A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat (generally above 200 °C (392 °F)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives.
Others are solids like that of the molding compound used in semiconductors and integrated circuit (IC). Once hardened a thermoset resin
cannot be reheated and melted back to a liquid form.
According to IUPAC recommendation: A thermosetting
polymer is a prepolymer in a soft solid or viscous state that changes irreversibly
into an infusible, insoluble polymer network by curing. Curing can be induced
by the action of heat or suitable radiation,
or both. A cured thermosetting polymer is called a thermoset.
Process
The curing process transforms the resin into
a plastic or rubber by a crosslinking process. Energy and/or catalysts are added that cause the molecular
chains to react at chemically active sites (unsaturated or epoxy sites, for
example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger
molecular weight, resulting in a material with a higher melting point. During the reaction, the
molecular weight has increased to a point so that the melting point is higher
than the surrounding ambient temperature, the material forms into a solid
material.
Uncontrolled reheating of the material
results in reaching the decomposition temperature before the melting point is
obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. This
implies that thermosets cannot be recycled, except as filler material.
Properties
Thermoset materials are generally stronger
than thermoplastic materials due to this three
dimensional network of bonds (cross-linking), and are also better suited to
high-temperature applications up to the decomposition
temperature. However, they are more brittle. Many thermosetting polymers are
difficult to recycle.
Examples
Some examples of thermosets are given below:
§
Polyester fibreglass systems: sheet
molding compounds and bulk molding compounds)
§ Vulcanized rubber
§
Duroplast,
light but strong material, similar to bakelite used for making car parts
§
Urea-formaldehyde foam used in plywood,
particleboard and medium-density fiberboard
§
Melamine resin used on worktop surfaces
§
Epoxy resin used as the matrix component in many fiber reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)
§
Polyimides used in printed circuit boards and in
body parts of modern airplanes
§
Cyanate esters or polycyanurates for
electronics applications with high demands on dielectric properties and high
glass temperature requirements in composites
§
Mold or mold runners (the black plastic
part in integrated circuits or semiconductors)
Some methods of molding thermosets are:
§
Reactive injection molding (used for objects such as milk bottle
crates)
§
Extrusion
molding (used for
making pipes, threads of fabric and insulation for electrical cables)
§
Compression molding (used to shape most thermosetting
plastics)
§
Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines,
emblems as well as production and replacement parts)
MATERIALS
Thermosets
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