ABOUT POWDER COATING

Recently increased requirements for anti-corrosion protection of metal surfaces, their decorativeness, as well as ensuring high specific operational characteristics have made the issues of creating new types of coatings, developing methods and technologies for their application urgent.

The need to solve these problems is especially acute when aluminum structures are widely implemented in buildings with a large area of glazing.

Polymer powder coating, which is rapidly developing, meets the new requirements most fully. The accumulated experience of its application confirms high technical and economic efficiency, and, first of all, compared to alternative types of special coatings.

The essence of the powder coating process is the application and fusion of polymer particles on the surface of the product. A wide variety of spraying methods are known, which differ in the material used (thermoactive, thermoplastic), the principle of powder application (in a fluidized bed, pneumatic spraying, electrostatic, gas flame, plasma), by the type of equipment used (chamber, chamberless, induction, etc.).

The division into groups, or the classification according to the principles used, is very conditional, but it allows us to identify the general laws that underlie the flow of the coating formation processes. Knowledge and accounting of these regularities allows not only to develop and implement new equipment, but also to be a guarantee of obtaining high-quality coatings.

Recent successes in the powder coating technique have made it possible to significantly improve the quality of painting, as well as to significantly expand the scope of its application. This is evidenced by numerous publications, both in general technical and special literature /1-10/.

There you can get acquainted with the basics of technology, economic and ecological aspects, as well as problems arising in the process of working with powder materials.

At the same time, far from all the possibilities of polymer coatings, both functional and decorative, as well as the methods of their formation have been sufficiently explored.

In particular, the practical application of thermoplastic powders, gas-flame and plasma spraying, and the application of powder spraying on materials such as MDF, PVC, and glass remain open.

Research in these areas is mainly experimental in nature and, as practice shows, can always be used effectively.

LITERATURE:

Yakovlev A.D., Zdor V.F., Kaplan V.I. Powder polymer materials and coatings based on them. L., Chemistry, 1979, 256 p.
Yakovlev A.D. Powder paints., L., Chemistry, 1987, 216p.
Powder paints Coating technology. Under the editorship Yakovleva A.D., St. Petersburg , Khimizdat , 2001., 256p.
Bily V.A., Dovgyalo V.A., Yurkevich O.R. Polymer coatings. Minsk, Science and Technology, 1976, 416p.
GOST 9.410-88 Polymer powder coatings. 1988.
GOST 9.402-80 Surface preparation before painting. 1981.
US Patent 6825240.
US Patent 7159535.
US Patent 5824373.
Paint coating. Technology and equipment. Handbook., M., Chemistry, 1992, 415p.

WHAT IS POWDER PAINT

Powder paint is a solvent-free, micron-sized paint made from a solid resin-like raw material. All raw components of powder paint, such as pigment, hardener, filler and additives are used in a solid state and homogenized by mixing. After the answer to the question “What is powder coating?” found, it is necessary to consider the history and scope of this type of paint. Powder paint, produced as a safer alternative to liquid paint, was first developed in Germany in 1953 by Dr. Erwin Hemmer . Particles of powder paint are attached to the surface using Hemmer’s fluidized bed technology . In the following years, epoxy, epoxy-polyester, polyester and polyurethane paints were developed. Powder coating, often preferred around the world, gained popularity in the 1980s.

Environmentally friendly and ready-to-use powder paint, which creates a decorative surface, is currently used in many industries – from architecture to the automotive industry, MDF coating to glass, ceramics and household appliances.

APPLICATION OF POWDER PAINT

Of all the powder coating methods listed in the “Basics of powder coating” section, pneumatic electrostatic spraying is the most common. Today, this is the main coating method in industry. Its popularity is due to the high operational characteristics of the equipment, the simplicity of the constructions, ease of use, the ability to easily control the thickness of the coatings and, what is important, spray guns have been traditionally used for years in the paint industry.

The essence of the pneumatic electrostatic spraying method is quite simple: using the laws of mechanics and electrostatics, charge the particles, transport them to the surface to be covered, distribute them in the form of a uniform layer and keep them in this state until the product is placed in the polymerization furnace.

The simplest structural diagram of technological equipment for applying powder coating is presented below.

