300-350 microns. In the case of putties, sealants or composite materials, the layer thickness is in the range of 500 - 2000 microns or more.
The need to apply paints and varnishes in several layers is due in many cases to the impossibility of obtaining coatings with good protective properties, tk. when applying one thickened layer, the evaporation of the solvent and other film formation processes are hindered, and a coating with smudges and sagging can be obtained. Thick putties, thixotropic varnishes and enamels, as well as materials containing reactive solvents, such as polyester varnishes and enamels, can be applied in a layer thickness of more than 350 microns.
The upper layers of the coating provide the surface with the desired decorative properties, hiding power and resistance to the effects of the external environment. Enamels and paints are mainly used for applying top coats. A layer of varnish is sometimes applied to the top coating layer, giving the coating a shine or dullness.
Between the primer and top coat, if necessary, intermediate layers for various purposes are applied, for example, putty layers for leveling the surface and sealing welds and rivets, to prevent swelling of the primer or other previously applied layer in the solvent contained in the top coat. Depending on the type of material, the operations of applying individual layers are called priming, filling, painting or varnishing, respectively.
The main stages and methods of application.
Surface preparation
Surface preparation prior to painting is essential to obtain a high quality coating and ensure longevity. Surface preparation consists in cleaning from corrosion products, old paint, grease and other contaminants. Surface preparation methods are divided into two main groups: mechanical and chemical.
Mechanical cleaning methods include: cleaning with a tool (brushes, grinders), cleaning with sand, shot, a mixture of sand and water. Using these methods, you can get a well-cleaned surface with a uniform roughness, which contributes to the best adhesion of the paint film.
Chemical surface cleaning methods primarily include degreasing the surface, which is carried out using alkaline detergent compositions or active solvents (washes), depending on the type of contamination.
When updating the paintwork, it is necessary to carefully inspect it. If the old paintwork is firmly attached to the surface in the form of a continuous layer, it should be washed with warm water and detergents and dried. If the coating does not hold firmly, it must be completely removed.
Padding
The first operation after surface preparation is priming. This is one of the most important and responsible operations, since the first primer layer serves as the basis for the entire coating. The main purpose of the primer is to create a strong bond between the surface to be painted and subsequent paint layers, as well as to ensure a high protective ability of the coating. Priming should be carried out immediately after the completion of surface preparation work. The primer can be applied with a brush, spray gun or other method. The primer layer should be thin compared to the outer coats of paint. Soil drying should be carried out in accordance with the regime provided by the technology.
Puttying
This operation is needed to level surfaces. Thick and insufficiently elastic putty layers may crack during operation, as a result, the protective properties of the coating will be reduced. Therefore, the putty should be applied in a thin layer. Each layer of putty must be well dried. The number of layers should not exceed three. The recommended thickness of the putty layer is no more than 3 mm.
grinding
The putty surface after drying has irregularities and roughness. Grinding is used to remove irregularities, specks and smooth out roughness. In the process of grinding, the treated surface is exposed to many tiny abrasive grains, as a result of which risks are formed and it becomes matte. This significantly improves the adhesion between the coating layers. For grinding, an abrasive paper on a paper and fabric basis is used. The grit (number) of the sanding paper is selected depending on the type of coating to be processed.
Coloring
Enamels, paints, varnishes are applied to the primed surface using a paint sprayer, roller, brush or other methods.
If we consider the influence of the previous coverage on the quality of the next one, then the rule: "similar to similar" applies here.
However, it is possible to deposit materials of different chemical nature on top of each other.
Methods for applying paint and varnish coatings
The first and simplest method of applying paint is with a brush. Unfortunately, the brush, in addition to its indisputable advantages, has many disadvantages, first of all, a low painting speed (about 10m2/hour).
The use of a roller instead of a brush can significantly increase the speed of painting, in particular large and flat surfaces, but with its help it is difficult or even impossible to paint with quick-drying varnishes or materials having a high relative viscosity.
The first step towards a noticeable increase in the speed of painting and improving the decorative properties of paint coatings was made when creating a pneumatic liquid sprayer.
