Thermoplastic Powder Coating

• The early work on polyamides resulting in production of the first industrial polymers known under the name nylon was done by Carothers in the laboratories of DuPont.
• Although the experimental work on polyamides began with amino acids capable of self-condensation to form linear polymers, the first polymer produced on an industrial scale was the so-called nylon-6,6, prepared from adipic acid and hexamethylene diamine.
• Normal baking temperatures for powder coatings are below 200 °C. Therefore, those polyamides having low temperatures of melting are given preference over the others, because of energy saving, measures.
• Nylon-11 has a relatively low melting point of 185 °C. Together with nylon-12 which has a melting point of 178 °C they are practically the only polyamides that have found wide applications as binders for powder coatings. But nylon-6, nylon-6,6 and nylon-6,10 with melting points of 215, 250 and 210 °C respectively, have not been significantly accepted by powder coatings producers.
• Nylon-11 is a condensation product of 11-aminoundecanoic acid. And the production of nylon-12 is based on self-condensation of lauryl lactam.
• Both, nylon-11 and nylon-12 are practically insoluble in the common organic solvents. However, they are soluble or easily attacked by phenols, formic acid, mineral acids and similar compounds even at room temperature. At higher temperatures they are soluble in mixtures of alcohols and halogenated hydrocarbons, nitro alcohols and calcium chloride-methanol mixtures.
• Nylon-11 and nylon-12 are unaffected by water at room temperature, or even by boiling water. Stability against the action of alkalis is also very good. However, the stability in acidic media is not a strong point of polyamides in general.
• The first nylon powder coatings were introduced in Europe using nylon-11 and nylon-12 as a binder system.
• Several unique properties of nylon based powder coatings give them an advantage over other powder coatings.
  • Very good toughness,
  • High abrasion resistance combined
  • Superior impact resistance even at low temperature.
  • Their low coefficient of friction and unusual wearing properties make them excellent interfacial coatings to reduce metal-to-metal wear or noise.
  • Good thermal insulation
  • Exceptionally good resistance against most of the solvents commonly used in practice. The resistance to organic acids, inorganic salts and alkalis in low concentration is also good.
• The possibility of stabilization of nylon makes this material attractive not only for indoor functional coatings, but also for outdoor use.
• The fluidized bed method is the most commonly used application technique for nylon powder coatings. In this way, thick coatings in the order of 200 to 700 ?m can be applied in a one-step coating procedure.
• An advantage of nylon powder coatings concerning the application properties is the high line speed, that, until recently, could not be achieved with thermosetting powders.
• Nylon powder coatings are non-toxic and odour and taste free. This, combined with the fact that they are not attacked by fungus, and do not facilitate bacterial growth, makes them good for medical equipment, coating machine parts and piping used in the food processing industry. Powder coatings based on nylon have approval in all industrial countries for use with potable water and foodstuffs.
• Another strong point of the use of nylon powder coatings is their excellent impact resistance, which is retained over a broad temperature range from -40 to 80 °C.
• An interesting application of nylon powder coatings is in the coverage of handles of all types. Their low thermal conductivity gives the handle a warm feeling, which makes these materials very interesting for coating of tool handles, door handles, steering wheels, etc.

Problems with Nylon based powder coatings:

• When exposed to light, nylon-11 and nylon-12 like all other polyamides have a tendency to lose tensile strength.
• Problems which could be encountered with nylon powder coatings derive from their easy contamination by other materials such as PVC, epoxies, polyethylene etc.
• Like most thermoplastic powder coatings, the use of a primer is recommended in order to improve adhesion to the substrate.

• Powder coatings based on thermoplastic polyesters as binders are prepared from linear high molecular weight polymers produced by polycondensation of dibasic acid and diols.
• There are two main methods by which high molecular weight polyesters can be produced.
  1. Direct esterification of diacids with diols results in the formation of polyester and water as a by-product.
  2. Transesterification of diesters of diacids and diols under vacuum at high temperature, leading to a formation of polyester and corresponding diol as a by-product.
• Thermoplastic polyester powder coatings are based in most cases on high molecular weight crystalline or semi crystalline linear polyester resins, alone or in combination with other binders, in order to achieve desirable properties of the fused films.
• Another important characteristic of polyester resins for thermoplastic powder coatings is their ability to crystallize. A certain degree of crystallinity is necessary to assure a high enough melting point,
  Thermoplastic polyester powder coatings are characterized by
  • very high decorative properties giving a porcelain-like finish.
  • surface of the coating has a high gloss and excellent flow.
  • Adhesion to metal substrates, in contrast to other thermoplastic powder coatings, is excellent, so previously priming the substrate with an adhesion promoter is not necessary.
  • The abrasion resistance, scratch resistance and hardness of the coatings are excellent, combined at the same time with good flexibility and impact resistance.
  • They possess excellent electrical properties, considerable chemical resistance, long-term durability and an unlimited shelf-life
• Because of their excellent aesthetic properties, thermoplastic polyester powder coatings are used for metal furniture, guardrails, inside architectural elements and panels, shelves, aluminium extrusions, tools and machine components.
• Good dielectric properties make them suitable for applications including transformers, housing for electrical equipment and instruments.
• Although the weathering resistance of some polyester resins is satisfactory, thermoplastic powder coatings still cannot compete with the thermosetting counterparts with respect to exterior durability. They are therefore used mainly for inside applications, or on articles which are not permanently exposed to weathering conditions.
• Polyester powder coatings have the lowest solvent resistance among thermoplastic coatings.
• Problems which have to be overcome, when thermoplastic polyester resins are used as binders, are poor adhesion to the substrate, crack formation upon rapid cooling or bending, and poor lustre in spite of the fact that the polyester resins offer reasonable weathering resistance. Addition of cellulose esters such as cellulose acetate butyrate or propionate improves the gloss of the coating to a considerable extent.