The dip painting method is a highly economical and environmentally friendly painting method that is one of the most modern surface treatment technologies. The main area in which this method is used is the automotive industry (painting car bodies and their parts).
This method is also widely used for painting 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-based 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. Their advantage lies in the ability to apply various paints and varnishes and obtain sufficient coverage good quality when 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 the 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 shortcomings, however, are eliminated if 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 them decorative look is achieved by keeping freshly painted products in solvent vapors. This method, as a variation of the pouring method, called Jet dousing received widespread in industry. Likewise Dip coatings can be improved. 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 products will thus be equal to:
Wx = Wu -Wp .
Under the condition of laminar movement and exclusion of gravity, the speed individual layers changes evenly 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 the solvents included in its composition and for different types varnishes and paints 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).
|
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.
|
|
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 environment Such baths are usually equipped with an 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 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
|
6 |
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, ethyl cellosolve, butyl cellosolve), thixotropic additives (aluminum alcoholates, bentonite, etc.), polyorganosiloxanes are introduced into the composition of such materials, and measures are also taken 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 high voltage is supplied to it, 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. Most Applications The pouring method was obtained in the jet application version. Exposing products to solvent vapors helps improve the appearance of 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, mainly 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 the same amount of material per unit surface, precisely the amount that prevents its runoff and at the same time achieves good leveling (spreading) onto the surface. horizontal surface. For this purpose, varnish or paint is applied to the surface in the form of a flat jet (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 used for the varnishing and painting of many types of products: panel furniture, particle boards and fiberboards, 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, 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; 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.
Paint filling 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 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 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 necessary to paint reverse side or the 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 subsequent transportation of products to the dryer, solvents or monomers evaporate. 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) can be 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 a 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 convective 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, feeding into the drum is provided warm air and removal of solvent vapors 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 paints and varnishes are used - bitumen and cellulose nitrate varnishes and enamels, as well as water-based 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.
For mass production, dip painting is a simple and cost-effective process that does not require highly skilled labor or complex equipment. Products to be painted by dipping must have a streamlined shape, due to which the paint is evenly distributed over the entire surface in an even layer.
Products that have various recesses in a complex configuration should not be painted by dipping, since the paint may linger in the recesses and be unevenly distributed on the surface.
Products of simple configuration, light weight and size are manually dipped into small baths (Fig. 33).
When mass painting small items using the dipping method, special mesh baskets or boxes are used (Fig. 34).
Pneumatic lifts are used to load baskets into the bath and unload them, as well as for heavier products.
For products arriving for painting in a continuous flow along a conveyor, the baths are placed in a chamber equipped with ventilation and combined with a drying system. Products are hung on conveyor hangers, which, along the conveyor, are immersed to a certain depth in a paint bath (Fig. 35).
After dipping, the painted products gradually leave the paint bath along the conveyor and enter the drying chamber.
Thus, the conveyor dip painting method is completely mechanized.
Maintenance of the paint bath is reduced only to hanging the products on the conveyor and removing them from the conveyor after drying.
The size and capacity of the bath must correspond to the size of the products being painted, ensuring their complete immersion in the paint.
It is not advisable to increase the size of the bath beyond the required ones, since the solvent quickly evaporates from a large mirror of paint, the viscosity changes, the paint thickens and the concentration of vapors in the room increases, which can cause a fire.
Large-capacity baths are equipped with driven mixers to eliminate pigment sedimentation. The stirring blades of the mixer are located under the safety grid.
Mixing of paint in the bath is carried out by circulation, i.e. by pumping paint out of the collection bath and pumping it into the upper receiving tank, as shown in Fig. 36.
From the receiving tank, the paint flows by gravity through filters into the working paint bath, in which a constant working viscosity is maintained.
If the viscosity of the paint changes, an appropriate amount of a particular solvent is added to the bath. The working viscosity of the paint in the bath is checked at least twice per shift.
It is unacceptable to mix the paint with air using the bubbling principle, since in this case the film former is oxidized, the solvent quickly evaporates with the air, and the paint thickens and gelatinizes.
Dip painting
In dip painting, the pieces are immersed in a bath of paint for a specified period of time; After rising from the bath and draining of excess paint, a film forms on the surface of the product.
