The dipping method is used to create thin films and apply coating. Technically, the method is based on immersing the substrate in a container with the coating material, after which the material is fixed to the substrate and then allowed to drain. Part of the coating can be removed by drying or heating.
Stages of immersion
The dive can be divided into three main stages:
- The substrate is immersed in the solution at a constant speed;
- Keeping the substrate in the solution in a stationary state;
- The substrate is removed at a constant speed. The faster the substrate is removed from the solution, the thicker the layer of material on the substrate will be.
Cons and pros
The method is quite simple, which makes it easy to automate. Film thickness is controlled by coating viscosity and release rate from the container. The containers used in this method can be different in shape and size. This makes it possible to coat larger substrates.
One of the disadvantages is the fact that the film thickness in the lower part of the plate may be greater than in the upper part (“wedge effect”). The coating may flow unevenly at the edges of the substrate, resulting in a thicker coating at the edges. Also, solvent vapors can carry away particles of the coating, causing it to become uneven.
Brief theory
Dip coating method is a process in which the substrate is immersed in a liquid and then removed under controlled environmental conditions, ultimately resulting in coating. The thickness of the coating is determined by the rate of rise of the substrate, the viscosity of the liquid and the content of solid components. If the rate of rise of the substrate is selected taking into account that the state of the system will be in the Newtonian regime, then the film thickness can be calculated using the Landau-Levich equation.
h - coating thickness, η - viscosity
γ LV - liquid-vapor surface tension, ρ - density
g - specific gravity
The work of James and Strawbridge showed that the experimental values of the thickness of acid-catalytic cremosol correlate well with the calculated values. An interesting effect occurs in the dipping method: by selecting the appropriate viscosity, the thickness of the coating can vary with high accuracy from 20 nm to 50 microns while maintaining high optical quality. A diagram of the immersion process is shown in Figure 1.
Picture 1. The steps in the dip coating process are: immersing the substrate in a solution, forming a wet layer by removing the substrate, and converting the layer into a gel by evaporating the solvent.
If reactive systems are chosen for coating, for example, as is the case with sol-gel coatings for which alcoholates or pre-hydrolyzed sols are used, then it is necessary to control the state of the environment. The environment influences solvent evaporation and can destabilize this process, leading to gelation and the formation of transparent film because of small size particles of sols (nm). This is shown schematically in Figure 2.
Figure 2.The gelation process during dip coating, obtained by evaporation of the solvent and subsequent destabilization of the sol (Brinker et al.)
Sol particles are stabilized by surface charges, which is why it is necessary to consider the conditions of Stern stabilization. According to Stern's theory, the gelation process can be explained by the approach of a charged particle to a distance at which the repulsive potential occurs. This potential results in very rapid gelation. This process occurs at the gelation point, as shown in Figure 2. The resulting gel is subjected to heat treatment, and the sintering temperature depends on its composition. However, due to the fact that the gel particles are extremely small, the system is characterized by the presence of excess energy, which is why in most cases a decrease in sintering temperature is observed compared to systems bulk materials. However, it should be taken into account that alkaline diffusion in ordinary glasses, such as for example glasses made from slaked lime, starts from a few hundred degrees Celsius and, as shown by Banj, alkali ions diffuse into the coating layer during compaction. In most cases, this is not a significant disadvantage, since layer adhesion improves, but when calculating optical systems, the effect on the refractive index must be taken into account.
The dip painting method is highly economical and ecological way painting, which is one of the most modern surface treatment technologies. The main area in which it is used this method is the automotive industry (painting of car bodies and their parts).
This method is also widely used for painting home household appliances(refrigerators, washing machines, radiators, etc.), agricultural machinery, metal furniture, building structures etc.
Compliance with very stringent requirements for surface quality can be achieved using electrophoretic painting methods (mainly the KTL cataphoresis method), which, due to the properties of the process and the properties of the resulting coating, currently have no comparative competition in certain industries.
KOVOFINIŠ offers a complete line of equipment for immersion painting, both by dipping in classic or water-borne paints and by electrophoresis (cataphoresis and anaphoresis). At the customer's request, our company is ready to offer equipment of both clock action and continuous operation.
Clock-type lines are characterized by high operating flexibility. They are advantageous to use for small volumes, frequently changing shapes of products, as well as for products large sizes. Continuous lines are convenient to use when high productivity, mass production or large-scale production of similar parts are required.
Our company supplies this equipment on a turnkey basis, including equipment for pre-treatment, varnish (paint) firing, exhaust air purification, handling equipment and transportation systems, control systems, visualization technological process as well as equipment for water preparation for processes and waste water treatment.
We supply:
- cataphoresis staining lines (KTL)
- anaphoresis staining lines (ANL)
- Dip and spray painting lines
Dipping and pouring are the simplest and longest-used methods of coloring. They allow you to apply various paints and varnishes and obtain sufficient coverage good quality when using simple equipment. By dipping the product into paint and varnish material, or pouring it over the product, it is possible to paint almost all areas of the surface, including those hidden from the human eye, which is not achieved using other methods.
Dipping and pouring are used mainly to obtain primer and single-layer coatings on small and medium-sized products of varying complexity. They are used in many industries (automotive, instrument-making, agricultural engineering, etc.), since they allow mechanization and automation of painting processes.
