JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014 - 47

be lower than the curing temperature of the epoxy matrix. If the
yarn does not completely melt, then the improvement in impact
toughness falls off sharply. Since the cross-linking temperature
of the RTM systems approved for aircraft applications is around
180°C, thermoplastics with a relatively high melting point are
used, which scarcely affect the glass transition temperature (Tg)
of the cured epoxy matrix.
In the case of isothermal processing at lower temperatures,
which is more typical of resin systems for automotive applications, it is more difficult to secure the required increase in
impact toughness, and this is the object of current research.
Fig. 4: The left hand side shows the effect of PA12 addition on delamination
for the two resin systems, while the right-hand side shows how PA12 affected compressive strength after impact (CAI)

Samples cut out of the laminates to 100 x 150 x 4 mm were
struck in a drop weight impact tester according to the CAI
standard, in the centre and normal to the surface, with an
absorbed energy of 30 Joules. After the depth of penetration
had been determined, the samples were subjected to ultrasonic
testing, the degree of delamination being assessed by the halfvalue method (Figure 3). The samples were then mounted on
end in a standardised jig and subjected to increasing vertical
pressure until failure in order to evaluate the residual compressive strength after impact.
Figure 4 shows the striking reduction in the delaminated area:
the thermoplastic laid scrim leads to a reduction of up to 80%,
independently of the resin system. The residual compressive
strength shows a similar improvement: with RTM6, the thermoplastic laid scrim provides a 28% increase, while with Cycom
890 the increase is even greater at 69%.
The effect of the addition of polyamide 12 on the epoxy resin's
glass transition temperature (Tg) was investigated by differential scanning calorimetry. This showed that for Cycom 890
with 20% PA12 by weight, the Tg fell by 3% to 204°C, while for
RTM6 there was no change at all, the Tg remaining at 218°C.
The results of testing for interlaminar shear strength after impact
(ILSS) are not so favourable. In this case, the samples show a
clear reduction in shear strength, ranging from 20% (RTM6) to
45% (Cycom 890) at an elevated temperature of 120°C.

Conclusions and findings
The experimental results demonstrate in the most striking way
that the thermoplastic laid scrim significantly reduces the extent
of the damage to the fibre-reinforced samples. This laid scrim,
initially a semi-finished textile, is transformed into an integral
part of the resin during the RTM process, increasing the fracture toughness of the otherwise brittle resin matrix.
Optimal linking of the copolyamide with the epoxy matrix is a
decisive factor, and this takes place only when the thermoplastic
melts during the cross-linking of the thermosetting matrix. It
follows therefore that the melting temperature of the yarn must

Further outlook
Despite these outstanding results, there is still much to be done
before thermoplastic laid scrims are commonly used in the
manufacture of structural parts for aircraft. The biggest obstacle
is to get the new material system officially approved. However, it
is an advantage in this regard that, up to now, few resin systems
suitable for the RTM process have been accepted and approved
for use in aircraft, which limits the number of possible material
combinations that will require such acceptance and approval.
From the standpoint of textile technology, integrating thermoplastic yarn in a carbon fibre fabric presents no great challenge.
The thermoplastic laid scrim structure can either be prefabricated (for instance by BAFATEX Bellingroth GmbH & Co. KG
Germany) and paid off from the roll into the fabric, or else the
thermoplastic can be spooled off as yarn and integrated directly
as the fabric itself is being produced.
As mentioned in the introduction, epoxy resins are typically
used in the RTM process. However, various raw material manufacturers recently begun to propose injectable polyurethane
resin systems. A special feature of a matrix of this material is -
among others - its high fracture toughness, due to the specific
interactions between the polyurethane molecules, which also
result in the chemical cross-linking. This means that innovative
resin systems themselves are opening up new ways of producing
impact-resistant fibre-reinforced components. How far and with
what benefits can these systems in turn be modified by integrating thermoplastics remains to be seen. n

Main features
Integrating a thermoplastic laid scrim of polyamide 12 into the interlaminar layers of carbon fibre fabrics has the following advantages:
- Outstandingly good impact toughness of the epoxy matrix
- Binding effect in preforming and debulking
- Improved permeability during resin infusion
- No reduction in electrical conductivity
- Possibility of integrating other additives into the resin matrix

More information:
www.iwk.hsr.ch
Contact:
mhenne@hsr.ch

No90 June - July 2014 /

jec composites magazine 47

http://www.iwk.hsr.ch

JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014