SAMPE Journal - July/August 2016 - 56

Feature Article
Interlayer Toughened Prepreg System (P2352W-19)
The interlayer toughened prepreg system
adhered to the processing cure concept similar to
the non-interlayer prepreg system. Although when
an interlayer toughener is present in the interlayer,
it has greater chance to block the air paths in
the composite in the xy plane, z-direction, or
both. Therefore, during cure if a high compaction
pressure, such as autoclave pressure, is not
available, trapped air pockets and/or volatiles may
not be effectively removed or minimized and may
remain after cure, leading to a high void content.
To overcome this, it was necessary for the material
to dwell at a lower temperature before increasing
to a higher temperature cure cycle in an effort
to maximize consolidation and minimize voids.
In addition, it was anticipated that an interlayer
toughened system would pose a greater challenge
than a non-interlayer toughened system due to
increased viscosity, and the system might not be
designed for fast ramp capabilities. Similar to noninterlayer toughened panel fabrication, a series
of cure experiments as shown in Figure 6 were
performed in the QS process to establish optimal
processing conditions and bagging schemes to
achieve a void-free part. Once a void-free part was
achieved, the following cure profile was established
and utilized to cure all flat laminates on the spar
tool for mechanical testing:
1. PVT held within 30 min prior to start of cure
2. Ramp from room temperature to 140°C, dwell
for 120 min, ramp rate up to 30°C/min
3. Ramp to 180°C, dwell for 120 min, cool down to
room temperature with similar ramp rate

Cycle Time
As shown in Figure 6 total cycle time reduction
was gained through ramp up and ramp down
segments only, for interlayer toughened system
two hour dwell times were necessary to obtain
full conversion of the material in order to retain
mechanical properties.
Table 3 summarizes the total cycle time including
PVT between QS, VBO, and autoclave. For the QS
process the data shows marginal reduction in total
cycle time under 5 hr, which includes a PVT less
than 30 min compared to the standard autoclave
cure at a total cycle time of about 6 hr. VBO again
had the longest total cycle time requiring longer
PVT, more than 3 hr on top of a 6 hr cure for a total
cycle time of over 9 hr.
Process variable temperature overshoot was
also kept to a minimum to control actual part
temperatures which were maintained within 5°C to
avoid excessive heat generation. It was found that
cured plied thickness in the QS process was also
equivalent to an autoclave cured laminate, and the
process was able to produce significantly faster
ramp rates, up to 30°C/min.
Void Content
After cure, all spars and panels showed minimal
to no resin bleed out due to an optimal bagging
scheme similar to non-interlayer toughened
material, and no defects were observed on all
surface areas of the laminates. Figure 7 shows
a representative ultrasonic scan image with
corresponding cross sectional micrographs taken
at various central locations of the CAI panel. The
CAI panel is representative for all laminates due to
it being the thickest laminate fabricated, a 24 ply

Figure 6. Overlay of actual
cure profiles for series of
experiments for interlayer
toughened system.

SAMPE Journal, Volume 52, No. 4, July/August 2016

Table of Contents for the Digital Edition of SAMPE Journal - July/August 2016