JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 48

RTM
rials involves
electromagnetic
curing processes.
Because of the
high frequency
of microwaves,
transmission
through free
space is possible.
Therefore, threedimensional
curing is possible
without specific
electrodes.

Fig. 2: Infrared thermograph of a
microwave-cured polyester part

homogeneous exothermal reaction may
appear along the thickness of the part.
This results in curing variations, externalinternal solidification, thermal degradation and residual stresses related to the
process. Thus, it is necessary to work at
low temperature and low heating speed.
This involves long curing cycles and
external-internal curing that produces a
tension and contraction of the resin, and
can cause delamination and/or matrix
breaking.
Microwaves are electromagnetic radiation in the frequency range from 300
MHz to 300 GHz. They are mainly used
for radar systems, communication technologies, plasma generation and material
heating. 2.45-GHz microwaves are
the most commonly used in industrial
applications. Microwave curing of mate-

Microwave
power radiation Fig. 3: AIMPLAS´ facilities for microwave curing of composites
is known as
an incredibly rapid and selective, clean
for microwave processing. However,
and safe, versatile and energeticallymany polymers contain groups that form
convenient heating method. It can be
strong dipoles. The efficiency of microapplied to polyester resin to accelerate
wave radiation interactions with polymer
polymerisation processes. Nowadays,
materials depends on the dipole strength,
this technology is used industrially to
its mobility and mass.
cure thermoset elastomers (SBR, NBR,
natural rubber, etc.) in a continuous
Thermoset resins in general are polar
extrusion process.
materials which are capable of absorbing the microwaves applied to them.
In contrast to conventional thermal sysThe absorption of microwaves by the
tems, the main mechanism of microwave
materials is dependent on their dielectric
absorption in a polymer is the reorienproperties, leading to activation of the
tation of dipoles in the imposed electric
polar groups, and thereby to a heating
field; due to dipole movement the energy effect. The penetration capability of the
is dissipated as if heated. Energy is transmicrowaves allows volumetric heating of
mitted volumetrically, so temperature
the material, which is not possible with
gradients are reduced and internal-exterthermal heating (Figure 1). Dielectric
nal curing is possible, as can be observed
additives can be employed to improve
in Figure 1. In Figure 2, an infrared therthe interaction between microwaves
mography image shows different steps of
and resin. Dielectric additives are polar
polyester resin curing using microwave
substances than can be added to the resin
radiation. Volumetric heating can be
mixture in very low proportions (up to
observed along the curing time.
2.5%) to absorb microwave radiation and
transform it into heat, which is transmitNot all polymer materials are suitable
ted to the resin.

More information
AIMPLAS is a Centre for Innovation and Technology (CIT) recognised by the Spanish Interministerial
Committee of Science and Technology. The centre is a non-profit research association working at the
national and international level.
Since its foundation, AIMPLAS has been striving to promote direct contact with companies in all
sectors related to the plastic industry in order to detect their needs and determine the required
actions to meet them.
The centre provides a customised and integral solution for companies belonging to the sector
through coordination of technological services (testing and laboratory services) and R&D projects.

48 jec composites magazine / No78 January - February 2013

Processing and performance
Research on microwave curing (Figure
3) was carried out to highlight its benefits for composite processing. Multifunctional polyester, vinyl-ester and
epoxy resins suitable for RTM process
and microwave curing were modified to
have a low viscosity for optimal filling of
the mould. After the filling process was

JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

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