In almost all pneumatic spray guns, air moving at a speed of about 30 m/s breaks the liquid flow into drops with a diameter of 40-120 microns, which allows painting at a speed of 30 m2/hour. However, in the process of using pneumatic spraying, negative aspects were quickly discovered: large losses of paintwork materials, which increase with increasing air velocity in the gun, difficulties in applying high-viscosity materials, and high volatility of organic solvents.
The need to limit the evaporation of organic solvents into the atmosphere, dictated by modern environmental protection legislation, has contributed to the intensification of the search for new ways of coloring. For the application of high-viscosity paints, hydrodynamic painting technology - airless spraying - has received great development. Airless spray painting is a complex process that requires highly skilled operators. This technology differs from pneumatic spraying, where the paint is applied in stripes that only slightly overlap each other. For airless spraying, cross-guided gun control is required. The high productivity of hydrodynamic painting (200-400 m2/h) is effective when painting large surfaces (for example, the sides or decks of ships), but is inconvenient for painting small elements or when frequent changes of surfaces to be painted are required.
Properties of paints and varnishes and coatings.
Grinding degree
Particles of fillers or pigments included in paints, enamels, primers and fillers differ in their size. The particles have the smallest size in the composition of enamel (5-10 microns) and the largest size in putties (40-60 microns or more). The reduction in particle size occurs in the process of grinding fillers in mills of various devices (paint mills, ball mills, bead mills).
Time and degree of drying of the coating
The drying time is taken as the time during which a coating of a certain thickness applied to a plate reaches the required degree of drying under given drying conditions.
The degree of drying characterizes the condition of the coating surface at a certain temperature and drying time under standard test conditions:
Drying from dust - the moment when the thinnest surface film is formed on the surface of the coating;
Practical drying - the film loses its stickiness and the product with a paint and varnish coating can be subjected to further operations;
Complete drying - the end of the formation of the coating on the painted surface.
Nominal viscosity
When choosing a coating method, the conditional viscosity of the paintwork material is of decisive importance. Relative viscosity is the time of continuous flow in seconds of a certain volume of material through a nozzle of a certain size.
hiding power- the most important technological indicator that characterizes the consumption of paintwork material per 1 m2 of the painted surface. The value of this indicator determines the uniformity of applying a layer of paint and varnish material, which determines its economic efficiency. Hiding power depends on the optical properties of the pigment, its dispersion and volumetric concentration in the binder. The chemical composition, color and physicochemical properties of the binder, the type of solvent, etc., also have a significant effect on the hiding power.
However, hiding power is mainly due to optical phenomena occurring in the film.
Hardness- the resistance provided by the coating when another body penetrates into it. The hardness of the film is one of the most important mechanical properties of the paint coating, which characterizes the strength of the surface.
The bending strength of a coating indirectly characterizes its elasticity, i.e. brittle property.
Adhesion- the ability of paint and varnish coatings to adhere or adhere strongly to the painted surface. The mechanical and protective properties of coatings depend on the magnitude of adhesion.
Water resistance - the ability of a paintwork to withstand prolonged exposure to fresh or sea water.
weather resistance- the ability of the paintwork to maintain its protective and decorative properties for a long time in atmospheric conditions. The service life depends on the climatic and specific conditions of the area. The types of destruction associated with the loss of the decorative properties of paint and varnish coatings include: loss of gloss, discoloration, whitishness, dirt retention, etc.
The technological process of painting CONSISTS OF the following three main operations: priming, puttying, final painting.
Priming on prepared metal is the application of the first layer of paint and varnish material on a cleaned, degreased, washed, and phosphated metal surface. The primer layer is the basis of the coating. It provides reliable adhesion to the metal prepared for painting and the subsequent layer of paint, has high anti-corrosion properties, mechanical strength.
For better leveling, one or two layers of a second primer are often applied to the previously primed and putty surface, which differs from the first in composition, properties, color, application and drying methods. For preliminary priming, water-borne primers are most often used, applying them by the method of electrodeposition. Epoxy, epoxy ester and other types of primers are used to apply the second coat, using various spray methods.
