Thermoplastic
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 differ from
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.
Thermoplastics are elastic and flexible above a
glass transition temperature Tg, specific for each one â€" the midpoint of a
temperature range in contrast to 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, as is also the case for non-thermoplastic
fibrous proteins such as
silk. (Elasticity does not mean they are particularly stretchy; e.g., nylon
rope and
fishing line.) Above
Tm all crystalline structure disappears and the chains become randomly interdispersed. As the temperature increases above
Tm,
viscosity gradually decreases without any distinct
phase change.
Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name.
Animal horn, made of the
protein α-keratin, softens on heating, is somewhat reshapable, and may be regarded as a natural, quasi-thermoplastic material.
Thermoplastics are useful between
Tg and
Tm, a temperature range in which most are neither brittle nor liquid. If a plastic with otherwise desirable properties has too high a
Tg, it can often be lowered by adding a low-molecular-weight
plasticizer to the melt before forming (
Plastics extrusion;
molding) and cooling. A similar result can sometimes be achieved by adding non-reactive
side chains to the
monomers before
polymerization. Both methods make the polymer chains stand off a bit from one another. Before the introduction of plasticizers,
plastic automobile parts often cracked in cold
winter weather. Another method of lowering
Tg (or raising
Tm) is to incorporate the original plastic into a
copolymer, as with
graft copolymers of polystyrene, or into a
composite material.
Although modestly vulcanized natural and synthetic rubbers are stretchy, they are
elastomeric thermosets, not thermoplastics. Each has its own
Tg, and will crack and shatter when cold enough so that the
crosslinked polymer chains can no longer move relative to one another. But they have no
Tm and will decompose at high temperatures rather than melt. Recently,
thermoplastic elastomers have become available.
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Acrylonitrile butadiene styrene (ABS)
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Acrylic*
Celluloid*
Cellulose acetate*
Ethylene-Vinyl Acetate (EVA)
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Ethylene vinyl alcohol (EVAL)
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Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE)
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Ionomers
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Liquid Crystal Polymer (LCP)
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Polyacetal (POM or Acetal)
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Polyacrylates (Acrylic)
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Polyacrylonitrile (PAN or Acrylonitrile)
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Polyamide (PA or Nylon)
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Polyamide-imide (PAI)
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Polyaryletherketone (PAEK or Ketone)
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Polybutadiene (PBD)
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Polybutylene (PB)
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Polybutylene terephthalate (PBT)
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Polyethylene terephthalate (PET)
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Polycyclohexylene dimethylene terephthalate (PCT)
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Polycarbonate (PC)
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Polyketone (PK)
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Polyester*
Polyethylene (PE)
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Polyetheretherketone (PEEK)
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Polyetherimide (PEI)
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Polyethersulfone (PES)- see
Polysulfone *
Polyethylenechlorinates (PEC)
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Polyimide (PI)
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Polylactic acid (PLA)
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Polymethylpentene (PMP)
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Polyphenylene oxide (PPO)
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Polyphenylene sulfide (PPS)
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Polyphthalamide (PPA)
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Polypropylene (PP)
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Polystyrene (PS)
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Polysulfone (PSU)
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Polyvinyl chloride (PVC)
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Spectralon*
Thermosetting plastic
