JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016 - 27

Recent advances in powder coating technology for the RTM process

Fig. 1: (a) Thickness comparison, liquid/powder gelcoat

(b) microscopic section of a powder gelcoat

Tab. 1: Ecogel target values vs. results obtained for powder gelcoat developments

Properties

Target values

Obtained values

Peak cure temperature (°C)

110-130°C

130°C

Gel time (at 120°C)

20-60 seconds

Around 60 seconds

Storage stability

Tg ≥ 40°C

Electrical conductivity: resistivity Ω cm

≤ 10+5

Chemical resistance

As good as high-quality
liquid gelcoat

Cross-cut tests

104-105

UV resistance
Curing time

50% lower than liquid gelcoat

Fig. 2: The Ecogel RTM process

cost-effective powder gelcoat formulations suitable for case studies requirements. Promising results were obtained,
fulfilling the Ecogel targets regarding gel
time, storage stability, electrical conductivity, adhesion, chemical resistance, UV
resistance, temperature to film formation
and curing time (Table 1).
Moreover, the incorporation of electrically-conductive additives in the powder
gelcoat formulation allows the development of electrically-conductive gelcoats
that will contribute to the growth of the
demand for electrostatic-painted plastic
parts in the automotive sector. In this
case, the conductive powder gelcoat is
used as in-mould primer for painting,
avoiding the secondary phases needed

when working with plastics before applying the final coat. In the Ecogel Cronos
project, a fast-curing electrically-conductive powder gelcoat (electrical surface
resistivity around 104-105 Ω cm) was developed based on the use of conductive
carbon fillers.
Carbon fibres as a resistivity
heating element
Taking all this into consideration, the current RTM process needs to be modified
in order to integrate powder gelcoat technology in a suitable way to obtain a highproductivity RTM line.
The proposed RTM process (Fig 2) consist in introducing replaceable/reusable
electrically-conductive temperature-controlled skins that can be prepared offline.
No109 November - December 2016

This makes it possible to apply the release
agent, gelcoat and fibre on the skin while
another one is being injected. This way,
production can be increased for a relatively small additional investment compared
to RTM tooling. The skin design should
include an electrically-conductive surface and heated systems to fully optimise
production, allowing the introduction of
powder gelcoats in RTM production.
The use of innovative powder gelcoats in
RTM production is only possible if new
moulding technologies are developed.
The technology used to heat up the powder gelcoat is one of the key elements
improving productivity in the Ecogel
Cronos project. The moulds for the project are heated using the Fibretemp®
technology. This technology is based on
the fact that carbon fibres are electrically
conductive. They heat up when connected to an electric power supply, very much
like a copper strain. Woven fabrics were
used to act as a structural component as
well as a heating element in the mould's
composite laminate. Due to the narrow
weave of the fabric used, heat distribution
is very consistent across the whole surface
of the heated mould. Because it is part of
the mould structure, the heating layer
is located very close to the surface. This
makes it possible to heat up the mould
very quickly. A special development of
this heating technology in the Ecogel
Cronos project was the development of
RTM Light moulds in order to test the
technology at high temperature and in
complex shapes. Two different moulds
were prepared for the Ecogel case studies.
Different resin systems were employed to
obtain a high-quality surface and homogeneous heating at high temperature. The
whole manufacturing process was similar
to standard moulding processes. Only the
layup of the electrically heating carbon
layers and the electrical contacts for the
integrated layers were added. Moreover,
an insulated hood was designed to improve film formation during powder
gelcoat curing. The consistent heat distribution and electrical conductivity
properties on the mould surface were essential prerequisites to apply and cure the

/ jec composites magazine

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016