Compressor.
Control unit.
Receiver.
Air cleaning unit.
Pressure regulator.
Powder supply device.
Recovery system.
Spray gun.
Powder charging unit.

Blocks (1 – 7) are typical and are used in many technological processes.

Specific, which are used only in powder spraying, are powder charging devices and the spray guns themselves.

CHARGING THE POWDER PARTICLES TAKES PLACE IN TWO MAIN WAYS:

electrostatic (corona);
tribostatic (contact, friction).

The charging mechanisms in each of the methods are fundamentally different, and put forward a number of specific requirements, both to the characteristics of the materials being sprayed, and to the conditions of spraying. Currently, a fairly large theoretical and experimental database has been accumulated, which allows to objectively assess the advantages and disadvantages of each of the methods, to identify the most effective areas of their application.

Nevertheless, it must be recognized that charging by corona discharge is used more often than the tribostatic method in large and small production. And, according to practitioners, mainly because it is more effective in terms of electrification than most existing powder paints.

At the same time, practitioners consider and compare domestic and foreign serially produced devices. In our opinion, it is more objective to analyze the potential, theoretically justified possibilities of those methods that take into account the promising development of powder compositions.

We use both electrostatic and tribostatic atomizers of our own development (see photo and section “Equipment – Atomizers “).

At the same time, we prefer tribostatic systems.

IT IS KNOWN THAT THE EFFICIENCY OF TRIBOCHARGING OF POWDER PAINTS IS DETERMINED BY THE FOLLOWING FACTORS:

the ratio of the area of the charged surface to the volume of the powder, which, in turn, depends on the trajectory of the particles and the degree of turbulence of the air-powder mixture;
distribution of powder particles by size;
optimal selection of the material used as a charging surface;
air humidity.

For the most part, all modern tribostatic atomizers are of the “acceptor” type, i.e., materials capable of accepting a charge when in contact with a particle of powder are used as charging systems.

For thermosetting powders, the most optimal material used as a charging surface inside the tribosprayer is fluoroplastic (see “tribostatic series”).

Control of the trajectory of particle movement in such systems is usually limited by the design features of charging tubes.

In our opinion, this method has not fully explored the possibilities of controlling the trajectory of partially charged powder due to the imposition of additional adjustable electrostatic or magnetic fields. So, for example, if the design of the charging system ensures the movement of the air -powder mixture in such a way that each particle acquires a radial component of velocity, then the longitudinal magnetic field will force the particles to move in a helical trajectory (Lorentz force). As a result, the effective area of the contact surface inside the atomizer increases, thereby increasing the specific tribocharge.

Recently, in connection with some revival in the field of application of thermoplastic powders, especially on the basis of polyethylene (see “triboelectric series”), the task of developing ” donor ” tribocharged atomizers, in which the charging surface does not accept, but gives electrons, has become urgent. Our experiments on the tribocharging of thermoplastic powders developed for application by the fluidization method allowed us to identify a number of features and practical recommendations.

In particular: methods of controlling the trajectory have been determined, as well as optimal conditions for the application of microparticles of powder have been chosen (see photo).

Since the average size of these particles (median) is within 250 μm, it was necessary to completely revise the issue of particle transportation, taking into account changes in the interaction of electric, hydrodynamic, inertial, and gravity forces, which in turn depend on the size of the particles, their mass, dielectric constant, and flow density .

In conclusion, it should be noted that the main requirement that all chargers must meet is the stability of parameters during the application process.

A criterion of stability can be a constant specific charge of particles, which for most industrially produced powders is within 0.5 – 2.5 µC /g.

We also consider it our duty to note that the issues of powder application, which are directly related to the general problem of powder painting, despite the progress achieved in this field, have not yet been fully studied.

BIBLIOGRAPHY:

Polyakova K.K., Paima V.M. Technology and equipment for applying powder coatings., M., Mashinobuduvaniya, 1972.
S.S. Nemchatov Technology of obtaining polymer coatings. Tashkent, 1975 232p.
Yakovlev A.D. Mashlyakovsky L.M. Powder paints and coatings. St. Petersburg , Khimizdat , 2000, 64 p.
Technology and equipment for applying polymer coatings in an electrostatic field, edited by Vereshchagina I. P. and others, M., Publishing House, 1990, 240 p.