To obtain a coating of the desired quality when painting by immersion, you must correctly select the viscosity of the paint and varnish material. Working viscosity is determined experimentally by adding solvents and diluents to the starting material. Since the paint compositions in the bath gradually thicken due to the evaporation of the solvent, it is necessary to periodically (preferably 1-2 times per shift) check the viscosity of the paint composition and adjust it. Due to the increased evaporation of solvents, quick-drying nitro and perchlorovinyl paints are not used for immersion painting. Dip painting is suitable for simple shaped items that allow excess paint to completely drain off. If parts have internal cavities or pockets, then special technological holes (drains) are provided in them to allow paint to drain.
When immersed, the product must be completely covered with paint without air bubbles; when removed from the bath, excess paint must drain without forming smudges. The optimal position of the product when immersed in a bath should be selected experimentally in each case.
To hang painted products on the conveyor, you should use the simplest devices - hooks, “herringbones” different designs; It is not recommended to use baskets, shelves and devices with a large surface area, as they carry away a significant amount of paint.
Electrodeposition painting
Essence this method is the process of deposition of paint on the surface of a metal product when the latter is immersed in a bath with the simultaneous application of an electric current.
Any paint can be applied using the electrodeposition method, but the most suitable are water-based and water-based paints based on various water-soluble resins. Under the influence of electric current, particles of resin (film former) and particles of pigment included in the water paints, receive a negative charge, move to a positively charged product - the anode and are deposited on its surface.
This process is called electrophoresis; At the same time, the processes of electrolysis and electroosmosis occur.
Electrophoresis determines the rate of sediment formation into the thickness of the coating film. As a result of electroosmosis, water is removed (displaced) from the sediment; The paint particles are compacted and adhere to the surface of the product, forming uniform, dense layers of coating. Electrolysis of salts in water interferes with the deposition process, therefore, in the preparation of solutions for electrodeposition, demineralized water - condensate - is used.
Installation for electrodeposition coating. The process is carried out in a bath, most often made from stainless steel. The cathode is either the bath body or carbon or steel rods introduced into the bath. To improve the quality of the coating, the bath is sometimes equipped with a device for mixing paint.
At the beginning of the electrodeposition process, surface areas where the highest density of field lines is observed (for example, edges) are painted.
As separate areas are covered with a layer of paint, the insulating effect of the applied layer increases, and other areas of the surface of the product gradually begin to be painted; As a result, a dense, non-porous film is formed, having the same thickness on all surface areas.
Painting methods
TO category:
Cellulose varnishes
Painting methods
By the oldest method painting is painting with a brush.
Painting with a brush
When trying to apply this painting method to apply nitrocellulose and other cellulose varnishes, significant difficulties were encountered, which is why cellulose varnishes were not initially successful. At that time, slow-evaporating solvents and additives were not yet known that would slow down the varnish from drying too quickly. In practice, cellulose varnishes are currently applied primarily not by brush, but primarily by spraying.
The history of the development of cellulose varnishes, and especially nitrocellulose-based varnishes, shows that it was these varnishes that contributed to the development of the spraying method as a new method of painting. Therefore, both concepts - cellulose varnish and spraying - are historically and practically related. This explains why the name "spray varnish" primarily means cellulose varnish.
At modern conditions choosing solvents, plasticizers and resins, making brush varnish is no longer difficult. When making such varnishes, you need to pay attention to two significant points, namely:
1) the use of a large amount of slowly evaporating solvent and
2) the use of a chemically drying film former, such as an oil-modified alkyd resin.
Meeting both of these conditions is almost always not possible. Since a slowly evaporating solvent is always more expensive than one evaporating at an average or high speed, slowing down drying by introducing a slowly evaporating solvent is often economically unprofitable.
In practice, solvents are characterized not by the rate of evaporation (slow and fast), but by the boiling point (high, medium and low). It was already indicated on page 45 that the evaporation rate and the boiling point of the solvent are completely unrelated. But the rate of solvent evaporation is the quantity that determines the drying rate of the varnish and the associated painting method. Therefore, it is practically more expedient to distinguish solvents not by their boiling limits, but by the rate of evaporation.
The second way to obtain a varnish that is easy to apply with a brush is to add resins that dry completely or partially as a result of chemical processes. The most commonly used resins for nitrocellulose varnishes are oil-modified alkyd resins, urea resins and other similar types of resins. Since in this case film formation occurs as a result of chemical processes and does not yet end at the end of the evaporation of the solvent, such a film can be shaded with a brush for a relatively long time. True, even in this case it is almost impossible to avoid increasing the cost of varnish, since these resins are of high quality and their cost is relatively high.