Each layer of the applied primer is dried in accordance with the technical requirements. Then the surface is subjected to grinding with abrasive waterproof skins with abundant wetting of the surface with water. Grinding is done manually or with the help of special grinding machines. In mass and large-scale production, in order to reduce labor intensity and improve the quality of surface finish, the grinding operation is mechanized.
Then, the sections ground to the metal are primed. Usually, primers are used for this, which dry quickly at room temperature.
Water-borne primers used for preliminary priming are dried at a temperature of 180-190°C, primers for the second and subsequent layers are dried at a temperature of about 160°C.
Puttying is the process of leveling identified minor defects on pre-primed surfaces of the body. As the main tool for this operation, rubber, plastic, wooden and metal spatulas are used. Sprayers are used to apply liquid fillers. The thickness of the putty layer applied to the primed surface should not exceed
0.5 mm. The exception is epoxy fillers, which can be applied to both primed and metal surfaces with a layer thickness of up to 15 mm.
The final painting is carried out on the primed, puttied and sanded surface of the body (cabin). Due to the high resistance during operation and the lower labor intensity of application, synthetic enamels of the ML grades are most widely used. Synthetic enamels allow various methods of application, however, to obtain a high quality enamel surface, it is recommended to apply with pneumatic spraying or spraying in a high voltage electric field. An important indicator that determines the quality and service life of the paintwork is the total film thickness. A thin coating is not resistant to abrasion, does not provide the required protection against corrosion, and does not provide the necessary gloss. An excessively thick coating becomes brittle and loses its properties with sudden changes in temperature. The total thickness of the coating is considered optimal from 80 to 120 microns.
There are various ways to apply paint and varnish coatings.
Hand painting with brushes is used in the final finishing, when it is necessary to touch up small defects on the non-facial surface of the body, assembly unit or part.
Dip painting has become widespread in the industry. The part is immersed in a bath with paint and varnish material, then removed from it, kept for some time over the bath to drain excess paint from the surface and dried. In mass production, painting by immersion is carried out using overhead conveyors equipped with pendants of various shapes in the form of hooks, Christmas trees, rakes, etc.
18-9. METHODS FOR APPLYING PAINT COATINGS
Paints and varnishes are applied to the surface of products by various methods: pneumatic spraying, spraying under high pressure, spraying in an electric field, aerosol spraying, etc. electroplating, flow coating, dipping, pouring, rollers, drumming, brush and spatula.
The most effective method of applying a paint and varnish material for a particular electrical apparatus is selected from the requirements for coating, dimensions and configuration of the electrical apparatus, assembly unit or part, production conditions, economic feasibility, production volume.
Pneumatic spray painting. About 70% of produced paints and varnishes are applied by this method. Pneumatic spraying is used mainly without heating.
High pressure spray painting (airless spray). For spray painting with heating, paints and varnishes are heated to 40 - 100 ° C and fed to the spray device under a pressure of 4 - 10 MPa with a special pump. The spray torch is formed due to the pressure drop when the paint material exits the spray nozzle and the subsequent flash evaporation of a part of the heated solvent. Loss of paintwork material is 5 - 12%. The advantages of this method "-compared to air spray painting are as follows:
1) losses on paintwork materials are reduced by 20 - 35%;
2) the consumption of solvents is reduced;
3) the painting cycle is shortened.
This method is recommended for painting medium, large and extra large devices in serial and single production.
When painting by spraying under high pressure without heating, the paintwork material at 18 - 23 ° C is fed to the spray device under pressure.
Spray painting without heating has several advantages over spraying with heat:
installations are simpler in design and lower energy consumption.
Spray painting in a high voltage electric field. This method is based on the transfer of charged paint particles in a high-voltage electric field created between a system of electrodes, one of which is a corona-spraying device, the other is an electrical apparatus or its part to be painted. The coating material enters the corona edge of the atomizer, where it acquires a negative charge and is sprayed under the action of electrical forces, after which it is directed to a grounded product, depositing on it.
surfaces.
