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CLASSIFICATION AND APPLICATION

p1000340

CLASSIFICATION AND APPLICATION

Thermoplastic powder coatings

Thermoplastic powder is one kind of industrial powder paints. Thermoplastic powder coatings include paints based on polyvinyl butyral, polyfluoroolefin, polyethylene.

The starting materials for obtaining such paints are polymers with relatively high molecular weight. Therefore, formation of these coatings requires high temperatures (150-200 °C).

During heating, applied thermoplastic powder melts and combines the particles in order to form a solid polymer slick on product, which has protective properties after cooling. After reheating thermoplastic coatings melt again.

Due to high temperature polymer degradation is not excluded; therefore the preparation of coatings should strictly follow the established technological modes. The coatings of thermoplastic powder paints generally have low adhesion. That is why it is required to conduct a thorough surface preparation: apply shot blasting or sand blasting of surface, and in some cases — its priming.

Characteristics of thermoplastic powder coatings

We present coating’s descriptions which were produced from paints based on most frequently used thermoplastic polymers.

Polyamides. The most common polyamide powder compositions and coatings are produced on a polymer known under the trade name nylon. Coatings of nylon possess many valuable properties. They have a beautiful appearance, high hardness, strength, resistance to abrasion, resistant to chemicals and solvents. There are compositions that do not require a primer, and inks which are applied to the pre-primed surface. In the case of critical surfaces (protection of dishwasher’s basket, pipes, valves, pumps, chemical equipment and medical instruments) priming is usually carried out. With an appropriate choice of raw materials polyamide coating is approved for food contact. Polyamide paint is used not only for interior but also for exterior use, such as painting of lighting, poles or benches.

Polyolefins. Powder compositions of polyolefins — polyethylene, polypropylene, as well as polyamides, have a long history of use in coatings. Polyethylene forms a soft-touch coating. Polypropylene, and especially some of its polymers, forms quite elastic coating. Those and other coatings have good chemical resistance. However, these coatings may crack due to stress arising when they exposed to some solvents. Another disadvantage of these coatings is low adhesion to metals. Improvement of these properties can be achieved by appropriate surface preparation, modification of compositions or application of more adhesive polymers instead of individual ones. (This process is described in a separate section).

Plasticized polyvinyl chloride. Coatings based on plasticized polyvinyl chloride, being soft and rubber-like, exhibit good resistance to detergents and water, retain adhesion even when they are used in such products as dishwashers baskets. These compositions are suitable for exterior coatings. With the proper paint composition it is possible to obtain coatings allowed for contact with food.

Polyesters. The coatings based on thermoplastic polyesters resemble a nylon coating. But they do not have many of properties inherent in nylon, in particular, resistance to solvents, resistance to abrasion. However, they are characterized by good adhesion to different materials and they do not require priming. Many polyester coatings have good resistance to external influences; they are used, for example, to protect the garden furniture. Due to some difficulties polyester coatings have not gained wide popularity in the market.

Polyvinylidene fluoride. PVDF-based coatings have excellent resistance to external influences. By weathering they surpass all other coatings obtained from the powder paints. They also have good electrical insulating properties and resistance to various chemicals, except for certain solvents. They are used mainly in the chemical industry to protect pumps, valves, piping, and other equipment. Due to the reduced adhesion of coatings the priming of surface is recommended.

Now, briefly describing coatings obtained from paints based on the most frequently used thermoplastic polymers, we can summarize:

Polymer type Coating properties Extras
Polyamides high hardness;
strength;
abrasion resistance;
resistance to chemicals.
The compositions are divided into two groups: require priming and do not require priming
Polyolefins; elasticity and softness of the coating;
resistance to chemicals.
Low adhesion to metals: recommended surface preparation
Plasticized PVC soft cover, like rubber;
resistance to detergents and water.
With proper paint composition it is possible to obtain coatings allowed for contact with food
Polyesters low resistance to chemicals;
low resistance to abrasion;
good adhesion;
resistance to external influences.
Does not require priming before application.
Its difficult for applying
Polyvinylidene fluoride high resistance to external influences;
good electrical insulation properties;
resistance to chemicals.
Low adhesion: recommended priming of surface

Methods of applying

Currently the main methods of thermoplastic powder coatings application are:

  • electrostatic spraying (with a corona or tribo-charging powder);
  • immersion in a fluidized (boiling) bed;
  • flame spraying.