In addition to modified alkyd resins, there are a number of resins that extend the drying time, but not due to chemical processes of film formation, but due to longer retention of some solvents, especially those that evaporate slowly. Such resins include, for example, some polymerization resins, such as polyvinyl ethers, polyacrylic acid esters, polyvinyl acetate, etc. These resins give the film of cellulose varnish a viscous consistency after application, which can be noticed by the ability of the film to stretch with threads. It is difficult or even impossible to apply such varnishes by spraying, but for brush varnishes this consistency should be considered normal.
Cellulose brush varnish must have a certain viscosity - approximately between 130-140 seconds DIN funnel at 20°, and the drying time of such varnish must be adjusted so that the film does not dry out of dust too quickly.
In the most important way painting with cellulose varnishes is
Spray painting
This painting method, typical of cellulose varnishes, was originally developed in America; over the past decades it has been modified, but even now it has not yet received its final development. This is evidenced by the new devices and methods that have appeared recently.
The simplest spraying installation consists of an apparatus for producing compressed air, a spraying device and a ventilation unit.
The compressed air must force the material entering the spray gun through the nozzle under a certain, uniform and controlled pressure. The installation for producing compressed air consists of a compressor or (in the simplest case and for small and rarely performed painting jobs) of a steel cylinder with a reducer that reduces the pressure of the air leaving the cylinder. In a motor-powered compressor, air is sucked in, compressed and then supplied under constant, adjustable pressure to the spray gun. The compressor can be mobile or stationary, installed at a specific place in the painting room. The motor of this installation is driven by electric current, i.e. it is connected directly to the network (especially in stationary installations) or driven by gasoline or oil.
Recently, a motorless compressor has been developed in which air is supplied to air chamber sprayer not in a roundabout way as a result of the operation of the motor and air compression, but directly in an electrodynamic way. The advantages of such a compressor are obvious, since its use eliminates the loss of rotational energy of a high-speed motor and the conversion of electricity into the movement of compressor pistons. When a selenium rectifier is connected to the circuit, only positive impulses are received from the alternating current network, as a result of which a force field appears in the coil 50 times per second, which sets the piston, which is the armature, in motion. Due to the periodicity of alternating current, the piston makes 50 movements per second, resulting in a uniform air flow. It is characteristic of this new compressor that when it is fully loaded and even overloaded, its current consumption is less than when it is underloaded. This depends on the fact that its coil, at full stroke of the piston, works like a choke coil. Thus, the current consumption in this case is reduced. Such compressors are manufactured for alternating current of various voltages with 50 periods (Urach pump factory, Urach-Württemberg).
The performance of a compressed air installation depends on how many spray guns are connected to it. Oil or gasoline powered compressor units are more mobile than those powered by electricity, but electrically powered compressor units provide clean and virtually continuous operation. Performance compressor unit characterized by the type of drive, number of cylinders, motor power, size of the compressed air tank, weight, dimensions and design of the spray gun. The operation and performance of the compressed air installation is also influenced by the viscosity of the varnish.
Spraying devices are available in a variety of designs.
When spraying, a distinction is made between working with high pressure (2-4 atm), medium pressure (1-2 atm) and low pressure (below 1 atm). The pressure is set by a pressure reducing valve connected between the compressed air unit and the spray gun.
The nozzle through which the paint and varnish material is sprayed can be of various sizes and shapes; the nozzle for a round jet has a diameter of 0.5-3 mm; The flat jet nozzle, from which the varnish exits through an oval hole, has a diameter of 1-3.5 mm.
Spray guns that go on sale are equipped with nozzles for a round or flat jet. Many types of spray guns are designed to replace one nozzle with another, and to accommodate nozzles with holes of different diameters.
The spray gun is equipped with a glass from which the paint and varnish material is sucked into the nozzle by air pressure and squeezed out of it. Ordinary spray guns are equipped with a vertically mounted glass with a capacity of 300 to 500 ml for feeding varnish into the spray gun by gravity. The glass has to be periodically filled with varnish. Such breaks in work to fill the glass are naturally inconvenient and therefore spraying devices are currently being designed that spray large quantities of material without interruption. Such devices include paint containers operating under pressure (R. S. Walther, Wuppertal-Wohwinkel, Josef Mehrer, Balingen-Württemberg, etc.). As needed, they are manufactured with a capacity of 20 to 120 kg of sprayed material and are equipped with a device that supplies the material to the spray gun under constant pressure. Thus, these devices are spare containers for varnish, from which varnish can be directly applied with an attached spray gun; For the convenience of changing the applied material, they are equipped with replaceable insert vessels. Pressure paint containers are available as portable (capacity up to 7.5 kg), transportable or stable. To prevent heterogeneity of the varnish due to pigment deposition, these containers are sometimes equipped with stirrers that are rotated manually or by electric drive(Josef Mehrer).