Disadvantages of the methods: uneven thickness of coatings along the height of the products, inability to paint products with pockets and internal cavities, relatively large losses paint and varnish materials, often reaching 20% or more. Many of these disadvantages, however, are eliminated if flat products are the object of coloring ( wooden boards, metal sheets, roll material), laid horizontally. The paint and varnish material is applied using paint-filling (paint-coating) machines. It is when painting such products, especially panel furniture, that the pouring method has found application.
Reducing losses of paints and varnishes and variations in coating thickness while simultaneously improving them decorative look is achieved by keeping freshly painted products in solvent vapors. This method, as a variation of the pouring method, called jet pouring, has become widespread in industry. Dip coatings can be improved in a similar way. Other variations of the dipping method include dyeing long items by drawing and coating small items in rotating drums.
Dipping and pouring in any embodiment are of particular interest when applying water-based paints and varnishes due to the possibility of organizing in-line fireproof technological processes.
Basics of methods. The principle of application by dipping and pouring is based on wetting the surface to be painted with a liquid paint and varnish material and holding it on it in a thin layer due to the adhesion and viscosity of the material. The quality and thickness of coatings when painting by dipping and pouring are determined by the properties of the surface, as well as the structural and mechanical characteristics of the applied material.
Let's look at the application process liquid paint by dipping a product, for example, a flat plate, into it (Fig. 8.13). The initial act is immersion of the product in liquid material, i.e. establishing adhesive contact. Depending on the viscosity of the material and the nature of the surface, the duration of this process can be seconds or minutes. Simultaneously with the establishment of contact, adsorption interaction of the liquid with the solid surface occurs.
Rice. 8.13. Diagram of forces acting on a liquid when removing a product from it
Rice. 8.14. Velocity distribution in the liquid layer when removing a product from it
When removing a product, for example, at a speed w0, not only a layer of adsorbed liquid will be entrained; due to adhesion and internal friction F, motion will be transmitted to parallel layers of liquid, but at a speed wп. In addition to the force F, these layers will experience the force of gravity P, causing the liquid material to sink (drain) at a speed wр. the total speed of movement of each elementary layer wx located at a distance x from the surface of the products, i.e. will be equal to:
wх = wп - wр. (8.10)
Under the condition of laminar movement and the exclusion of gravity, the speed of individual layers changes uniformly as they move away from the product and becomes equal to zero. In this case, the dependence wп =f(x) is linear (Fig. 8.14), and the velocity gradient dwn/dx = const.
IN real conditions when the force of gravity P is applied, the nature of the dependence changes, the volume of liquid extracted by the product is always less (in Fig. 8.14 it is shown as a shaded area).
If we take the width of the layer as one, and the thickness as dx, then dV will be: dV= wх×dx, and the volume of all liquid entrained by such a product per unit time will be equal to:
V=wх×dx. (8.11)
After removing the product from the liquid, part of it drains, and if it is a non-volatile liquid, then, regardless of the extraction rate, a layer remains on the surface, the thickness of which is determined by the viscosity, density and energy factors of the interaction of the liquid with the surface of the solid.
When dipping into paints and varnishes, the process is complicated by the continuous change in the viscosity of the layer applied to the product, as a result of which its flow slows down and then stops completely.
It is easy to verify that the thickness and degree of unevenness of the film will be greater, the higher the rate of removal of the product (Fig. 8.15), the viscosity of the paint and varnish material and the rate of its increase at the moment of dripping. Low-viscosity materials (20 s according to VZ-4 and less) form relatively thin coatings with a small variation in thickness over the height of the product. The same effect is achieved at low speeds of extraction of products from paint and varnish material - 0.1 m/min or less (Fig. 8.16). However, in practice, this leads to a decrease in the efficiency of painting: with a decrease in the viscosity of materials, the consumption of solvents increases and in some cases it becomes necessary to apply several layers of coating. Reducing the speed of product extraction reduces the productivity of installations.
Rice. 8.15. Dependence of the thickness of coatings from oil paint on the speed of removal of products from the bath at different paint viscosities (according to VZ-4) at 20°C
Rice. 8.16. Change in the thickness of cellulose nitrate varnish coatings along the length of the product at different speeds of its removal from the bath
When applying paints and varnishes using the pouring method, the patterns characteristic of dipping are preserved. The layer of liquid supplied per unit surface during pouring, in contrast to spraying, exceeds the maximum thickness at which the liquid can be retained on vertical surfaces due to the forces of adhesion and internal friction. Therefore, its excess necessarily flows off, leaving a layer of uneven thickness on the substrate, and deposits in the form of drops on its descending edge. The duration of drainage is mainly determined by the viscosity of the paint and varnish material and the rate of evaporation of the solvents included in its composition and for different types varnishes and paints is 5-15 min.
Evaporation of solvents can be slowed down or eliminated by placing the coated product in an atmosphere containing relatively high concentrations of solvent vapors. As a result, the increase in viscosity and surface tension of the paint and varnish material slows down or stops and conditions are created for its spreading and removal of excess from the surface (Fig. 8.17). By changing viscosity source material, the concentration of solvent vapors, the duration of exposure of painted products in it, it is possible to widely regulate the thickness of the resulting coatings, while simultaneously improving their uniformity (Fig. 8.18).
