Thermosets
Composites offer designers real
application advantages
Due to the intrinsic limitations of metals and thermoplastics, many industrial
designers have turned to high performance thermoset composites for
solutions.
hermoset composites often provide
more strength, dimensional stability,
and corrosion resistance than other commonly used materials, while also increasing design flexibility and manufacturing
efficiency. In addition, thermoset composites utilize strong molecular bonds
that help the materials maintain excellent
properties during prolonged exposure to
chemicals and temperature.
Composite materials consist of fiber
reinforcement in a polymer resin. The
fiber provides strength and stiffness,
while the resin protects the fibers and
gives the material its shape. Composite
reinforcement is typically fiberglass, but
high-strength fibers such as aramid and
carbon are used to meet demanding performance requirements.
The properties of a composite can be
changed by varying the type and quantity of its ingredients. Fiber type, length,
and mix proportion help determine
properties such as strength and rigidity.
In addition, resin characteristics can be
changed to provide the desired processability, durability, heat resistance, and
corrosion resistance.
Exposure to thermal energy causes
the formation of three-dimensional covalent bonds between the polymer molecules. This process, known as crosslinking, is irreversible. This means that
crosslinked materials cannot be remelted
and reshaped.
Users might choose vinylester resin
for corrosion-resistant products, epoxy
for high-strength applications, or polyester when overall performance and cost
are the driving factors.
As for reinforcement, many types of
glass fiber can be used in thermosets,
depending on the molding process and the
product’s strength requirements. Glass
A thermoset crosslinked
covalent bond provides
higher strength than a
typical thermoplastic
monomer bond (left).
reinforcement options
include chopped strand, mat
with random fiber orientation, light textile fabrics,
heavy woven materials,
knitted materials, and unidirectional fabrics.
BMC and SMC
A significant portion of the
world’s thermoset production comes in the form of
BMC and SMC. In BMC
(bulk molding compound),
a resin, fiber reinforcement,
and several other ingredients blend to form a viscous, putty-like material. By weight,
BMC normally includes 5-25% reinforcement, which typically consists of
chopped-strand glass fibers measuring
1/32-1/2 inch (. 75-12. 7 mm) in length.
BMC is suitable for compression,
transfer, or injection molding. When BMC is injection
molded, cycles can be as
fast as 10 sec/mm of part
thickness.
Depending on the application, BMC variations can
provide tight dimensional
control, flame and track
resistance, superior dielectric strength, corrosion and
stain resistance, excellent
mechanical properties, minimal shrink, and color stability. Available in numerous colors, BMC also provides surfaces receptive to
powder coating, paint, and other coating technologies.
SMC (sheet molding compound),
while similar in chemistry to BMC, is
manufactured by a much different
process. SMC is produced in sheets that
Thermoset composites are
finding increasing application in healthcare because
of their high chemical
resistance to aggressive
cleaning fluids.