A combination of a container and a spraying unit is also a device developed in the USA, known as “Nu-Spray”.
A spray bottle has also been designed in the USA, which makes it possible to simultaneously apply two solutions. This spray design is especially useful for applying varnish consisting of two components.
A sprayer developed in England, the operation of which is based on the action of centrifugal force, is known as “Egaspray”. It is powered by a small motor. This spray gun can work in very small spaces.
New products in the field of equipment for applying varnishes and enamels also include the electric sprayer “Sprivi” (Eichenauer, Frankfurt am Main). It works without compressed air, fan or motor. This sprayer can be powered by a lighting network and its power consumption is only 30 watts.
Correct Application And right choice atomizers are mandatory prerequisites for economical operation. Air consumption depends on its pressure in the network (working with high, medium or low pressure), the size and shape of the nozzle, the viscosity and temperature of the paint and varnish material. With the correct selection of these parameters, you can significantly reduce the formation of paint mist, which occurs due to the spraying of tiny particles of varnish on the sides that do not reach the surface to be painted. You should also pay attention to the correct choice of distance from the spray gun to the surface to be painted.
For spraying nitrocellulose enamel with a viscosity of 20-40 sec. you can accept the following given in table. 42 the relationship between the nozzle diameter, air pressure and the distance from the spray gun to the surface to be painted.
With the data given in this table and with a material consumption of 100 g/m2, the productivity can be achieved: for a flat jet - 1.4 m2/min; for a round jet - 0.9 m2/min.
The normal distance between the spray gun and the surface to be painted is considered to be 20-25 cm. If this distance is smaller, then so-called “drips” are formed, and when it is larger, then the so-called “dry spraying” occurs. It is recommended to place the product to be painted at a sufficient height from the floor so that spraying is carried out at an angle of 30-45°.
Fog formation generally increases with increasing air pressure, and at low air pressure virtually no fog is formed. With some varnishing methods, the formation of a mist is even desirable, in particular, for example, when the top layer of varnish is finally coated with tiny droplets of varnish to obtain a good appearance and a shiny surface. Fog for these purposes can be obtained by appropriately adjusting the spray gun.
The third part of a complete spraying installation is the cabin and the ventilation installed with it. The size and shape of the painting booth depends on the requirements of each enterprise. Air is removed from the cabin exhaust fan. The suction device must be mounted so that the ink mist is sucked out of the booth symmetrically and above the middle of the booth. Paint droplets are filtered out from the sucked air. Filtration of the sucked air is carried out by a so-called reflective sheet or a built-in layer of porous material, for example wood wool, etc. The air is sucked out of the cabin without the formation of vortices. To prevent clogging of the suction device, special attention should be paid to the possibility of easy cleaning. Choice of cabins and ventilation units so diverse that it is always possible to choose a plant that satisfies all production requirements.
Significant development of the spraying method in recent years is the so-called
Hot spray
This method consists in heating the varnish to 40-80°, and in this state it is sent for spraying. Obviously, this method of operation has significant advantages, namely: the viscosity of cellulose varnish decreases significantly with increasing temperature. Thus, cellulose varnish, containing about 50% dry residue, still has a fairly low viscosity at 80°. Therefore, with a single spray application of heated varnish, an even thicker film is obtained. In most cases this results in a film with good appearance and high gloss. It should be noted that with hot spraying there is also a saving in solvent. The film formed during hot spray painting is denser and less porous due to its significant thickness. In this case, it dries relatively quickly, since its drying occurs as a result of not only the evaporation of volatile components, but also the hardening process.
Hot spray varnish should, of course, contain only those solvents that evaporate in noticeable quantities at temperatures above the spraying temperature, i.e. 40-80°. This is the second advantage of hot spraying, since there is no need to use flammable solvents that evaporate at low temperatures, but it also reveals the economic inexpediency of the hot spraying method, since high-boiling solvents, as mentioned above, are much more expensive than low-boiling ones and boiling at medium temperatures.
