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FLAME SPRAY POWDER COATING

FLAME SPRAY POWDER COATING

The principle of polymer’s flame spray application (FSA) was originally borrowed from metal flame spraying technology. Coating touches more decorative properties and has restrictions on type of used polymers. Spraying is more generic definition which emphasizes the functionality of the coating.

The most widely used in industry are the following methods of polymer powder materials applying:

  • fluidized bed;
  • electrostatic application;
  • jet thermal application.

Each of these methods has its own advantages and disadvantages that determine its effective scope. It is based on the geometric parameters of coated parts and products, their design, technological features and future operating environment. The only factor that brings together all of this ways is heat treatment, which is necessary to form a stable adhesive bond between primer and polymer. The first two methods involve a complex of operations and presence of special booths, bathtubs and ovens. Therefore, the first and the main limitation of their use concern fixed and large-sized products. In this case, the only cost-effective and simple are methods of thermal jet spray, allowing forming a polymer coating in a single operation.

The methods of thermal jet spraying are:

  • heat radiant spraying;
  • melt spraying;
  • gas-thermal (flame, plasma).

Of these, the most technologically and economically feasible, is method of flame spraying. Its main advantages include:

  • the ability to produce spraying without dismantling the structures;
  • flexibility of technology and mobility of equipment;
  • lightness and ease of maintenance;
  • it is possible to form layers from most polymer materials with structure changing;
  • absence of power supply sources.

The essence of the flame spraying method is as follows:

  • formation of directional flow of dispersed powder particles, providing their transport to product’s surface;
  • creation of conditions for stable reaction of mixture «flammable gas-air» combustion, followed by temperature increasing and speed increasing;
  • reacting the combustion products with powder particles, whereby they are heated to the melting temperature of liquid phase;
  • converting the internal (thermal) and the kinetic energy of particles into work of deformation during the formation of adhesive contact with primer surface;
  • optimization of temperatures and flow of gas and powder rates to form a spray layer satisfactorily.

At present, many important questions about FSA are not yet fully clear. Difficulties in its studying are created because of complexity of combustion and heat transfer processes, parameters dependence on a variety of factors.

Before we come closer to development of equipment for the thermal jet powder coating, we conducted a systematic analysis of the issue. We have studied in detail the structure and experience of both modern facilities and first created in the USSR in late 80-ies. The first gas-oxygen thermal sprayings were very good at powder spraying of polyvinyl butyral, polyamide powders, and a select group of foreign powders. In recent years a greater interest have gas thermal spraying, which provides powder coating by polymers with melting points from 365 K to 670 K.

We have conducted technical consultations with Paton Electric Welding Institute, with well-known manufacturer of flame spraying equipment Xiom Corporation (USA). We have analyzed applications, modern and advanced level of industrial implementation of this technology in developed countries (USA, Britain, Italy). By the way, the US has implemented more than 400 installations.

We establish a sufficiently cooperation with the largest producer of thermoplastic powder coatings — Pulron (Pulver -Turkey). The goal is to achieve technical advice on development of technological processes and creation of new compositions of thermoplastic powder coatings for special purposes. Currently, we offer Plascoat Systems Limited products in Ukraine, and consider it the main material in industrial application of flame spraying method.

Conclusions:

  1. The demand and urgency of solving issues dealing with development of technology of flame spraying is not in doubt.
  2. There is no overall design of FSA equipment, which could serve as a basis for technical specifications development to powder manufacturers.
  3. There are no general guidelines for choosing optimal values ​​of heat flux for thermal spraying torch and melting conditions. Each equipment designer optimizes their systems relying primarily on their own experimental results with a certain group of polymers. The foregoing gives grounds to believe that development of specialized powder coating is a prerequisite for progress, both the method and technology.

Undoubtedly, the quality of formed coating largely depends on the chemical nature of polymeric material, its size and shape of dispersed particles, their density, moisture, thermal and electrical properties. Moreover, maintaining of strict technological rules of surface preparation is very important from the point of view of adhesive contact formation. However, as the theory and practice show, the main reason lies in the initial heat treatment cycle. Its task is to give an adequate supply of heat and kinetic energy to powder particles.

Energy efficiency means optimizing the combustion process of torch, and it is achieved through the coordination of space-time and thermal parameters of flame with thermal characteristics of used polymer, namely:

  • ratio of fuel and oxidant gas in the mixture;
  • nature of temperature distribution along the axis of flame;
  • time spent by particles in the core of propane-air flame;
  • geometric and thermal parameters of spray particles.

Based on our analysis of literature data and results of mathematical modeling of complex engineering calculations, industrial device MFSS (p) -1-50 was designed and built. It is used for manual application of powder polymeric coatings on surfaces prepared by flame spraying.

General view of installation is shown in the photo

MSPU (p) -1-50 is currently in an active stage of industrial testing.

Results of laboratory and field tests established:

  1. In the formation of polymer coatings (Plascoat PP571) with the help of FSSD (p) -1-50 thermal degradation of surface is low.
  2. Improving operational performance of applied coating may be achieved by:
      • activation of coated surface;
      • modulating of time-temperature conditions to form a coating;
      • modifying of polymeric material by introducing additives-fillers.

    Consider the points.
    Activation of coated surface is achieved by exposing. It means temperature rise, creation of stress and increase of surface roughness.
    It is much harder to control spraying regimes. This problem is more related to the technical implementation of the equipment.
    Effective modifiers may be dispersed metals and their oxides.

  3. If you have a large (d≥300mkm) and small (d≤ 100um) particles in powder it is desirable to separate them. Otherwise, the heating and melting of these particles will occur unevenly.
  4. Nozzle design influences on uniform flow of powder spray. It was developed a special its design — parallel flow. It provides a narrow stream of particles with a small angular divergence.
  5. Practically it was found that the best results, especially in terms of visual quality, is achieved while preliminary heating of coated surface.
  6. Experimentally demonstrated the possibility and optimal operating parameters are shown concerning applying of protective and decorative polymer coatings by flame method for non-metallic materials (plastics, ceramics, MDF, concrete, etc..).

The validity of these findings and conclusions is confirmed by necessary volume and repeatability of experimental data.