SAMPE Journal - September/October 2016 - 53

Feature Article
be 80°C initially, which represents the die temperature
and does not take into account any exothermal effects
of the curing reaction of PU. It is further assumed that
the limit of the PU matrix for a short-time exposure to
high temperatures is, dependent on the actual material,
between 180°C and 210°C. The calculation is based on a
semi-infinite plate with a constant outer temperature (of
230°C). As the temperature reduction of the thermoplastic
top layer during heat transfer is neglected, this is a worstcase-estimation of the maximum temperature influence.
Figure 6 shows the development of the temperature
over time and the penetration depth of a certain
temperature at a certain time. It is most interesting to
take a look at the upper line indicating an exposure time
of 7.5 s, which is likely to be the exposure time at the
defined haul-off speed (1.0 m/min) and the expected
length of the extrusion die. One can see that a critical
temperature of 180°C penetrates the pultruded profile
almost 1.0 mm or at 210°C almost 0.4 mm, respectively.
Specimen of PU (type Elastocoat® C 6226/105 by BASF
Polyurethanes GmbH, Lemfoerde, Germany) exposed to
this temperature show now visual signs of degradation.
However, future material testing must show whether this
temperature influence leads to a degradation of the PU
matrix when exceeding the degradation temperature, or
whether the PU matrix can stand this thermal influence.
As stated, the high pressure level and flow type of the
extrusion process might damage a pre-cured pultruded
profile. Thus it is crucial to raise the viscosity of the
thermoset matrix to a level, where damage is unlikely
to occur. Furthermore, from joining thermoplastics it is
known that the joining process of two partners yields
best results when the viscosity ratio is close to 117.
This means, for joining the viscosity of the PU matrix
should be similar to the viscosity of the thermoplastic.

To predetermine the point where the curing thermoset
matrix has a similar viscosity to a thermoplastic melt,
rheometric trials have been conducted on a Haake
Rheostress RS75 by Thermo Fischer Scientific Inc.,
Karlsruhe, Germany, at a representative shear rate of 10
seconds-1 and a gap of 0.5 mm. The results are shown
in Figure 7. The viscosity development of an exemplary
PU is shown at different measuring temperatures. For
comparison and as target figure, the range of processing
viscosities of a typical thermoplastic extrusion melt is
also given. For each temperature, two trials have been
conducted indicated with #1 and #2.
It can be seen that at low temperature (60°C) the target
viscosity range is reached after 105 seconds and lasts
about 60 seconds. With a rise of 20°C to 80°C, the target
range is already reached after 44 s and joining time is
shortened to 8 seconds. The development continues
with a further rise of the temperature to 100°C, where
the target range is reached after 24 seconds and lasts
only 5 seconds. One can derive two main findings from
the measurements:
1. The time when the target viscosity is reached as
well as the joining time can be carefully adjusted as
necessary by adjusting the temperature. This can also
be done during the continuous process if necessary.
2. There is a potential to speed up the process further,
as the temperature transition from the hot thermoplastic
melt might lead to a snap-curing effect of the thermoset
matrix system.
However, one must keep in mind that these rheometric
trials provide just an estimation of what happens inside
the pultrusion and extrusion die. It must be examined
if the predictions based on rheometric trials meet real
process conditions.

Figure 7. Viscosity development of an exemplary PU matrix (Puropreg 569 made by Ruehl Puromer GmbH, Friedrichsdorf (Ts.))
compared to viscosity of a typical thermoplastic melt.
SAMPE Journal, Volume 52, No. 5, September/October 2016

Table of Contents for the Digital Edition of SAMPE Journal - September/October 2016