According to reputable experts, especially the railway department, hot spraying does not create economic benefits; The advantages of this method include savings in time and labor due to the application of single-layer coatings, storage of smaller quantities of solvents, simplified safety measures when working with flammable solvents, more high quality varnish layers, etc.
Hot spray plants are manufactured by a number of companies. In the Therm-o-Spray unit (Kurt Freytag, Hamburg-Wandsbeck), compressed air is supplied to the distributor via an electric heater, the temperature of which is controlled by a rheostat. The air heater is designed to be explosion-proof; the air temperature in it can rise to 150°. The air heated in the heater is supplied to the heat exchanger of the varnish heater, where it transfers its heat to the varnish and is then used in the spray gun to spray the heated varnish. The varnish passes through the heated apparatus in just 30 seconds. The varnish heater and supply hoses contain approximately 0.2 l of varnish. In contrast to devices operating according to the principle. Due to the circulation principle, the varnish in this device is exposed to heat only for a short time, as a result of which it practically does not deteriorate. The varnish is under constant pressure. This increases the boiling point of the solvent and reduces the tendency of low boiling components to the formation of bubbles.
Thus, the hot spray method, like any other, has its advantages and disadvantages. For many purposes, this method has gained a strong place in the paint industry.
Electrostatic spraying
The main difference between the electrostatic spraying method and those described above is that when working with this method, the varnish is not sprayed onto the varnished product. congestion, but is attracted by electrostatic forces to the varnished product in the form of individual particles ejected by a spray gun. In accordance with this, an installation for electrostatic spraying consists of: 1) spray guns that spray varnish in the varnish space; 2) positive and negative poles to obtain electric field and 3) devices for moving the varnished product through the spray booth.
It should be added that it is advisable to install such an installation with a large number sprayers so that the varnish is sprayed more evenly in space from all sides. To create an electric field, the varnished product, which is one of the poles, is grounded, and the second pole in the form metal mesh placed at a distance of 1 m from the first pole. The voltage between both poles is several thousand volts. If the product to be painted is not made of metal and therefore cannot serve as a pole of the electric field, then a special metal device must be placed behind the product so that it attracts varnish particles.
Soon after the success that accompanied the discovery of this new method of spraying, it became clear that in order to obtain an impeccable varnishing by this method, a number of conditions must be observed, some of them difficult to implement.
In addition to the fact that in a chamber of a certain size it is practically possible to varnish only objects of the same size and shape, often the shape of the surface of the varnished product creates significant difficulties. Electrostatic attraction depends on the distance between the electrodes, and therefore, on recesses, convexities and generally on rounded places of different radii of curvature, paint particles are deposited with different intensities, depending on the distance of these places to the second pole of the electric field. As a result, the varnish layer is uneven. Such varnishing irregularities can be corrected: for example, by switching the poles, the varnish can be “removed” from such places, but this complicates the method too much. The entire installation must be adjusted to the product being coated. The adjustment consists of setting: the required viscosity of the varnish, the distance to the product being painted, electrical voltage, intensity of spraying, creating a certain temperature in the chamber, the desired speed of movement of the varnished product and a number of other factors. When changing the varnish, all these factors must be set again. This method can be used for production mass varnishing of certain products. Its significant advantage lies in the continuity of the spraying process. The installation works almost without interruption, since the varnish is supplied evenly and without delays; its maintenance can be done by auxiliary labor, since varnishing occurs completely automatically. Electricity consumption is negligible. With a commonly used voltage of 100-120 kV, the current is only 1 -1.5 mA. The formation of fog during operation is almost completely eliminated, since only a very small part of the varnish does not reach the product being varnished. Varnish utilization reaches 95% or more. The productivity of the installation is seven times greater than with manual spraying; the cost of its operation is negligible. By adjusting it, it can be adapted for the application of other materials applied by spraying, such as passivators, oils, etc.
In new designs of electrostatic spraying apparatus, the varnish is supplied by a pump to a rotating washer. The washer is connected to high voltage and electrostatically sprays the varnish in the form of a thin mist curtain towards the product. In practice, this so-called Ransburg method No. 2 is successfully used. Details about it are given in the relevant literature.