(Fig. 18-11). This method consists in the fact that the product, painted with paintwork material from the nozzles of a dousing device, is placed in an atmosphere containing a controlled amount of organic solvent vapors. Exposure of the applied layer of paintwork material in an atmosphere of solvent vapors makes it possible to slow down the process of volatilization of the solvent from it at the initial moment of coating formation. This allows the excess amount of paint material to drain off the product, and the remaining amount to be evenly distributed over the surface. In comparison with painting in an electric field, the best quality of coating of parts of any configuration is provided.
The jet flow method is used for priming and painting products in serial and mass production (Fig. 18-11).
Coloring aerosol spray. The method is effective in repair work, as well as in the application of stencils and inscriptions and other painting operations of a small volume. Paint spray cans are produced with a capacity of 0.15; 0.3; 0.5; 0.6l.
The technological process of painting includes the following operations: surface preparation for painting, priming, puttying, grinding, painting, drying, coating quality control.
For parts of tractors and combines that experience strong vibrations during operation, puttying is not used, since the putty layers are destroyed and peeled off.
Padding- one of the most critical operations, which creates a strong bond between the painted surface and subsequent paint layers, and also provides the protective ability of the coating. Prime the surface immediately after preparation. The primer is applied with a brush, spray gun or other means. When painting equipment operated in high humidity or atmospheric conditions, priming is recommended to be done with a brush to remove the water film (if any on the surface) in the process of shading the paint. The primer is applied in an even layer with a thickness of 15 ... 20 microns. With a glossy surface, the primer should be lightly cleaned with fine sandpaper (sandpaper).
When choosing primers, their purpose, physical and painting characteristics, compatibility of primers with the surface to be protected, putty and enamels are taken into account.
Puttying used to level the primed surface. Putty should be applied with a layer of no more than 0.5 mm, otherwise thick layers of putty lose their elasticity and may crack during operation, as a result, the protective properties of the coating are reduced. The total thickness of the putty layer can be 1…1.5 mm. First, a local putty is applied to the primed surface, and then a solid one. Each layer of putty is well dried. The number of layers should not exceed three. If more layers are used, a primer layer is applied between them.
grinding. The rough, putty surface is sanded after drying to smooth out irregularities. When grinding under the influence of abrasive grains, the treated surface becomes matte. Grinding can be dry and with coolant. When grinding a coating based on oil-lacquer and alkyd paints and varnishes, water is used as a coolant; based on perchlorovinyl, epoxy and nitrocellulose materials - water or white spirit.
For grinding the coating, a paper or fabric-based sandpaper is used, the grain size of which, depending on the type of coating being processed, is given in Table. 16.
Table 16
Grain size of skins for grinding coatings
Coloring. One or two layers of enamel are applied to the primed and sanded surface. The painted surface must be even and shiny. It is not allowed to see through the primer or putty, smudges, weediness and damage to the layer.
The painting of cars is divided into capital, repair and preventive.
Repair and preventive painting is carried out without disassembly. Preventive painting is performed in case of minor damage before storage, repair - in case of damage to the paintwork material up to 50% of the total surface; capital - in case of destruction of more than 50% of the protected surface. During the overhaul, the machines are disassembled into components and parts. When choosing paintwork materials for painting, they are guided by the requirements of GOST 5282-75.
Drying. To obtain a solid film, the paintwork must dry well. During the drying process, the solvent or diluent is first rapidly evaporated, and then a film is formed with the formation of complex molecules.
Higher drying temperatures shorten the process time and improve the quality of the coating. The drying temperature is determined by the properties of paints and varnishes. Apply natural, convective, thermoradiation drying of paints and varnishes.
The duration of natural drying is 24...48 hours, while not all paints and varnishes pass into an irreversible solid state. Convective drying is the most common, but not effective enough. Thermoradiation drying (irradiation with infrared rays) is the most perfect, it is characterized by a reduction in the duration of the process, simplicity and ease of adjustment.
Control the quality of the coating visually in normal daylight or artificial lighting.
The appearance of paint and varnish coatings of combines for harvesting grain crops must comply with class III, other agricultural machines - class IV.
The color of the coatings is compared with approved color standards or reference samples.
The thickness of the coatings is determined using ITP-1 thickness gauges on the surface of products or witness samples. For this purpose, KI-025 micrometers, type 636 devices (from 10 to 1000 microns), TPN-IV devices, TLKP, etc. are also used.