Rice. 8.17. diagram of leveling a layer of paint and varnish material when exposed to solvent vapors: 1 - coating profile during conventional dipping; 2 - coating profile during dipping with exposure to solvent vapor
Rice. 8.18. Dependence of the thickness of alkyd coatings on the duration of exposure to solvent vapors at a viscosity of the paint and varnish material of 20 s according to VZ-4 and different vapor concentrations (a), solvent vapor concentration of 18 g/m 3 and different viscosities of the paint and varnish material
As can be seen from Fig. 8.18, the thickness of the coating decreases the more intensely, the higher the concentration of solvent vapors in the steam chamber. Naturally, materials with lower viscosity form thinner coatings. Established empirically optimal time exposure of coatings in solvent vapors, which maintains sufficient thickness and at the same time ensures satisfactory uniformity of coatings along the height of the products.
With a viscosity of paints and varnishes of 20-40 s according to VZ-4 and a solvent vapor concentration of 15-25 g/m3, this time is 8-14 minutes.
Application of paints and varnishes by dipping. Dipping coloring options are very diverse in terms of hardware and technological design.
Fig.8.19. Schemes of installations for dip painting:
a - with manual immersion of products; b - with immersion of products on a pulsating conveyor using a lowering mechanism; c - with immersion of products on a continuous conveyor;
1 - bath; 2 - pump; 3 - pocket; 4 - waste tray; 5 - product
In small-scale production conditions, stationary baths are used; products are immersed in them using lifts, hoists or manually (Fig. 8.19, a). To prevent evaporation of solvents in environment Such baths are usually equipped with on-board suction. In mass production, products are fed into the bath by a periodic or continuous conveyor (Fig. 8.19, b, c), while the bath (stationary or rising) is placed in a chamber equipped exhaust ventilation. The continuous bath has a drain tray for collecting paint and varnish material flowing from products and pumps for mixing (in the case of pigmented compositions). Mixing of paints is carried out by selecting them from the top of the bath or from a pocket and feeding them through a pipe with holes in bottom part; the rate of material circulation is 3-5 rpm. You can also mix the paint in the bath using stirrers or compressed air; the latter method is not common.
Dipping with exposure to solvent vapors is carried out in baths equipped with a steam tunnel. Depending on the dimensions of the products being painted, the volume of the baths ranges from several liters to several tens of cubic meters. Especially large baths are used for painting welded structures, power transmission masts, floors of car bodies and cabins, and panel products. Dipping baths with a volume of 0.5 m3 or more are equipped with an emergency drain - a pipe and an underground tank for the evacuation of flammable paint and varnish material in the event of emergency situation. The speed of continuous conveyors for dip painting usually does not exceed 2.5 m/min.
Using the dipping method, you can apply any storage-stable paint and varnish materials: bitumen, glypthal, pentaphthalic, urea and melamine-formaldehyde, epoxy, etc. When painting small products, cellulose nitrate varnishes and enamels are often used. Non-pigmented paints and varnishes are more convenient for dipping application.
Coloring flat products using the pouring method. Pouring is a type of pouring method in which paint and varnish material is applied to flat (or slightly curved) horizontally laid products in strictly dosed quantities. Dosing involves supplying an equal amount of material per unit surface, precisely one that prevents its runoff and at the same time achieves good leveling (spreading) on a horizontal surface. For this purpose, varnish or paint is applied to the surface in the form of a flat stream (veil), covering the entire width of the product. Such a curtain can be obtained by draining liquid through a horizontal threshold (dam), or a narrow slit in the wall or bottom of the vessel. If the curtain is carried evenly over the product at a certain speed, or the product is passed through the curtain (which is technically more convenient), then the surface will be covered with a uniform layer of paint and varnish material. This principle is used for the varnishing and painting of many types of products: panel furniture, particleboard and fibreboards, cardboard, plywood, door leaves, skis, timber materials, etc.
The distinctive features of the filling method - high productivity, low losses of paints and varnishes, the ability to apply coatings of different thicknesses (up to 300 microns) in one layer - make it one of the most promising painting methods.
For bulk application, paint filling machines of different designs are used. The principle of their operation is clear from Fig. 8.20.
Rice. 8.20. operating diagram of the lacquer filling machine: 1 - filling head; 2 - product to be coated; 3 - transport devices; 4 - receiving tray; 5 - settling tank; 6 - pump; 7 - filter
The paint and varnish material is supplied to the product from the filling head. The material that does not fall on the product (the length of the curtain is usually greater than the width of the product) flows through the receiving tray into the settling tank, from where, freed from the air bubbles entrained by it, it returns to the cycle. The process is carried out continuously. The products to be painted are moved automatically using transport devices. The most important part of paint filling machines is the filling head. It determines the profile of the flowing jet and the consumption of paint and varnish material. Filling heads with a bottom slot (the most common type), with a drain dam, with a drain dam and a screen have been used; optimal distance from the filling head to the product is 50-100 mm.
Regulation of the supply of paints to products in paint filling machines is carried out by changing the width of the slot, pressure or volume of material entering the filling head.
The thickness of the coatings can also be changed by changing the speed of movement of the devices transporting the product. When painting and varnishing furniture products, the LM-3 varnish-filling machine is widely used. It has two filling heads and allows you to paint both flat parts and edges of products up to 2.2 m wide; the speed of movement of products can be varied between 10-170 m/min.
Lacquering machines (LM-3, LM-140-1, LMK-1, etc.) are very productive and economical look painting equipment. With automated feeding and removal of products from the conveyor, productivity on the surface to be painted can reach tens of thousands square meters at one o'clock.
