JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe - 103

the B-stage. According to Arrhenius'
law, a formulation curing at 120°C in
several minutes reacts at room temperature in several days. So most prepregs
are either not stable when stored at
room temperature and must be stored
at -18°C, or offer unsatisfactory long
curing times at elevated temperatures.

The PU-based solution
With Evonik's polyurethane-based
prepreg system, both issues can be
solved (Figure 1). Polyurethanes are
becoming more and more important in
the composite industry. They offer fast
curing cycles combined with very good
mechanical properties, not to mention
the excellent toughness of PU-based
composites over epoxy systems. But
being highly reactive, and thus fast
curing, makes these systems unsuitable
for prepreg systems, where a certain
latency is needed to ensure storage
stability.
Evonik's PU prepregs based on aliphatic diisocyanates Vestanat® IPDI
offer the required latency due to the
use of reversibly blocked isocyanates
via blocking-agent-free uretdione
chemistry. Aliphatic isocyanates can be
transferred into stable dimers (uretdiones, Figure 2); the back reaction into
the isocyanate usually starts at temperatures around 180°C. This temperature
can be lowered to about 120°C by the
use of suitable catalysts.
An uretdione of a diisocyanate has
four isocyanate groups in total. Two of
them are blocked in the uretdione ring,

Fig. 2: Uretdione formula and structure

while two isocyanate groups remain
free. When formulating this uretdione
with a stoichiometric amount of polyol
component to react all four isocyanate
groups, the overall reaction to the
cured polyurethane takes place in two
well-separated steps: the free isocyanate
groups instantly react to polyurethane
chains with blocked isocyanate groups
inside. This resembles the pre-polymer
or the B-stage. The second step can
only occur if the remaining isocyanate
groups in the uretdione rings are
unblocked - which happens only at
temperatures above 120°C.
The result is a prepreg which is truly
storage stable at room temperature
and still offers fast curing at elevated
temperatures.
However, the rate-limiting step for the
curing process is not the polyurethane
formation but rather the ring opening
reaction. This leads to curing times of
about 30 minutes at 140°C or 2 minutes
at 190°C. These times are strongly
related to the amount of catalyst added
in the formulation - as does the storage
stability. The fully catalyzed prepreg
is stable for about 10 weeks at room
temperature and even 4 weeks at 40°C.
Formulations with less catalyst cure
significantly slower (30 minutes at
180°C) but have significantly improved
storage stability of over 2 years at room
temperature.

Suitable for
automation...
The unique combination of fast curing cycles
with improved storage
stability is not the only
thing setting Evonik's
PU prepregs apart.
Another important
property with significant influence on the
composite manufacturing process is that the
B-stage pre-polymer is
solid at room temperature. This causes the

prepreg to be relatively form-stable and
non-tacky. Together with a relatively
low Tg of ~50°C, this offers a variety of
processing options for the PU prepreg.
By heating it moderately to 60-80°C,
the system becomes drapeable and
tacky, and thus can be handled like
any other prepreg, especially since the
curing reaction will not kick in at these
temperatures. This is important for
hand lay-up processes, which can take
several hours. For automated production of composites, this means the
prepreg can be formed thermoplastically at moderate temperatures. Only
in the final curing step is the thermoset
formed. So the prepreg can ideally be
used for preforming steps without the
need for additional binders and return
to the original solid, form-stable and
non-tacky state after cooling down
(Fig.3, left); drastically easing automated handling, i.e. with robots. So in comparison with RTM processing, which is
often used in the automotive industry
for large-volume applications, PU
prepregs hold two major processing advantages. First, the fabric does not need
any additional binder to be pre-formed.
This saves one processing step in the
production chain and the saved binder
does not influence the matrix system,
may it be in wetting the fibres or in the
curing reaction itself. The second major
advantage is that the infusion process
itself is no longer required. This is even
more important since this saves time in
the actual RTM press. Since the curing
reaction is comparably faster than state-

Fig. 3: PU prepreg preform (left) and pressed parts (middle, right)

No87 March 2014

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JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe

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