JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013 - 57

applied after attaining the matrix’s molten state. The fifth, and
final, phase is used to cool the laminate, which is also performed
with a ramp of 10K/min. The cooling rate influences the composite’s degree of crystallinity, which turns out lower the faster
the pressing machinery is cooled. Low degrees of crystallinity
cause a decrease in tensile strength and dimensional stability at
high temperatures [12]. In order to avoid the growth of air locks
in the thickness direction and ensure dimensional accuracy, the
applied pressure is maintained during the cooling phase. Additionally, the interior air pressure can be modified with a vacuum
pump. The standard value of choice for the production of FRP
sheets is 0.05 MPa.

Fig. 7: Direct hybrid yarn – thread cross section and manufacturing schematics (left) Microsection of Twintex® TR PP 60 B 1870 T thread cross section
(right) Hybrid yarn manufactured by direct process (Source: IPF e. V.)

Results and discussion
Homogeneous distribution of hybrid yarns
Many optimization measures were taken to minimize thread
strain during transportation of the filament yarns to the commingling zone. Based on the experience gained in the manufacture of hybrid yarns with air texturing, the following process
parameters were ascertained as optimal for GF/PP manufacture
at TUD:
- Component delivery speed GF: 102 m/min; PP thermoplastic: 104.5 m/min
- Hybrid yarn take-up speed: 100 m/min
- Nozzle design: type LD 5.05 (TEMCO)
- Air pressure: 0.4 MPa
The machine settings were selected to achieve a reinforcement
thread ratio of at least 60 per cent mass fraction. For the GF/
PP hybrid yarns, the assay of reinforcement thread ratios was
based, in accordance with DIN EN ISO 1172, on a one-hour
incineration at 625° C, for which representative samples were
stored in a Nabertherm-controller muffle furnace until reaching
constant weight. The thread cross sections were sampled in order
to allow for comparison
of the commingling
hybrid yarns and spreadmingle hybrid yarns. The
main focus was on the
assessment of the mixing
rate of reinforcement
and matrix component.
Figure 6 illustrates the
excellent mixing grade of
the hybrid yarn variant
Fig. 6: Micro-sections of the thread cross
(PP) obtained with the
sections of the GF/PP hybrid yarn made with
above parameters.
the commingling process
The mixing rates attained are comparable to those of direct hybrid yarns. Figure 7
shows the homogenous mixing and the open architecture of a
commercially available Twintex reference yarn designated TR
PP 60 B 1870 T, achieved by interlinking the spinning of thermoplastic filaments and glass fibre. The narrow product range
with regard to hybrid yarn fineness and component ratio is a

disadvantage of this technique. So far, Twintex technology has
been restricted to the mixing of glass fibre and polypropylene.
It is unrealistic to find a fully homogeneous distribution, as this
is an idealized view. Furthermore, this condition is not guaranteed to offer the best performance. On the one hand, the matrix fibres are a protection against abrasion. On the other hand,
it would be ideal for a matrix filament to lie beneath every glass
filament, to ensure best impregnation. For best performance,
the glass filaments have to be aligned straight in the hybrid yarn.
However, due to the manufacturing process and the shrinking
of the matrix material, a compromise is needed.
Tensile force testing of GF/PP hybrid yarns
The stresses placed on the hybrid yarns during processing are
directly influenced by the composite’s mechanical properties.
Therefore, it is necessary to first test the thread damage which
occurs during the weaving process. To this end, individual yarns
were removed from multilayer woven fabrics. The yarns were
tested for tensile force in the warp (w) and pick (p) direction
in various weft densities (7, 10 and 14 weft yarns per cm) on
a Zwick Z100 yarn testing machine (Zwick GmbH & Co. KG,
Germany) according to DIN EN ISO 2062. The diagram in
Figure 8 shows the yarn damage results.

Fig. 8: Effect of GF/PP hybrid yarn processing on tensile force for various
weft densities

No81 May - June 2013 /

jec composites magazine 57

JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013

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