Недостатки:

  1. для загрузки ванны необходимо большое количество порошка;
  2. обрабатываемая деталь должна быть предварительно нагрета;
  3. этот метод нанесения используется только в тех случаях, когда необходимо получить толстослойное покрытие;
  4. окрашиваемые изделия должны быть простой формы.

The method of electrostatic spraying

The method of electrostatic spraying is the most convenient. It ensures better control of coating thickness. This method has become the main in coating industry.

Electrostatic spraying is based on connection of dry powder particles and an electric charge. There are two methods of charging:

  • corona charging in an electric field;
  • tribostatic (friction) charge.

Both methods are effectively used in powder coating technology. However, there are differences that make some cases more suitable for corona charging, and others — for charging by friction.

The method of immersion in a fluidized (boiling) bed

Powder coating is carried out in fluidized bed. Purified compressed air is fed through a special porous bottom of bath. Fluidized layer is generated under the influence of air. Before immersion into the bath the product is heated to a temperature above the melting point of powder paint. The coating thickness depends on how long the product was in the bath and its temperature. If the product is large, the accumulated heat will be enough to complete the curing of powder paint. For metal-products post-curing in oven should be carried out.

Coatings applied in the fluidized bed typically have a thickness of 0.25 to 0.50 mm, but at repeated heating and immersion coating may have a thickness of 2.5 mm. It is more convenient to cover products of the same type and equal thickness. Small and medium-sized pipes, spiral springs for automobiles, electric motors, transformers, wire, metal mesh are mostly coated in such way.

Powder material in fluidization state can be applied not only to heated body, but also to cool. In this case, powder is moved to aerosol state in fluidization chamber and simultaneously there it is charged, creating a cloud of charged particles. When a grounded object is placed in this cloud, the charged powder quickly settles on its surface.

Benefits:

  • one cycle of application and post-curing can create durable coating with high corrosion resistance;
  • layer thickness uniformity can be adjusted by observing the technological cycle;
  • low initial cost of equipment.

Disadvantages:

  • you need to download large amounts of powder into bed;
  • the work detail must be preheated;
  • this application method is used only in cases when it is necessary to obtain thick coating;
  • painted products should have simple form.

The method of flame spraying.

Traditional polymer powder coating technology, a prerequisite of which is the presence of curing oven, faced the problem of impossibility to paint large and remote products, such as power lines, oil and gas pipelines, bridges and water towers.

An excellent solution of this important problem was the use of flame spraying method, which uses a jet of flame for application of polymer powder. This principle is borrowed from technology of metal flame spraying. Gun creates a flame, which is fed powder paint. The flame heats the ink and sends them to the surface of coated products. Temperature effect lasts only a few seconds, thereby it doesn’t harm the surface and does not lead to thermal deformation. Cooling of painted surface with applied paint lasts a little time, after which the surface is ready for operation.

For flame (thermal) powder coating is not required to charge the product and powder particles to create an electrostatic field. This means that almost any surface can be covered, not only metals, but also plastics, glass, ceramics, wood and many other materials that would be deformed or burned in curing oven.

Flame coating eliminates the need to use bulky ovens, and outputs powder painting to a new level of application. Spray equipment is portable and universal. It is used not only for heating the surface and powder spraying but also for reheating in order to equalize the surface.

Among the disadvantages of this technology — is that the coating does not always have a smooth surface, and their value is more functional than decorative. But for such objects as bridges, ship hulls or water towers protection against corrosion and rust is more important than slight unevenness in the coating.

Modified thermoplastic powders.

The need for polymer modification is dictated by necessity to create materials which would have unique combinations of physical properties. For example, combining tough and flexible polymers in different proportions may create the final polymer composite with desired properties.

Unfortunately, in most cases, these materials are inherently incompatible on chemical level, and require complex and costly process of preliminary chemical modification before mixing.

In addition to the long-known and widespread chemical treatment, other methods of surface modification, such as corona discharge, ultraviolet radiation, gamma radiation, electron beam, reactive gases, and low-temperature plasma have been researching.

Currently, in addition to conventional chemical modification, commercially used only two types — a process of plasma chemical modification and modification of reactive gases.