When applied using the pouring method, there are fundamentally no restrictions on the use of any liquid materials. Since the pouring method is used mainly for finishing wood products, the application of furniture varnishes and cellulose nitrate (I) and polyester (II) varnishes and enamels has been mastered primarily. Below are the main technological parameters of their application:
Working viscosity according to VZ-4, from 80 55-100
Product movement speed, m/min 60-90 50-80
Average consumption of materials, g/m2 120-200 400-500
Thickness of single-layer coatings, microns 25-40 200-300
The components of polyester varnishes are mixed immediately before application (in the case of machines with one filling head) or during the application process (when using machines with two filling heads). The pouring method can apply single-layer and multi-layer, homogeneous and heterogeneous coatings. When applied, only one side of the product is painted - the top. If it is necessary to paint the back side or ends (edges) of the products, they are turned over and the process is repeated. The most common coating defect is gas filling. It occurs as a result of air entering the paint stream or its microdispersion upon contact with a fast-moving surface. Elimination of this and other defects is achieved by changing the parameters of the paint and varnish material (viscosity, surface tension) and machine operating modes. During the filling process or during subsequent transportation of products to the dryer, evaporation of solvents or monomers occurs. Therefore, the designs of paint coating machines provide for local suction, and the rooms where painting is carried out are equipped with general ventilation.
Dyeing of long products using the broaching method. Long products having a constant cross section along the length (pencils, baseboards, cornices, wire, sections of pipes of small diameter), it is convenient to paint by pulling them through a bath of paint and varnish material (Fig. 8.21).
Rice. 8.21. Scheme for coloring pencils using the drawing method:
1 - feed rollers; 2 - pencil; 3 - bath with paint and varnish material; 4 - limit washers; 5 - drying conveyor
Excess material is removed by restrictive rings (rubber washers) blocking the entrance and exit of products from the bath. The role of the bath can be a porous material (foam rubber, felt, fabric bag) that tightly compresses the surface to be covered. Dispensing of varnish or paint onto a porous material is carried out through a tube or using the wick method. When pulling products through a porous material impregnated with varnish, the latter is deposited in a thin layer on the surface of the product. This method is used, in particular, for varnishing wires at electrical industry enterprises.
For varnishing and painting using the drawing method, both fast- and slow-drying paints and varnishes are used: cellulose nitrate, oil, alkyd, polyester, epoxy (single-pack), etc.
Thus, pencils are coated with cellulose nitrate varnishes and enamels with a relatively high viscosity and a solids content of 50-60%. The pencil pushed out from the bathtub enters the receiving (drying) conveyor.
To coat the wire, varnishes with low-volatile solvents (kerosene, white spirit, cresols, etc.) are mainly used; coatings are cured in convective or induction type at high temperatures. The thickness of single-layer coatings when drawn is small, 2-5 microns, so it is necessary to apply several layers - from 2 to 12.
The drawing method is productive, quite economical, allows you to mechanize and automate the painting process, but has great limitations on the shape of the products being coated.
Application of paints and varnishes in drums. The simplest and most cost-effective method for small mass-produced items (shoe caps, hooks, loops, buckles, bolts, nuts, tool handles, typewriter parts, etc.) is the drum dyeing method. Mechanically driven drums are used. They ensure drainage of paint and varnish material and often drying of products during rotation. In the latter case, feeding into the drum is provided warm air and removal of solvent vapors from it. Products are usually loaded into the drum to 1/2-2/3 volume. The paint and varnish material is poured in such a way as to completely wet the products. Drum rotation time is 5-7 minutes, rotation speed is 75-120 rpm. If the coatings are dried outside the drum, then the products are unloaded onto meshes and, after the excess paint has drained off, they are sent to the dryer.
There are drum designs in which products are painted not by immersion in the paint and varnish material, but by spraying it. At the same time, in the case of thermosetting varnishes and paints, it is possible to apply them in multiple layers with each layer drying directly in the drum as it rotates. Centrifuges can be used instead of drums. The products are loaded into a perforated basket of a centrifuge, lowered into a container with paint and, after being removed from it, the centrifuge is rotated to remove excess paint and dry the products.
For application in drums and centrifuges, predominantly quick-drying paints and varnishes are used - bitumen and cellulose nitrate varnishes and enamels, selected experimentally in each specific case. The coatings have a low finishing class (not higher than III), there are defects in places where the products come into contact, and drips are possible.
Dipping and pouring are the simplest and longest-used methods of coloring. Their advantage lies in the ability to apply various paints and varnishes and obtain coatings of fairly good quality using simple equipment. By dipping (immersing) the product in paint and varnish material or pouring it over the product, it is possible to paint almost all areas of the surface, including those hidden from the human eye; this cannot be achieved through many other means.
Dipping and pouring are used mainly to obtain primer and single-layer coatings on small and medium-sized products of varying complexity. Both methods are used in many industries (automotive, instrument-making, agricultural engineering, etc.), as they allow mechanization and automation of painting processes.
The disadvantages of dipping and pouring methods are: uneven thickness of coatings and height of products, the inability to paint products that have pockets and internal cavities, relatively large losses of paints and varnishes, often reaching 20% or more. Many of these disadvantages, however, are eliminated if the object of painting is flat products (wooden panels, metal sheets, rolled metal) laid horizontally. The paint and varnish material is applied using paint-filling (or paint-coating) machines. It is when painting such products, especially panel furniture, that the pouring method has found the greatest application.
Reducing losses of paints and varnishes and variations in coating thickness while simultaneously improving their decorative appearance is achieved by aging freshly painted products in solvent vapors. This method, as a variation of the pouring method, called Jet dousing has become widespread in industry. Dip coatings can be improved in a similar way. Other varieties of the dipping method are dyeing long items by pulling and dyeing small items in rotating drums.
