Characteristics of Thermoplastics
Thermoplastics (also referred to
as thermo-engineering plastics) are high molecular weight polymers, which become
liquid upon heating and glassy solids on cooling. Bonding between molecules of
a polymer could be of varying strength, thus resulting in different type of
thermoplastics (such as polythenes that have weak vander-wall bonds, or
stronger dipole-dipole hydrogen bonds in nylon, or very strong stacked aromatic
ring bonds in polystyrene). Thermoplastics are mostly addition polymers
(meaning that they are formed by an addition reaction where smaller molecules
combine together to form a larger molecule) and are therefore chemically inert,
non-biodegradable and difficult to recycle.
Characteristics of Thermoplastics
Molecular Arrangement of
Thermoplastics
Thermoplastics can be broadly
classified as either being amorphous or semi-crystalline, based on their
molecular arrangement.
Amorphous thermoplastics are characterized
by their high brittleness and stiffness nature. They are also clear solids in
their normal state. However, they can undergo a transformation of their
molecular arrangement at high temperatures and on application of stress. Examples
of amorphous thermoplastics are polyamideimide, polyethersulphone,
polyetherimide, polyarylate, polysulphone, amorphous polyamide,
polymethylmethacrylate, polyvinylchloride, acrylonitrile butadiene styrene and polystyrene.
Semi-crystalline solids, on the
other hand, only partially exhibit such characterstics, given their partially
amorphous state. Examples of semi-crystalline are polyetheretherketone, polytetrafluoroethylene,
polyamide 6.6, polyamide 11, polyphenylene sulphide, polyethylene terephthalate,
high and low density polyethylene, polyoxymethylene and polypropylene.
Physical properties of
Thermoplastics
Specific gravity of various
thermoplastics typically varies from about 0.92 for polyybutylene to 1.72 for poly-vinylidene
fluoride.
The tensile yield strength (defined
as the engineering stress that causes irreversible deformation) of thermoplastics
however has a wide variance, with as low as 2.8 (*10^3 psi) for cross-linked
polythene, to as high as 8.0 (*10^3 psi) for polyvinyl chlorides.
Thermal conductivity of these
polymers can vary from 1.0 to 3.2 (Btu/ (h ft^2 C)).
Specific heat is in the range of
0.22 to 0.55 (kcal/kg C).
Characteristics of Specific
Thermoplastics
Acrylonitrile butadiene styrene
is a strong thermoplastic that is also resistive to most bases and acids.
However, chlorinated hydrocarbons can still corrode or damage this polymer. It
is usable only about 71C temperature, and is used in drainage and vent pipes.
Polybutylene and polyethylene
(normal as well as cross-linked varieties) are flexible polymers that find
their most common application in pressurized water systems. However, while polybutylene
doesn’t get affected by extreme water temperatures, polyethylene cannot be used
for hot water.
Polypropylene is a light-weight
polymer, and can be used till about 82C temperature, while also being highly
resistive to most acids, bases and solvents. It’s primary use in laboratory
plumbing.
Polyvinyl chloride is also a
strong and highly resistive plastic, with the maximum usable temperature being
60C. This finds applications in piping but should be avoided in hot water
applications. A variant polymer, chlorinated polyvinyl chloride, can however be
used for higher water temperatures (upto 82C).
Polyvinylidene fluoride is an
extremely strong and tough thermoplastic, which is also resistant and
non-abrasive. It can be used up to temperatures 138C, and its most common
application is in laboratory plumbing.
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