JEC COMPOSITES MAGAZINE - Issue #100 - Special JEC Asia - October 2015 - 79

Faster reaction,
shorter cycle times
When it comes to manufacturing and
processing prepregs, polyurethane offers a variety of advantages over epoxy
resins in terms of production costs,
energy savings, environmental and
resource conservation, and processing
flexibility. In addition, it also delivers
improved component properties. The
significantly lower initial viscosity of the
polyurethane reaction resin (less than
100 mPa*s) enables straightforward and
faster wetting of the reinforcing fibre
when manufacturing prepregs.
The polyurethane prepregs also react
faster during pressing than epoxy
systems, meaning that demoulding can
be performed earlier with shorter cycle
times. The benefit for the processor, in
addition to enhanced productivity, is
that fewer prepreg presses are required,
thereby lowering investment costs.
The chemical reaction when pressing
polyurethane prepregs is also markedly less exothermic than with epoxy
systems (see Figure 2). This prevents
heat damage or, worse still, burns, even
when working with thicker component
sections. This greatly extends the field of
possible applications for polyurethane
prepregs. Due to higher elasticity and
notched impact strength, polyurethane
prepregs are less inclined to brittle
fracturing arising from dynamic stresses

Fig. 2: The new polyurethane casting resin system
has significantly lower exothermic reactions than
corresponding epoxy systems (EP = epoxy) (Photo:
Covestro)

and therefore do not suffer from fatigue
as quickly as epoxy systems. In practical terms, components can therefore be
expected to last longer.

Higher glass transition
temperature makes annealing
unnecessary
Following forming, curing and demoulding at around 140°C, polyurethane
prepregs have a higher glass transition
temperature than traditional epoxy systems and are therefore more resistant to
high temperatures. They exhibit the same
mechanical properties as popular epoxy
systems but unlike these, the pressed
components do not require subsequent
heat treatment (annealing/curing) at
over 100°C. In addition, pressing can
usually be performed at lower temperatures. Both deliver noticeable energy
savings that help protect the environment and the climate. Yet, if polyurethane prepreg components are subsequently annealed, the glass transition
temperature of the composite increases
to as much as 175°C, and this is accompanied by an improvement in mechanical
properties.

Practical example - leaf springs
with 25% lower manufacturing
costs
The industrial-scale processing behaviour of the new polyurethane prepregs
was tested in conjunction with IFC
Composite GmbH, Haldensleben,
Germany, employing the type of mould
that could be used for epoxy prepreg
leaf springs for a van, for example. The
use of polyurethane prepregs led to a
manufacturing cost saving of up to 25%
(see Figure 3). Key factors in this were
shorter cycle times and reduced energy
consumption resulting primarily from
lower pressing temperatures and the
elimination of component annealing.
Pressing times could be reduced by
around 40%. The lower costs in prepreg
production, the reduced investment
required for presses and the elimination
of expenditures for prepreg freezers and
annealing ovens also played a part.

Fig. 3: Prepreg pressing system at IFC Composite
GmbH for series production of van leaf springs
(Photo: IFC Composite GmbH)

Outlook: Circuit boards and SMC
In addition to leaf springs, interesting
applications for the new polyurethane
prepregs include printed circuit boards
(PCB). These were previously the
domain of epoxy prepregs and, for cost
reasons, are manufactured in large quantities in highly automated processes. This
is an area where polyurethane prepregs
could help to significantly reduce
production costs further, and related
development projects have already been
launched.

Potential for SMC
Transferring the concept to the field of
SMC component manufacturing is also
a further option for polyurethane casting
resins. For instance, the low initial viscosity enables efficient impregnation of the
fibre mats used in the SMC method. The
faster polyurethane reaction should lead
to shorter cycle times and thus more
cost-effective component manufacturing. The lower exothermic energy of
the polyurethane reaction could pay
particular dividends in the manufacture
of large, thick-walled components such
as truck panels.
Combination with carbon fibres delivers
yet another quantum leap in mechanical
properties, meaning that polyurethane
is poised to tap into new markets and
applications in the future. n

No100 October 2015 / jec

More information:
www.covestro.com

composites magazine 79

http://www.covestro.com

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #100 - Special JEC Asia - October 2015