Dipping and pouring in any version attracts Special attention when applying water-based paints and varnishes due to the possibility of organizing in-line fireproof technological processes.
The principle of application by dipping and pouring is based on wetting the surface to be painted with a liquid paint and varnish material and holding it on it in a thin layer due to the adhesion and viscosity of the material. The quality and thickness of coatings when painting by dipping and pouring are determined by the properties of the surface, as well as the chemical and structural-mechanical characteristics of the applied material.
Let's consider the process of applying liquid paint by dipping a product, for example a flat plate, into it (Fig. 7.21). The initial act is immersion of the product in liquid material, i.e. establishing adhesive contact. Depending on the viscosity of the material and the nature of the surface, the duration of this process can be seconds or minutes. Simultaneously with the establishment of contact, adsorption interaction of the liquid with the solid surface occurs.
When removing a product, for example, at a rate of 1%, not only a layer of adsorbed liquid will be entrained; due to adhesion and internal friction P, the movement will be transmitted to parallel layers of liquid, which will also rise, but with a speed gu„. In addition to the power of Reti, the layers will experience the effect Powers
Rice.7.22. Velocity distribution in the liquid layer when removing a product from it
Gravity P, causing the liquid material to descend (flow) at a speed Wp. The total speed of movement of each elementary layer located at a distance x from the surface of the product will thus be equal to:
Wx = Wu -Wp .
Under the condition of laminar movement and the exclusion of gravity, the speed of individual layers changes uniformly with distance from the product and at a distance A becomes equal to zero. At the same time, dependence Wn -/(X) is rectilinear (Fig. 7.22), and the velocity gradient Dwn/ Dx- const. In real conditions, when gravity is applied RU the nature of the dependence changes, the volume of liquid entrained by the product is always less (in Fig. 7.22 it is shown as a shaded area). If we take the layer width as one and the thickness as Dxy then d V will be:
D.V. = W3Ax,
And the volume of all liquid entrained by such a product per unit time will be equal to:
A
V=jwxdx.
After removing the product from the liquid, part of it drains and (if it is a non-volatile liquid, then regardless of the extraction rate on the surface) a layer remains, the thickness of which is determined by the viscosity,
Rice.7.23. Dependence of the thickness of 5 alkyd enamel coatings on the speed of removing the product from the bath at different paint viscosities (according to VZ-246)at20 °C
Density and energy factors of interaction of a liquid with the surface of a solid.
When dipping into paints and varnishes, the process is complicated by the continuous change in the viscosity of the layer applied to the product, as a result of which its flow slows down and then stops completely. It is easy to verify that the thickness and degree of unevenness of the film will be greater, the higher the rate of removal of the product (Fig. 7.23), the viscosity of the paint and varnish material and the rate of its increase at the moment of dripping. Low-viscosity materials (20 s according to VZ-246 and less) form relatively thin coatings with a small variation in thickness over the height of the product. The same effect is achieved at low speeds of extraction of products from paint and varnish material - 0.1 m/min or less (Fig. 7.24).
However, in practice, this leads to a decrease in the efficiency of painting: with a decrease in the viscosity of materials, the consumption of solvents increases and in some cases it becomes necessary to apply several layers of coating; Reducing the speed of product extraction reduces the productivity of installations.
When applying paints and varnishes using the pouring method, the patterns characteristic of dipping are preserved. The layer of liquid supplied per unit surface during pouring, in contrast to spraying, exceeds the maximum thickness at which the liquid can be retained on vertical surfaces due to the forces of adhesion and internal friction. Therefore, its excess necessarily flows off, leaving a layer of uneven thickness on the substrate, and deposits in the form of drops on its descending edge. The duration of drainage is mainly determined by the viscosity of the paint and varnish material and the rate of evaporation of its constituent solvents and for different types of varnishes and paints it is 5-15 minutes.
Ras.7.24. Changing the thickness 5 of the cellulose nitrate varnish coating along the lengthI product at different speeds of its extraction from the bath
Rice.7.25. Scheme for leveling a layer of paint and varnish material when exposed to solvent vapors:
1 - coating profile during normal dipping; 2 - coating profile during dipping with exposure to solvent vapor
Evaporation of solvents can be slowed down or eliminated by placing the coated product in an atmosphere containing relatively high concentrations of solvent vapors. As a result, the increase in viscosity and surface tension of the paint and varnish material slows down or stops, and conditions are created for its spreading and removal of excess from the surface (Fig. 7.25). By changing the viscosity of the starting material, the concentration of solvent vapors and the duration of exposure of painted products in them, it is possible to widely regulate the thickness of the resulting coatings, while simultaneously improving their uniformity (Fig. 7.26).
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T, min |
As can be seen from Fig. 7.26, the thickness of the coating decreases the more intensely, the higher the concentration of solvent vapors in the steam chamber; Naturally, paints and varnishes with lower viscosity form thinner coatings. The optimal exposure time for coatings in solvent vapors has been established experimentally, at which sufficient thickness is maintained and at the same time satisfactory uniformity of coatings along the height of the products is ensured. With a viscosity of paints and varnishes of 20-40 s according to VZ-246 and a solvent vapor concentration of 15-25 g/m3, this time is 8-14 minutes.
