In reality, during powder coating, the purpose of the oven is to heat the surface of the material, or its coating, to activate physico-chemical processes, which include several stages: transition of powder particles to a viscous state, fusion, wetting of the substrate, spreading, removal of gas inclusions, hardening (for thermosetting combinations).

Curing ovens are the main links of any painting line. Therefore, the search for new methods and means of heat treatment, aimed at improving the process of polymerization of powder, is an urgent task in the creation of modern economic equipment for coating. According to the physical essence, the heating of any body occurs due to the mechanism of heat exchange. But the “channel” of energy transfer can be different: direct heat exchange, convective or radiant. Each of the specified methods is implemented in equipment of very wide application. Recently, in connection with a serious increase in the price of energy carriers, manufacturers of thermal equipment increasingly began to look for ways to effectively use the features of infrared radiation when supplying radiant energy. However, there are serious obstacles to the widespread industrial implementation of IR technology. On the one hand, these are problems of growth, and on the other hand, there is a lack of specialized research and design units working purposefully in this direction. The creation of new modern infrared devices requires both analytical and experimental in-depth study of energy transfer processes in absorbing media, as well as processes of heat exchange by radiation occurring in working ovens. Knowledge of these regularities, as well as shortening the transition period from experimental models to industrial devises and the development of reliable engineering calculation methods that take into account the specifics of the processes and the kinetics of their flow, will make it possible to scientifically solve the issue of the introduction of heat technologies using IR – energy supply.

It is clear that this issue is also relevant in paint and varnish production, where modern technologies require more advanced methods of heat treatment of both liquid and powder coatings.

For the first time, infrared rays were widely used for drying and baking paint coatings in 1934. at Ford plants. It is believed that from that time the foundations of the drying process were laid in the form of a selected spectral range of IR radiation.

Unlike liquid paints, where “drying” is performed, i.e. removal of moisture, which is in various connections with the material being dried, and “heating” of the surface to perform the hardening process, in case of powder coating, the purpose of heating is only to thermally activate the physico-chemical processes listed at the beginning of the article. Therefore, when using IR heating, first of all, it is necessary to determine the list and sequence of technological tasks to be solved.

The physical essence of IR – heating is explained by the corpuscular-wave nature of the electromagnetic field and is associated with the intensification of processes, as a result of the resonance effect of energy absorbed on the bonds of atoms in molecules, whose vibration frequencies coincide with, or are multiples of, the frequency of the incident radiation. The energy of individual chemical bonds can be compared to the energy of photons of IR radiation. Thus, at λ ≤ 1μm, the energy of the photon

E = hv ≤ 2 * 10⁻¹⁹J

and the energy of chemical bonds of the main groups of C-C and O-H polymers is

2 * 10⁻¹⁹J and (0,32 — 0,46) * 10⁻¹⁹J

in accordance. Therefore, IR radiation, causing an increase in the level of self-oscillations of certain groups of atoms in a molecule, which means the conversion of radiation energy into thermal energy, contributes to the acceleration of the technological process. This is the most difficult question to be still at the stage of study, since the irradiation of objects with IR rays cannot be considered only as a method of intensive heat treatment, it is also a process of deeper influence on the physical and chemical nature of the material.

The efficiency of IR heating is determined by optimizing the ratio of energy reflected, absorbed and passed through the sample. These characteristics depend on the range of wavelengths, type and physical properties of both the polymer and the substrate. It is known /1,2,3/ that most coatings of materials penetrate 85% of the radiant energy in the radiation spectrum to a depth of 30-50 microns. At the same time, the air environment practically does not affect the transmission of the radiant flux. This is the main, important difference in the mechanisms of IR and convective heating when forming a polymer coating. These are different directions of the temperature gradient. With convective heating, the direction of the gradient of the surface of the coating is towards the substrate, with IR it is the opposite, i.e. the coating is partially heated due to the heat given off by the substrate. As the analysis of literature data /4,5/, as well as our tests show, such a mechanism significantly affects the quality of powders of the coating being formed, primarily by increasing its adhesive strength.

When developing our equipment, the first task we faced was to take into account the above features of IR heating when choosing the optimal spectral range and, as a result, the type of emitter that generates wavelengths in the selected part of the spectrum. At the same time, an important criterion was the economic and operational characteristics of the equipment being developed.

In contrast to standard convection systems, rigorous mathematical modeling of the heating process in IR ovens is a very difficult task. Analytical description of the kinetics of heating of products by IR radiation is the finding of relationships between power density, maximum permissible temperature and maximum permissible heating rate (a very important characteristic in polymerization processes). In this situation, the methodological basis of the model can be the differential equation of the energy balance, with the help of which it is possible to establish the rules of conformity connecting the interaction of the “emitter-product” system, and then only for some individual cases under serious assumptions and approximations, which often reduce the practical the value of decisions. Moreover, a significant drawback of the approximate calculations is the complete lack of an analytical connection between the functions of temperatures and effective flows with optical properties and parameters of the relative location of the structural elements of the system.