AEG recently released a new electrostatic spray gun called the Electric Brush (Elektropinsel) (Figure 23). When using this device, the sprayed material turns into a fine dust, which is attracted to the product being painted by electrostatic forces. The material to be sprayed is in a cylindrical vessel, on the lid of which a circulation pump and a vessel for spraying varnish are mounted. The sprayed material is pumped into the spray vessel, the excess varnish flows from there through the overflow pipe back into the reserve vessel. When a voltage of about 100 kV is created between the edge of the vessel and the painted product, the applied varnish is sprayed and moves towards the painted product.
Other methods, such as spraying with superheated steam, as well as flame spraying, are of little use for applying cellulose varnishes and are practically not yet used. There are a number of articles in the specialized literature regarding the possibility of using these methods.
Dip painting
Dip painting makes it possible to obtain a uniform finish on the product being painted. varnish coating. This method of work is only suitable for painting easily movable products and gives good results only when painting products of a certain shape. An uneven surface of the product being painted can create significant difficulties when painting by dipping.
Proper dip painting depends on three conditions: the shape of the product, the consistency of the varnish and the speed at which the product is immersed in the varnish.
The shape of the product is a given factor and cannot be changed; therefore, only products of a certain shape can be painted by dipping. The product to be painted must be properly immersed in the varnish. Subject to certain essential conditions It is possible to paint by dipping such products for which this method initially seems inapplicable. First of all, it is important to hang the product to be painted so that the varnish can drain from all areas of the surface in the easiest and most convenient way. shortcut. The varnish flows best when lower parts the product has sharp edges or ribs. In these places, the varnish easily collects and runs off in drops, leaving no flaws on the painted surface.
The quality of dip painting is influenced by the consistency of the varnish and the speed at which the item being painted is immersed in the varnish. Both of these conditions must be selected accordingly. There is a relationship between them, which is as follows: the varnish covering the surface of the product being painted naturally flows down after it is removed from the bath. At the same time, the process of evaporation of the varnish solvent begins. As a result, the varnish cannot flow evenly: as it flows down, it thickens and, finally, hangs like a fringe at the bottom. Therefore, the product should be pulled out of the bath at a speed equal to or slightly less than the speed at which the varnish drains from the surface of the product. At this speed of removing the product from the bath, no fringe is formed, and the varnish slowly flows back into the bath, and the painted surface turns out to be completely smooth. Consequently, the relationship between the viscosity of the varnish and the speed of immersion of the painted product in the varnish undoubtedly exists, since varnish with a low viscosity naturally drains faster than a varnish with a high viscosity, and therefore, when immersing a product in a varnish of low viscosity, it can be removed from the bath accordingly faster . Thus, the higher the viscosity of the varnish, the lower the rate of immersion of the painted product into the varnish.
When a product is immersed in a thick, highly viscous varnish, a thick layer of varnish is formed on it, and in most cases, immersing the product in varnish twice or even once is sufficient for painting. When immersed in liquid varnish, the product remains thin layer laCa. Depending on production requirements, liquid or thick varnish can be used. When using the dip painting method rationally, bathtubs are arranged in such a way that they can simultaneously be immersed large number painted products.
When painting by dipping, it is necessary to carefully and at a certain speed not only remove the painted products from the baths, but also immerse them, since if the immersion speed is inappropriate, bubbles may appear on the surface to be painted.
You should also monitor the temperature of the varnish in the bath, since even small temperature fluctuations can significantly change the viscosity of the varnish, resulting in poor paint quality; It is often very difficult to find out the reasons for poor coloring.
From the varnish in the bath, part of the solvent evaporates over time. To avoid changes in the viscosity of the varnish, it is necessary to ensure, firstly, that the bath is opened only when the products to be painted are immersed in it and, secondly, that solvent is added to the bath in a timely manner to compensate for the evaporated part of the solvent.
Special attention should be paid to the addition of solvents. With such solvent additions, care must be taken not only to ensure that the original composition of the varnish is preserved, but also to take into account the different evaporation rates of the individual components of the solvent mixture. The component that evaporates faster should be added in quantities greater than even the original composition of the solvent mixture.
Dip painting units are manufactured in various sizes and shapes. The choice of the appropriate installation depends on the nature of the products to be painted and the required coatings. Dip painting plants are manufactured from small sizes from small products to large, fully automated plants combined with degreasing and drying plants. More detailed information about such installations is available in the manufacturer's brochures (Weppo Schilde A. G., Bad Gersfeld).