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Rice.7.26. Dependence of the thickness of alkyd coatings on the duration of exposure to solvent vapors at a viscosity of the paint and varnish material of 20 s to VZ-246and different vapor concentrations(A); solvent vapor concentration 18 g/m3 and different viscosities of paint and varnish material(b)
Dipping coloring options are very diverse in terms of hardware and technological design (Fig. 7.27). In small-scale production conditions, stationary baths are used; products are loaded into them using lifts, hoists or manually (Fig. 7.27, A). To prevent the spread of evaporating solvents into the environment, such baths are usually equipped with on-board suction. In mass production, products are fed into the bath by a periodic or continuous conveyor (Fig. 7.27, 6 , #), the bathtub (stationary or rising) is placed in a chamber equipped with exhaust ventilation. The continuous bath has a drain tray for collecting paint and varnish material flowing from products and a pump for mixing (in the case of pigmented compositions). Mixing of paints is carried out by selecting them from the upper part of the bath or from a pocket and feeding them through a pipe with holes into the lower part; the rate of material circulation is 3-5 rpm. You can also mix the paint in the bath using stirrers or compressed air; the latter method, however, is not common.
Dipping with exposure to solvent vapors is carried out in baths equipped with a steam tunnel.
Depending on the dimensions of the products being painted, the volume of dipping baths ranges from several liters to several tens of cubic meters. Especially large baths are used for painting welded structures of power transmission masts, floors of car bodies and cabins, and panel products. Dipping baths with a volume of 0.5 m3 or more are equipped with an emergency drain - a pipe and an underground tank for evacuating flammable paint and varnish material in the event of an emergency. Speed
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The movement of continuous conveyors during dip painting usually does not exceed 2.5 m/min.
Using the dipping method, you can apply any storage-stable paint and varnish materials: bitumen, glyphthalic, pentaphthalic, urea and melamine-formaldehyde, epoxy (hot-curing), water-borne, etc. When painting small items, cellulose nitrate varnishes and enamels are often used. Non-pigmented paints and varnishes are more convenient for dipping application. Of the pigmented ones, only compositions with high sedimentation resistance can be used. The working viscosity of paints and varnishes is 16-35 s according to VZ-246. To dilute them, predominantly high-boiling solvents are used: white spirit, solvent, xylene, turpentine, ethyl cellosolve, butyl acetate. This reduces their losses due to evaporation from the surface of the bath and facilitates the drainage of excess material from the parts. Special additives (ketoximes, aldoximes, substituted phenols) are added to oil-containing varnishes and enamels to prevent the formation of a film on the surface of the bath as a result of contact with air.
Water-based paints and varnishes - solutions and dispersions - are also suitable for dipping application. In particular, such materials have found application in painting auto parts, household appliances and other mass-produced products. To reduce surface tension and the tendency to form prices, water-miscible solvents (alcohols, ethylcellosolve, butylcellosolve), thixotropic additives (aluminum alcoholates, bentonite, etc.), polyorganosiloxanes are introduced into the composition of such materials, and measures are also used to mechanically extinguish foam ( installation of a defoamer, etc.).
Immersion of products into a bath of paint and their removal is carried out smoothly, without jerking and at a moderate speed. To level the paint layer on streamlined products, they are often rotated after removal from the bath. Large deposits and remaining drops from the lower edges of the products are removed electrostatically. For this purpose, an electrode grid is installed above the drain tray and brought to it high voltage, and the product is grounded. The thickness of single-layer coatings obtained by dipping is 15-30 microns. In appearance they correspond to classes III and IV. However, with well-established application and curing technology, coatings of a higher class can be obtained. Multi-color and multi-layer coatings are not obtained using this method.
Unlike dipping, pouring allows you to get by with a relatively small amount of paint and varnish material. Thus, when painting medium-sized products, the volume of paint and varnish material used is reduced by 5-10 times compared to dipping. Due to this, the pouring method has an advantage over dipping in terms of fire protection. The most widely used method of pouring is in the jet application version. Exposing products to solvent vapors improves appearance coatings and has a positive effect on saving paint and varnish materials. Paint losses when using jet spraying are reduced compared to dipping by 10-15%, and with pneumatic spraying - by 25-30%. It is convenient to use jet painting in large-scale and mass production, since the coloring of products using this method is carried out automatically.
To apply paints and varnishes, jet spray installations are used (Fig. 7.28), in which the products sequentially pass through a spray zone and a steam tunnel. The abundance of dousing when painting products, depending on the category of complexity, ranges from 10 to 20 l/m2 of surface.
To bring the paint and varnish material to a given viscosity and flush the steam tunnel, a solvent supply is provided from the appropriate tank. Installations have been developed that allow painting products with maximum size up to 1600 mm. With a conveyor speed of 0.6-1.0 m/min, they provide a productivity of 200-250 m2/h on the painted surface. For spray application, basically the same paints and varnishes are used,
As for application by dipping, organo- and water-borne primers and enamels. For dilution, individual solvents or their binary mixtures are used; in the case of water-soluble materials, water-alcohol mixtures are used. Optimal temperature paint and varnish materials 17-22 °C. The required concentration of solvent vapors in the steam tunnel is created as a result of the evaporation of solvents in the pouring chamber and from the surface of the painted products. The maximum concentration of vapors should not exceed 50% of their lower explosive limit. To control the concentration of vapors, automatic flammable gas alarms of the types SGG-2, SVK-3, etc. are used. Technological parameters for applying glyph primers - tallow, oil-phenol-formaldehyde (I) and pentaphthalic, urea-formaldehyde, melamine-alkyd enamels (II) are indicated below:
SHAPE \* MERGEFORMAT
Viscosity according to VZ-246, s Duration of pouring, min Solvent vapor concentration, g/m Duration of exposure to solvent vapor, min Coating thickness, µm
Using the jet coating method, many products are primed and painted for which a finish of no higher than class III is allowed: components and parts of combines, tractors, attachments and sanitary equipment, heating radiators, pipes, beams, welded structures, electrical equipment etc. The disadvantages of the method are the bulkiness of the installations and the increased consumption of solvents, reaching in some cases 150% of the amount of paints and varnishes used.
