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CATEGORIES (articles) > Chassis & Bodywork Construction > chassis construction > Thermoplastics as used in Powder Coating

Thermoplastics as used in Powder Coating


A thermoplastic is a material that is plastic or deformable, melts to a liquid when heated, and freezes to a brittle, glassy state when cooled sufficiently. Most thermoplastics are high molecular weight polymers whose chains associate through weak van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers are contrasted with thermosetting polymers (Bakelite; vulcanized rubber), which, once formed and cured, can never be remelted and remolded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers, such as polyethylene and polypropylene.

Temperature dependence

Thermoplastics are elastic and flexible above their glass transition temperature Tg, specific for each one. This is the midpoint of a temperature range, unlike the sharp freezing point of a pure crystalline substance like water. Below a second, higher melting temperature, Tm (also the midpoint of a range), most thermoplastics have crystalline regions, alternating with amorphous regions in which the chains approximate random coils. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity. Above Tm, all crystalline structure disappears and the chains become entirely random coils. As the temperature increases above this, the viscosity gradually decreases without any further phase change. Thermoplastics can go through both freezing and melting cycles repeatedly, but it is the fact that they can be reshaped upon reheating that gives them their name. Animal horn, made of the protein α-keratin, softens on heating and is somewhat reshapable, and may be regarded as a natural, quasi-thermoplastic material. Elasticity does not mean that a material is particularly stretchy; e.g., nylon rope, fishing line and guitar strings.

Thermoplastics are useful between Tg and Tm, a temperature range in which they are neither brittle nor liquid. If a plastic with useful properties has a Tg which is too high, it can often be lowered by adding a low-molecular-weight plasticizer to the melt before forming and cooling. A similar result can sometimes be achieved by adding non-reactive side chains to the monomers before polymerization. These methods make the polymer chains stand off a bit from one another. Plastic automobile parts often cracked in cold winter weather before the introduction of plasticizers. Another method of lowering Tg (or raising Tm) is to incorporate the original plastic into a copolymer or composite material. Modestly vulcanized natural and synthetic rubbers are elastomeric thermosets, but are not thermoplastic. Each has its own Tg, and will crack and shatter when cold enough so that the polymer chains can no longer move relative to one another; but they have no Tm, and will decompose at high temperatures rather than melt.

List of thermoplastics

  • Acrylonitrile butadiene styrene (ABS)
  • Acrylic
  • Celluloid
  • Cellulose acetate
  • Ethylene vinyl alcohol (E/ VAL)
  • Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE)
  • Ionomers
  • Liquid Crystal Polymer (LCP)
  • Polyacetal (POM or Acetal)
  • Polyacrylates (Acrylic)
  • Polyacrylonitrile (PAN or Acrylonitrile)
  • Polyamide (PA or Nylon)
  • Polyamide-imide (PAI)
  • Polyaryletherketone (PAEK or Ketone)
  • Polybutadiene (PBD)
  • Polybutylene (PB)
  • Polybutylene teraphthalate (PBT)
  • Polyethylene terephthalate (PET)
  • Polycyclohexylene dimethylene terephthalate (PCT)
  • Polycarbonate (PC)
  • Polyketone (PK)
  • Polyester
  • Polyethylene/Polythene/Polyethene
  • Polyetheretherketone (PEEK)
  • Polyetherimide (PEI)
  • Polyethersulfone (PES)- see Polysulfone
  • Polyethylenechlorinates (PEC)
  • Polyimide (PI)
  • Polymethylpentene (PMP)
  • Polyphenylene oxide (PPO)
  • Polyphenylene sulfide (PPS)
  • Polyphthalamide (PPA)
  • Polypropylene (PP)
  • Polystyrene (PS)
  • Polysulfone (PSU)
  • Polyvinyl chloride (PVC)




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