Drum painting
Drum painting is based on the same methods previously used for washing, cleaning, degreasing and removing rust from small metal parts. Over time, it turned out that the advantages of this working method can be successfully used for painting.
Painting in drums is carried out mainly at elevated temperatures, i.e. with hot-drying varnishes. But this method can also be used for painting with cellulose varnishes at normal temperatures.
When working using this method, the varnished products, mostly very small ones, such as buttons, etc., are loaded into an apparatus that looks like a perforated drum. This device is placed in a metal container, at the bottom of which there is a certain amount of varnish. Using a special device, a perforated drum can be immersed in varnish so much that the loaded products are covered with it. Then the apparatus is rotated, as a result of which the products roll and excess varnish comes off; a heating device combined with the apparatus dries them.
When painting products with cellulose varnishes, as a result of prolonged rotation of the drum, sometimes lasting several days, a silky shine of the painted surface can be obtained. A different appearance of the painted surface can be obtained due to additional processing of products with mordant, wax or other substances.
Drum painting is used to coat wood, steel and other materials. This method is especially suitable for painting mass-produced items. If possible, the products to be painted should not have large flat surfaces, since in the presence of such surfaces they can sinter. The products should also not differ greatly in shape, since in this case they interlock.
The varnish used for painting in drums must be low-viscosity so that the products immersed in it are quickly wetted. Acceleration of solvent evaporation is achieved special device to heat the drum. The solvent should evaporate as easily as possible. The advantage of drum painting is primarily the saving of varnish. -To work using this method, a very small amount of varnish is sufficient, since the film on the painted product is very thin. When working with this method, various effects can be achieved. For example, when painting wooden balls with paints containing aluminum and bronze powder, the result is a surface that is difficult to distinguish from metal.
The company Carl Kurt Walther (Wuppertal-Wohwinckel), which produces a number of models for painting in drums, has developed a model called “Lackier-Tauchzentrifuge” (submersible coating centrifuge). In this model, the immersion and centrifugation devices are combined in such a way that the loading basket is immersed in the varnish using a special handle, and then raised to remove excess varnish by centrifugal force. This design marks the first time that loading baskets are inserted and removed from the top rather than from the side. They can be replaced very quickly and without loss of varnish. Devices of this design are equipped with a variator drive. They can be used to paint large parts various shapes, which was considered impossible before the invention of this device.
This new model is an improved example of devices used for spin-staining.
Spin staining
Spin painting differs from drum painting by the speed of rotation of the drum. If when painting in drums this speed is relatively low, then when painting by centrifugation, the rotation speed of the product with a drum reaches 500 rpm, therefore, when painting by centrifugation, the painting process ends in a much shorter time. The material consumption with this painting method is also very small.
Push painting
This painting method is suitable for finishing long and straight items such as pencils, sticks, rods, etc. When painting using this method, the items are passed through a container filled with varnish. The product leaves the container through a device that removes excess varnish. The items to be painted are pushed or pulled through the varnish bath. Pulling is used primarily for painting bendable products, such as wires, cables, strips, etc. The paint and varnish material used for pushing coating should dry as quickly as possible, since this painting method is used mainly in continuous production. To obtain a film of sufficient thickness, the product to be painted must be passed through the bath two or even several times.
Spray painting
For painting some products, the spray painting method turned out to be the most appropriate. When working using this method, the varnish is supplied from the tank to the painting site with a hose and the worker only directs the varnish onto the product to be painted. The latter is installed in such a way that the flowing drops and jets of varnish are collected in a vessel, from which the varnish is returned to the tank. Another variation of this method is to varnish rotating products. As a result of the rotational movement of the painted product, an even coating is immediately formed on it.
Painting on roller machines
The varnish can be applied to a smooth, flat surface using a so-called roller varnishing machine. This machine has a large number of rollers that take the varnish from the tank and apply it in an even layer to the surface to be painted. By appropriately installing the rollers, you can obtain a layer of varnish or paint of any thickness. This method makes it possible to quickly paint first tape materials, such as metal strips.
In recent years, several new painting methods have been developed, which, however, turned out to be of little use for applying cellulose varnishes and therefore need not be mentioned here. These new methods include, for example, varnish rinsing, wire varnishing, flame spraying and pipe varnishing.
In accordance with the nature of the book, the descriptions of individual methods are given here as briefly as possible. A comprehensive comparison of individual painting and varnishing methods is available in the 1954 edition of the Paint Shop Handbook.
