JEC COMPOSITES MAGAZINE - Issue #104 - April/May 2016 - 47

microwave

Susceptors improve microwave curing
technology for thermosetting resins
The use of microwaves to initiate the curing process of resins is well known. Liquid resins that cure
at high temperatures need an increase in temperature to start the polymerization reaction and finally
become a solid material. Several methods can be
used to increase the temperature, such as conventional heating, ultraviolet radiation, electron beam
or microwave radiation. Microwave (MW) curing
has two main advantages: 1) heating is volumetric
and homogeneous, and 2) curing time is reduced to
seconds (so that VOC emissions are also reduced).

the resin in order to reduce the polymerization time and the
die length.
The main microwave absorption mechanism in a polymer is
the reorientation of dipoles in the imposed electric field. The
reorientation makes the polymer's molecules vibrate, thus
heating the material. As in a home microwave oven, the materials with the greatest dipole mobility will exhibit the most
efficient coupling [1].

Fig. 1: Aeronautics, an application sector for microwave curing technology

Microwave-absorbing additives (or susceptors)
noRa laRDiés miazza
ReseaRcheR at comPosites DePaRtment
aimPlas (Plastics tecnology centRe)

he applications of microwave curing technology include
the fabrication of composite parts where the resin cures
at high temperature in sectors like railroad, automotive,
aeronautics or construction. Due to the advantages of this
technology, the main application is the manufacture of largesized composite parts such as wind turbine blades.
Aimplas developed and studied microwave curing technology
in several projects such as MAC-RTM, Wavecom, Code or
the ongoing Coaline project. The Coaline European project
(www.coaline.eu) received funding from the European Union
Seventh Framework Programme (FP7/2007-2013) under
grant agreement no.609149. The project started in September
2013 and is coordinated by Aimplas. Its aim is to drastically
improve the pultrusion process as the gelcoat will be injected
inside the mould, resulting in an in-line, clean one-stage
process. In this project, microwave radiation is used to cure

Microwave processing can be used over a broad range of polymers and products, including thermoplastic and thermosetting resins, rubbers and composites. Many polymers contain
groups that form strong dipoles (epoxy, hydroxyl, amino,
cyanate, etc.), however not all polymer materials are suitable
for microwave processing.
In fact, many polymer systems that are candidates for microwave processing (polyester, vinylester, etc.) are typically not
conductive and have low polarity. For this reason, conductive
particles and fibres or un-reactive organic additives with high
electronegativity should be added to the polymer or resin to
improve microwave absorption. This mixture creates a complex combination of physical effects that may be included to
aid processing or to modify the mechanical, physical, or optical properties. The presence of these inclusions can strongly
influence the way the composite material interacts with the
microwave radiation. The effect of heating promoters depends
on the size, geometry, concentration and electric resistance of
the additive or susceptor. It is also very important to achieve
a good and homogeneous dispersion of the susceptor in the
polymeric matrix.
Some functional groups with a significant dipolar moment can
absorb microwave radiation. This dipolar moment represents
an electronic cloud distortion between two atoms with difNo104 April - May 2016 /

jec composites magazine 47

http://www.coaline.eu

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #104 - April/May 2016