Thermoplastic Powder Coating

Thermoplastic Powder Coating

Powder coating based on thermoplastic polymers employ relatively simple methods of manufacturing and application, no involvement of complicated curing mechanisms, raw materials that in many cases belong to commodity polymers, acceptable properties for many different applications etc. contributed to the popularity of these coatings in the market in very less time.

At the same time, however, weaknesses such as high temperature of fusion, low pigmentation level, poor solvent resistance and bad adhesion on metal surfaces etc. were also there. And due to such problems inherent to the thermoplastic powder coatings were successfully overcome later on by the thermosetting powders which very quickly took the largest part of the market.

Advantages of thermoplastic powder coatings

  1. Excellent solvent resistance (polyolefins),
  2. Outstanding weathering resistance (polyvinylidene fluoride),
  3. Exceptional wear resistance (polyamides),
  4. A relatively good price/performance ratio (polyvinyl chloride)
  5. High aesthetic appearance (polyesters).

These advantageous properties combined with the simplicity of the system, created a considerable market share for thermoplastic powder coatings.

  • By the use of coordination catalysts, or the so-called Ziegler-Natta catalysts, polypropylene and low pressure polyethylene are produced on an industrial scale, being two of the most important thermoplastic polymers of this century. They are the main representatives of the polyolefins used in the manufacture of powder coatings.
  • Being non polar, high molecular weight crystalline polymers, with carbon-carbon bonds in the polymer backbone, they possess a unique balance of toughness, chemical and solvent resistance.

2.1 Polyethylene:

  • The empirical formula of polyethylene –(CH2–CH2).
  • By the discovery of Ziegler in 1953 polyethylene was produced in an astonishingly simple process at atmospheric pressure and almost room temperature by using alkyl aluminium compounds combined with titanium tetrachloride as catalyst. It offered polymer that was highly oriented with a high degree of crystallinity, almost without side chains, and properties that differed very much from those obtained by the high pressure process.

Important differences between different polyethylene types with respect to their application in powder coating

  • When the density or crystallinity of polyethylene is reduced, then the impact resistance, cold flow, tackiness (blocking), clarity and permeability increase. On the other hand, a reduction in density produces a decrease in stiffness, tensile strength, hardness, abrasion resistance, brittleness temperature, softening point, fusion temperature, chemical resistance and shrinkage.
  • The high density grade Polyethylene is suitable for industrial application, while the low density is used for domestic applications.

2.2 Polypropylene(PP) :

  • Polypropylene is a thermoplastic “addition polymer” made from the polymerisation of propylene monomers.
  • The polymerization of propylene by the anionic coordination mechanism produces a stereo regular crystalline polymer with a melting point high enough from a practical stand point. Which became possible with the discovery of the Ziegler-Natta catalysts.
  • It has a number of desirable properties that make it a versatile material for production of powder coatings.
  • It possesses
    • good surface hardness and is scratch and abrasion resistant.
    • It is essentially unaffected by most chemicals and possesses excellent solvent resistance.
    • It is somewhat more brittle than polyethylene at normal temperature.
  • Considerable amounts of polypropylene are sold as copolymer containing 2 to 5 % of ethylene. The resulting polymer has increased flexibility, toughness, impact resistance and clarity, and somewhat lower melting point.
  • Because of the carbon-carbon bonds in the molecular chains, polyethylene and polypropylene powder coatings exhibit very good chemical resistance.
  • In the case of polypropylene powder coatings, high gloss can be obtained by water-quenching immediately after fusion.
  • Polyolefinic powder coatings are resistant to a variety of hostile environments. Solvent resistance at room temperature is excellent against all common solvents used in industry. Only very strong oxidizing agents, such as nitric acid, will attack the coatings at room temperature.
  • The main use of polyethylene and polypropylene powders, however, remains in the field of functional coatings with excellent protective properties which include appliance parts, such as dishwasher racks, refrigerator racks, washer tubes, metal containers, drums, pipes, electrical applications and industrial equipment components.
Problems with Polyolefinic based powder coatings:
  • Being inert materials, polyolefins have poor adhesion to metal or other substrates.
  • Polyethylene powder coatings are even more problematic with respect to adhesion properties than polypropylene powders. Priming of the surface is usually necessary prior to application of the polyethylene powder coating, since the adhesion promoting resins do not perform as well as in case of polypropylene.
  • Powder coatings based on polyolefin polymers have a low aesthetic appearance, and, therefore, are used mainly for protective rather than decorative purposes.