JEC COMPOSITES MAGAZINE - Issue #100 - Special JEC Asia - October 2015 - 78

polyurethane

Polyurethane prepregs for
cost-effective lightweight construction
For the first time ever, polyurethane (PU) prepregs suitable for
storage can be both produced and processed on an industrial
scale. These materials offer a cost-effective alternative to epoxy
prepregs as they deliver components with comparable mechanical
properties at significantly lower production costs.

he prepreg method is one of the established fibre composite manufacturing processes that also include
hand lay-up, infusion, resin transfer
moulding (RTM), pultrusion, filament
winding and the sheet/bulk moulding compound (SMC/BMC) process.
Prepreg is short for "pre-impregnated
fibres." The process comprises two stages.
In the first, rovings and fabric made of
glass and/or carbon fibre are impregnated with a reactive resin in a dip tank in
a continuous process. The wetted fibres
then undergo thermal conditioning and
consolidation before being transferred to
a storage-stable state where they are not
fully reacted (B state), wound onto rollers with separating foil and put into storage. In the second stage of the process,
the component itself is produced. The
prepregs are cut to size then pressed into
the required shape in a mould, generally
at an elevated temperature.
The benefit of the prepreg method
is that, unlike RTM technology for
instance, the separation of fibre impregnation and the pressing process makes
it possible to produce components with

extremely high, reproducible and consistent mechanical qualities. That said,
the relatively high manufacturing costs
that result from the two-stage process
are a drawback. In addition, commonly
used epoxy resin prepregs need to be
stored in freezers at temperatures down
to -18°C so that they do not gradually
react prematurely and become unusable.
Although epoxy systems that remain stable at room temperature for a longer time
are available, these are more expensive.

Two-stage curing reaction
with polyurethane
So far, aromatic polyurethane systems
have not been used in prepreg manufacturing as it was not possible to stop
the reaction between the resin starting
materials - isocyanate and polyol - at a

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defined point in order to obtain prepregs
with fully wetted fibres that are suitable
for storage. Covestro has now succeeded
in doing just that with its new casting
resin system from the Baydur® product
family. This system is based on polyurethane raw materials with varying reactive
OH functionalities. A special wetting
technology was developed using an
injection box (see Figure 1) that enables
the polyurethane reaction systems to be
thinly applied and the fibres completely
impregnated. The new polyurethane
prepregs based on aromatic isocyanates
can be stored at room temperature for up
to 30 days without reacting any further.
As production trials with development
partners have shown, these can be
manufactured on an industrial scale with
consistently high levels of quality.

Fig. 1: The injection box developed for the new polyurethane casting resin system enables reproducible,
controlled reinforcing fibre impregnation without air bubbles (Photo: Covestro)

78 jec composites magazine / No100 October 2015

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