Pouring is a type of pouring method in which paint and varnish material is applied to flat (or slightly curved) horizontally laid products in strictly dosed quantities. Dosing involves supplying an equal amount of material per unit surface, precisely one that prevents its runoff and at the same time achieves good leveling (spreading) on a horizontal surface. For this purpose, varnish or paint is applied to the surface in the form of a flat stream (veil), covering the entire width of the product. Such a curtain can be obtained by draining liquid through a horizontal threshold (dam) or
a narrow slit in the wall or bottom of a vessel. If the curtain is carried evenly over the product at a certain speed, or the product is passed through the curtain (which is technically more convenient), then the surface will be covered with a uniform layer of paint and varnish material.
This principle is the basis for varnishing and painting many types of products: panel furniture, particle boards and fiberboards, cardboard, plywood, door panels, skis, lumber materials, etc.
The distinctive features of the filling method - high productivity, low losses of paints and varnishes, the ability to apply coatings of different thicknesses (up to 300 microns) in one layer - make it one of the most promising painting methods.
For bulk application, different designs are used. Paint filling machines. The principle of their operation is clear from Fig. 7.29. The paint and varnish material is supplied to the product from the filling head. The material that does not fall on the product (the length of the curtain is usually greater than the width of the product) flows through the receiving tray into the settling tank, from where, freed from the air bubbles entrained by it, it returns to the cycle.
The process is carried out continuously; The painted products are moved automatically using transport devices. The most important part of paint filling machines is the filling head. It determines the profile of the flowing jet and the consumption of paint and varnish material. Filling heads with a bottom slot (the most common type), with a drain dam, with a drain dam and a screen have been used; the optimal distance from the filling head to the product is 50-100 mm.
Regulation of the supply of paints to products in paint filling machines is carried out by changing the width of the slot, pressure or volume of material entering the filling head. The thickness of the coatings can also be changed by changing the speed of movement transporting the product
and varnishing of furniture products, the JIM-3 varnish-filling machine is widely used. It has two filling heads and allows you to paint both flat parts and edges of products up to 2.2 m wide. The speed of movement of products can be varied between 10-170 m/min.
Lacquering machines are a very productive and economical type of painting equipment. With automated feeding and removal of products from the conveyor, productivity on the surface to be painted can reach tens of thousands of square meters per hour.
When applied by pouring, there are fundamentally no restrictions on the use of any liquid materials. Since the pouring method is used mainly for finishing wood products, the application of primarily furniture varnishes and enamels - nitrate-cellulose (I) and polyester (II) - has been mastered. Below are the main technological parameters of their application:
Working viscosity according to VZ-246, from 80-100 55-100
Product movement speed, m/min 60-90 50-80
Average consumption of materials, g/m2 120-200 400-500
Thickness of single-layer coatings, microns 25-40 200-300
The components of polyester varnishes are mixed immediately before application (in the case of machines with one filling head) or during the application process (when using machines with two filling heads). The pouring method can apply single-layer and multi-layer, homogeneous or heterogeneous coatings. When applied, only one side of the product is painted - the top. If it is necessary to paint the back side or ends (edges) of the products, they are turned over and the process is repeated. The most common coating defect is gas filling. It occurs as a result of air entering the paint stream or its microdispersion upon contact with a fast-moving surface. Elimination of this and other defects is achieved by changing the parameters of the paint and varnish material (viscosity, surface tension) and machine operating modes. During the filling process and during the subsequent transportation of products to the dryer, evaporation of solvents or monomers occurs. Therefore, the designs of paint-filling machines provide for local suction, and the rooms where painting is carried out are equipped with general ventilation.
Long products that have a constant cross-section along their length (pencils, baseboards, cornices, wire, sections of pipes of small diameter) are conveniently painted by pulling them through a bath of paint and varnish material (Fig. 7.30).
Excess material is removed by restrictive rings (rubber washers) that block the inlet and outlet of the products from the bath. The role of a bathtub can be performed by a porous material (foam rubber, felt, fabric bag) that tightly compresses the surface to be covered. Dispensing of varnish or paint onto a porous material is carried out through a tube or using the wick method. When a product is pulled through a porous material impregnated with varnish, the latter is deposited in a thin layer on the surface of the product. This method is used, in particular, for varnishing wires at electrical industry enterprises.
For varnishing and painting using the drawing method, both fast- and slow-drying paints and varnishes are used: cellulose nitrate, alkyd, polyester, epoxy (one-pack), etc. Thus, pencils are coated with cellulose nitrate varnishes and enamels with a relatively high viscosity and a solids content of 50- 60%. The pencil pushed out of the bath enters the receiving (drying) conveyor.
To coat the wire, varnishes with low-volatile solvents (kerosene, white spirit, cresols, etc.) are mainly used; coatings are cured in convection or induction dryers at high temperatures. The thickness of single-layer coatings when drawn is small - 2-5 microns, so several layers are applied - from 2 to 12.