The thickness of the film can be determined by the consumption of paintwork material (MRTU 6-10-699-67, MI-1). This method is used in cases where it is impossible to measure the film thickness by other methods.
Film adhesion is determined according to GOST 15140-78 by the peeling method (quantitative method), as well as by lattice and parallel cuts - a qualitative method.
With the correct execution of technological operations for the restoration of paint and varnish coatings, their durability should correspond to the service life of the machines before overhaul, subject to compliance with GOST 7751-85 (Technology used in agriculture. Storage rules.) And instructions for operating the machines.
Paints and varnishes in the conditions of repair production can be applied by pneumatic and airless spraying in a high voltage electric field, with a brush, hand rollers, etc.
Pneumatic spray. The method of pneumatic spraying can be used to apply almost all enamels, paints, varnishes, primers produced by the industry, including quick-drying ones and those with a short shelf life, on products of simple and complex configuration, various overall dimensions and purposes.
Main Benefits pneumatic spray method:
1) simplicity and reliability in the maintenance of painting installations;
2) obtaining good quality coatings on parts of complex configuration of various sizes;
3) application of this method in various production conditions in the presence of a source of compressed air with a pressure of 0.2 ... 0.6 MPa and an exhaust ventilation system.
TO shortcomings methods include:
1) large losses of paintwork material, ranging from 25 to 50%;
2) unsatisfactory sanitary and hygienic working conditions;
3) the need for a powerful exhaust ventilation system and cleaning devices;
4) high consumption of solvents for dilution of paints and varnishes to working viscosity.
The method allows you to apply quick-drying paints and varnishes (nitro-varnishes, nitro-enamels). In airless spraying, the paint is atomized in a jet of compressed air, forming a mist that is transferred to the surface to be painted. Productivity - 30 ... 40 m 2 / h.
Airless spray. The essence of the method is the spraying of the paintwork material under the influence of high hydraulic pressure created by the pump along the internal cavity of the spraying device and the displacement of the paintwork material through the nozzle opening. In this case, the volatile part of the solvent evaporates intensively, which is accompanied by an increase in the volume of the paint and its additional dispersion. The method is based on the well-known phenomenon in hydraulics of liquid crushing when flowing through a hole at a speed exceeding the critical one, below which crushing does not occur. The required critical flow rate for airless spraying is achieved by supplying the paint material to the sprayer nozzle under high pressure (4…10 MPa). One of the main features of this method is a painting jet with clear boundaries, almost the same density, uniform over the entire section with little fogging.
Advantages airless spray before pneumatic:
1) saving up to 20% of paints and varnishes;
2) saving solvents as a result of the use of more viscous paints and varnishes;
3) reducing the complexity of work in connection with obtaining thickened layers of the coating;
4) reduced operating costs of spray chambers as a result of easier cleaning and the ability to use less powerful ventilation;
5) improvement of working conditions.
TO shortcomings methods include:
1) the difficulty of applying the method for painting parts of complex configuration;
2) the method cannot be applied to paints and varnishes that cannot be heated, which contain pigments and fillers that easily precipitate; when painting products with a minimum torch and when obtaining highly decorative coatings.
electrostatic spray. The essence of the method is that paint particles, falling into the zone of the electric field, acquire a charge and are deposited on a grounded surface that has the opposite charge. To ensure the mobility of charged paint particles, a high voltage electric field (70…120 kV) is required, which is created between a negative charged corona electrode and a grounded conveyor with painted parts. A copper mesh or paint supply devices are used as a corona electrode.
The method has the following Benefits:
1) reduction in the consumption of paints and varnishes by 30...70% compared to pneumatic spraying;
2) reducing the cost of equipment for ventilation devices;
3) the possibility of complex mechanization and automation of the process;
4) improving the culture of production and improving sanitary and hygienic working conditions.
TO shortcomings methods include:
1) incomplete staining of products of complex configuration with deep cavities, combinations of complex interfaces and internal surfaces;
2) the paintwork material must have a specific volumetric electrical resistance of 10 ... 107 Ohm cm;
3) the need for highly qualified equipment maintenance.