The drawing method is productive, quite economical, allows you to mechanize and automate the painting process, but has great limitations on the shape of the products being coated.
The simplest and most cost-effective method for small mass-produced items (shoe caps, hooks, loops, buckles, bolts, nuts, tool handles, etc.) is the drum dyeing method. Mechanically driven drums are used to drain the paint and varnish material and often dry the products during rotation. In the latter case, warm air is supplied to the drum and solvent vapors are removed from it. Products are usually loaded into the drum to 1/3-2/3 volume. The paint and varnish material is poured in such a way as to completely wet the products. Drum rotation time is 5-7 minutes, rotation speed is 75-120 rpm. If the coatings are dried outside the drum, then the products are unloaded onto meshes and, after the excess paint has drained off, they are sent to the dryer.
There are drum designs in which products are painted not by immersion in the paint and varnish material, but by spraying it. Moreover, in the case of thermosetting varnishes and paints, it is possible to apply them in multiple layers with each layer drying directly in the drum as it rotates. Centrifuges can be used instead of drums. The products are loaded into a perforated basket of a centrifuge, lowered into a container with paint, and after being removed from it, the centrifuge is rotated to remove excess paint and dry the products.
For application in drums and centrifuges, predominantly quick-drying paint and varnish materials are used - bitumen and cellulose nitrate varnishes and enamels, as well as water paints. Their viscosity is selected experimentally in each specific case. The coatings have a low finishing class (not higher than III), there are defects in places where the products come into contact, and drips are possible.
Application of paints and varnishes by dipping- a simple and productive method that can be successfully applied in both mechanized and non-mechanized production.
Essence of the method consists in the fact that the products to be finished are immersed in a bath filled with paint and varnish material, then removed from the bath and kept for a certain time over a bath or tray to drain excess paint and varnish material from the surface. The quality and thickness of the coating are determined by the properties of the surface, as well as the chemical and structural mechanical characteristics of the applied material.
The condition for using this method is a simple, well-streamlined shape of the product, without internal nests and cavities in which paint and varnish materials could be retained. This method can be used to finish profile molded products, legs of chairs, tables, cabinet products, handles of knives, tools, spinning rods, bent-glued elements of sofas, armchairs, parts of agricultural machinery, wagons, cars, etc.
The scheme for applying liquid materials by dipping using the example of a flat plate is shown schematically in Fig. 4.13. When immersing, the speed of dipping parts into the bath should not be high, since during rapid immersion the part carries with it air, which forms bubbles on the coating of the part when it is removed from the bath.
When removing a product from a liquid at a constant speed v Not only the layer of adsorbed liquid is entrained; due to adhesion and internal friction F the movement will be transmitted to parallel layers of varnish
When dipping into drying liquids, such as paints and varnishes, the process is complicated by a continuous change in the viscosity of the applied layer, as a result of which its flow slows down and then stops. It is obvious that fast-drying paints and varnishes, under other conditions, form more uneven and thicker coatings than slow-drying ones.
Application of paints and varnishes by dipping can be carried out in various options. In conditions where the volume of painting work is small and the products being painted are light in weight and dimensions, baths are used in which products are immersed and removed manually.
Viscosity of fresh varnish should be 30 ... 40 s according to VZ-246, the viscosity of the varnish in the working bath during operation is 40 ... 70 s. The temperature of the varnish in the bath must be maintained by cooling at 16...20°C.
Finishing wood products using the dipping method has the following advantages: no sophisticated equipment is required, as well as highly qualified personnel to service the installations; possibility of full mechanization; simultaneous finishing of external and internal surfaces of a large number of different products; virtually no loss of paint and varnish materials; the ability to create a coating with high quality indicators in one technological operation without refining; combining priming and varnishing operations on the same equipment.
To the disadvantages The method may include the possibility of finishing products only with a streamlined shape without internal cavities and ledges; uneven coating thickness; the need to create special conditions to increase the viability of reaction systems; the need for large volumes of working solutions of paints and varnishes; loss of solvents from the open bath mirror.
For a product of simple shape without internal corners Paintwork materials can be applied using jet methods. Slow-drying paints and varnishes are used (alkyd, melanin). Primers, enamels, paints are used; varnishes are not used.
This method used for finishing building products (windows, doors).
Jet spray installation diagram
I – entrance vestibule, II – pouring chamber, III – steam tunnel.
1 – overhead conveyor, 2 – product, 3 – air curtain, 4- container with paint and varnish,
5 – pump, 6 – pipeline for supplying paintwork materials, 7 – stack for paintwork materials.
In the pore tunnels, drying does not occur, but on the contrary, liquefaction of the paintwork material occurs, for the excess paintwork material to drain away.
It is used for finishing products with a streamlined shape: the legs of chairs, cabinets, sideboards. Essence: the products are immersed in a container filled with paints and varnishes, and then, after a short soaking in it, they are removed and kept until the excess paints and varnishes finally drain off. The excess is collected in special containers after cleaning and diluting with a solvent to working viscosity and is reused. The quality of finishing depends on: the speed of immersion and removal of the part, the viscosity and temperature of the paintwork material, the dry residue of the paintwork material, and the shape of the part.
The method under consideration has advantages: losses of paintwork materials are reduced (since excesses after draining are used)
The number of applied layers is reduced (the use of coatings with a large dry residue), the finishing process lends itself to automation and mechanization.
The disadvantage is the difficulty of obtaining coatings of uniform thickness along the height of medium and large parts, especially those longer than 300 mm.